Creep Properties of Magnesium Alloys AE44 and AZ91

Transkript

1 Creep Properties of Magnesium Alloys AE44 and AZ91 HUVUDOMRÅDE: Materials science FÖRFATTARE: Nora Esho Faris HANDLEDARE: Nils-Eric Andersson JÖNKÖPING 2019 April Postadress: Besöksadress: Telefon: Box 1026 Gjuterigatan (vx) Jönköping

2 This thesis was completed at Jönköping School of Engineering. The author is responsible for opinions, conclusions and results presented in this thesis. Examiner: Mohammadereza Zamani Supervisor: Nils-Eric Andersson Credits: 15 ECTS (undergraduate) Date: Postadress: Besöksadress: Telefon: Box 1026 Gjuterigatan (vx) Jönköping

3 Abstract Abstract Cast components of magnesium alloy AZ91 has proven to have poor creep properties at operation. At the department of Materials and Manufacturing at Jönköping University the properties of AE44 are evaluated to see if it performs better as cast component than previous alloy. The purpose of this paper is to research the creep properties of AE44 as a cast component by creep tests and compare it to AZ91. Creep tests yields the activation energy for creep and the stress exponent whose values are associated with different creep mechanisms. Microstructural analysis of AE44 specimens is performed to compute the grain size to investigate how it differs in different positions in the cast component and how this effect the properties of the material. Through SEM analysis, the grain size of two AE44 specimens was determined to be 5.30 and 5.98 microns. Cast AZ91 specimens with the solidification rate of 0.3 mm/s and 6 mm/s are also tested and compared to AE44. Fast solidification rate leads to finer grains which is linked to poor creep properties. The AZ91 (0.3 mm/s) specimen performed therefore better than AZ91 (6 mm/s) when creep tested at 180 C and load 70 MPa were the steady state creep rate was lower. At 180 C and 90 MPa however, AZ91 (6 mm/s) performed better having lower steady state rate than AZ91 (0.3 mm/s). At both 70 MPa and 90 MPa, AE44 had lower steady state rate than both AZ91 specimens. Tensile tests showed AZ91 (6 mm/s) to have higher UTS than 0.3 mm/s at the rates of 0.1 s -1 and s -1 at 180 C. At the same rates and temperature, no big difference in UTS could be observed between AE44 and AZ91. The dominating creep mechanisms of AZ91 are according to other papers, grain boundary sliding and dislocation climb. This could not be verified since only creep test that would yield the activation energy were carried out in this paper. The stress exponents are needed as well in order to determine the creep mechanism, this is to compare the values to other papers. The activation energy of AZ91 with the solidification rate 6 mm/s was higher compared to other papers, however this can be due to higher test temperature in these creep tests. The activation energy of AE44 was approximately 222 kj/mol for test conducted at C and C, which was higher than AZ91 at same temperatures and loads. In conclusion AE44 specimens, taken from cast components, performed better than cast AZ91 specimens in creep tests at MPa and 180 C having lower steady state rates. The impact the solidification rate has on the creep properties of AZ91 could not be determined due to lack of data thus more tests are needed. i

4 Sammanfattning Sammanfattning Magnesiumlegeringen AZ91 är känd för att ha dåliga krypegenskaper och på Avdelningen för Material och tillverkning på Tekniska Högskolan i Jönköping vill man undersöka om AE44 presterar bättre som gjuten komponent. Syftet med arbetet är att undersöka krypegenskaperna av AE44 som gjuten komponent genom krypningstester och hur denna legering jämförs med AZ91. Genom krypningstester erhåller man aktiveringsenergi och stressexponent vilkas värden är associerade med olika krypmekanismer. Mikrostrukturell analys av AE44 prover för att beräkna kornstorleken utförs för att undersöka skillnaden i kornstorleken mellan de olika positionerna i den gjutna komponenten och hur detta påverkar krypegenskaperna. Enligt SEM analys var kornstorleken av två AE44 provstavar 5.30 och 5.98 µm. Gjutna AZ91 provstavar med stelningshastigheten 0.3 mm/s och 6 mm/s testas och jämförs med AE44. Snabb stelningshastighet som leder till mindre kornstorlek är sämre för krypegenskaperna, detta visade sig stämma för ett krypningstest vid belastningen 70 MPa där hastigheten av sekundär kryp var lägre för 0.3 mm/s än 6 mm/s men tvärtom vid 90 MPa. Vid både 70 och 90 MPa hade AE44 lägre kryphastighet än AZ91 i sekundärområdet. Vid dragprov hade AZ91 (6mm/s) högre brottgräns än AZ91 (0.3 mm/s) men ingen stor skillnaden kunde observeras mellan AZ91 och AE44 vid samma temperatur och hastighet. De dominerande krypmekanismerna, som enligt många artiklar är GBS och dislocation climb för AZ91, kunde inte verifieras i denna rapport på grund av att endast krypningstester som erhåller aktiveringsenergi genomfördes. Aktiveringseneringsenergi för AZ91 med stelningshastigheten 6 mm/s visade sig högre jämfört med andra rapporter men detta kan vara till följd av hög testtemperatur jämfört med andra krypningstester. AE44 prover hade höga aktiveringsenergier jämfört med AZ91 vid samma temperatur och belastning. Sammanfattningsvis presterade AE44 provstavar, som kommer från pressgjutna komponenter, bättre än gjutna AZ91 provstavar (provstavar med cirkulärt tvärsnitt som sedan frästs ner till provstavar med rektangulärt tvärsnitt) vid krypningstester med belastningen MPa och temperaturen 180 C då AE44 hade lägre kryphastighet i sekundärområdet. Hur stelningshastigheten påverkar krypegenskaperna för AZ91 kunde inte fastställas, mer tester borde därför utföras. ii

5 Innehållsförteckning Table of Contents 1 Introduction BACKGROUND PROBLEM DESCRIPTION PURPOSE LIMITATIONS DISPOSITION Theoretical framework CREEP DEFORMATION ACTIVATION ENERGY AND STRESS EXPONENT CREEP MECHANISM MICROSTRUCTURE Experimental SPECIMENS TENSILE TEST CREEP TESTS MICROSTRUCTURAL ANALYSIS VALIDITY AND RELIABILITY Results TENSILE TEST AE AZ CREEP TESTS AE AZ91, 0.3 mm/s AZ91, 6 mm/s CREEP TEST, ACTIVATION ENERGY Q AE44 Q AZ91 Q iii

6 Innehållsförteckning 4.4 MICROSTRUCTURAL ANALYSIS Optical microscope Scanning electron microscope Analysis WHAT IS THE ACTIVATION ENERGY FOR AE44 AND AZ91? WHAT IS THE MAIN CREEP MECHANISM FOR AE44 AND AZ91? WHAT IMPACT DOES THE MICROSTRUCTURE HAVE ON THE PROPERTIES OF THE ALLOYS? Tensile test Microstructure Steady state rate Discussion and conclusion CONCLUSION FURTHER WORK References iv

7 1 Introduction 1.1 Background There is a big interest in a material s creep properties of designs during high temperature operation. Creep deformation for metals is approximated to be substantial at temperatures T> T M where T M is the material s melt temperature. [1] Magnesium alloys are extensively used in the automotive industry due to their light weight leading to reduced fuel consumption, however the poor corrosion resistance and mechanical properties at elevated temperatures limit their use. One of the most common magnesium alloys include AZ91 however it is prone to creep deformation at applications where temperatures exceed 125 C which has led to this alloy only being utilized in low temperature applications. Addition of rare earth metals improves the mechanical properties of magnesium alloys to be used in elevated temperature applications. Many components are cast using magnesium alloy AZ91 which is the most commonly used die casting alloy because of its excellent corrosion resistance and mechanical properties at room temperature and castability [2]. This magnesium alloy, however, is known for its poor creep properties. Mg-Al-RE alloys are developed for high temperature applications such as the automotive industry where operating temperatures can reach 200 C. AE44 was developed as an alternative to AZ91 and has shown to have superior creep properties compared to Az91 at elevated temperatures [3]. The department of Materials and Manufacturing at the School of Engineering in Jönköping is interested in evaluating the creep properties of magnesium alloy AE44 as a cast component since AZ91, as a cast component, has shown to have poor creep properties. This paper will be investigating the creep properties of AE44 as cast components and comparing these results with Mg-alloy AZ91 which is currently used to cast the components in this research. 1.2 Problem description It can be hard to determine what properties to expect from materials since there are a lot of influencing factors to consider such as manufacturing and casting method as well as the geometry of the cast component. Components cast in AZ91 were shown to have poor creep qualities at elevated temperatures. The poor creep resistance of this alloy has resulted in loosening bolts during operation. Many studies have demonstrated the superior creep qualities of mg-alloys containing rare earth metals as opposed to AZ91. However, it is difficult to estimate the performance of the components cast in RE-alloys since several factors influence the properties. Manufacturing and casting process and the geometry of the cast component affect the microstructure which in turn affect the mechanical properties of the material. [4] 1.3 Purpose The aim of this study is to examine the creep properties of AE44 and AZ91 magnesium alloys. As several factors influence the creep properties of a material, it is interesting to investigate the creep properties of the alloys as cast components. This work will answer the following questions: What is the activation energy for creep for AE44 and AZ91? What is the main creep mechanism in die-cast AE44 components? What impact does the microstructure have on the creep properties of the alloys? 5

8 1.4 Limitations This paper will investigate the dominating creep mechanism for die-cast components of certain geometry and the results will be compared to AZ91 specimens of rectangular cross-section. The activation energy and stress exponents values of AE44 yielded in this paper applies to the cast components. For AZ91, the values apply to specimens cast by the department of Materials and Manufacturing at Jönköping University. In this work the creep properties of AZ91 as cast components will not be studied. Microstructural analysis will be performed to study the grain size of the specimens to investigate the effect the grain size has on the properties of the specimens. No analysis of the β- phases will be done in this paper. The microstructural analysis will focus merely on the grain size. 1.5 Disposition This paper will begin with briefly explaining creep deformation and how creep tests are carried out. In the theoretical framework research papers are reviewed and different theories regarding the subject will be compared and analyzed. The experimental part describes the steps taken to answer the questions of this paper. It is followed by the results of the experiments including tensile testing, creep testing and microstructural analysis. The results are then analyzed and compared to the results of other research papers and theories. In the last part of the paper the conclusions are presented, and recommendations of further work are stated. 2 Theoretical framework 2.1 Creep deformation Creep deformation is time-dependent plastic deformation under fixed stress at an elevated temperature greater than T m where T m is the absolute melting temperature of the material. Creep is more evident at elevated temperatures because work-hardening diminishes with increasing temperatures. Creep tests are most commonly conducted under fixed tensile load and fixed temperature using creep-testing machine which produces the results as a curve with the strain as a function of time, among other relevant data. The creep curve, with the strain as a function of time, is divided into three regions; Stage I, also called primary creep, where the material experiences hardening through changes in the dislocation substructure. The creep rate, as a result, decreases with time. In stage II, called steady state creep or secondary creep, the creep rate is almost constant and hardening, through dislocation formation, is balanced by dynamic recovery. In stage III, the tertiary stage, the creep rate accelerates rapidly due to cracking and cavitation until the material ruptures. The two most important engineering data obtained from this curve is the steady state rate and the total elapsed time to rupture. Tests conducted at higher temperatures or higher loads exhibit higher creep rates and shorter rupture times. [5-7] Figure 1- a typical creep curve with strain on y-axis and time on x-axis. ε ss stands for steady state rate [6] 6

9 2.2 Activation Energy and Stress exponent The steady state creep rate can be explained by ε = Aσ n exp ( Q RT ) (1) where A is the material constant, σ is stress, n is the stress exponent, Q is activation energy for creep, R is the gas constant and T is the absolute temperature. Equation (1) can be simplified to ln(ε ) = ln(a) + nln(σ) Q/RT (2) Depending on whether the stress or the temperature is constant, equation (2) can be rewritten into: ln(ε ) = C 1 + nln(σ) (3) ln(ε ) = C 2 Q/RT (4) where C 1 and C 2 constants. In Equation (3) the temperature is constant while in equation (4) the stress is constant. [8] The slope of a log ε versus log σ plot at a given temperature yields stress exponent n while the slope of the ln ε versus 1/T plot, called Arrhenius plot, yields the activation energy for creep Q. The stress exponent and activation energy for creep can be used to determine the creep mechanisms for a material. The values of Q and n vary depending on the temperature and stress, indicating the existence of different creep mechanisms at different temperature and stress [9]. 2.3 Creep mechanism Creep mechanisms can be determined based on the stress exponent and activation energy. In this paper the values of activation energy will be compared to the results of other research papers. Dislocation climb and grain boundary sliding are the two most common creep mechanisms for magnesium alloys. Dislocation climb is the climb of dislocations over obstacles with the help of diffusing vacancies. Grain boundary sliding, as the name suggests, is the displacement of grains against each other [10]. According to [11], a low value of stress exponent n in AE44 has been linked to grain boundary sliding at stresses MPa and temperature 175 C. Table 1- creep mechanisms of mg-alloys [10-13] Alloy Temp. [ C] Stress [MPa] n Q [kj/mol] Mechanism Reference AZ Dislocation climb [10] AZ Dislocation glide [10] AZ GBS [13] AZ GBS [10] AZ GBD/Pipe diffusion [13] AZ Coble creep [12] AE GBS [11] 7

10 Table 1 shows the dominating creep mechanism for specific loads and temperatures. The creep mechanisms are linked to different values of stress exponent and activation energy. 2.4 Microstructure The poor creep properties of AZ91 have been generally associated with the formation of Mg 17Al 12 phases near the grain boundaries and interdentritic regions [14]. The Mg 17Al 12 intermetallic phases in AZ91 experiences cracking due to their low melting temperature. Figure 2 - Microstructure of die cast AZ91 [15] Figure 2 shows the microstructure of die cast AZ91. The specimen was taken from die cast sample using a cold chamber machine. The microstructure of AZ91 is made up of primary α-mg and β-mg 17Al 12 phase. The Al rich α-mg forms last during solidification and surrounds the Mg 17Al 12 phase. [15] The addition of RE elements lead to formation of thermally stable intermetallic phases. These favorable intermetallic phases inhibit the dislocation motion and grain boundary sliding during creep since studies have shown GBS to be the dominating creep mechanism for AZ magnesium alloys [16]. AE42, which contains 4% aluminum and 2% rare earth metals, have better mechanical properties at high temperatures; this is related to the formation of Al-RE intermetallic phases during solidification which are thermally stable [15]. Creep properties of AE42 has been improved by adding more rare earth elements, creating AE44. There are numerous theories as to why AE44 is superior to AE42 in terms of creep resistance, first one being the formation of Al 2RE intermetallic phases which have better thermal stability than Al 11RE 3 phases which are the dominant intermetallic phases in AE42. At temperatures as high as 150 C and above, Al 11RE 3 releases Al atoms that react with Mg atoms to create Mg 17Al 12 resulting in a decline in creep resistance. Another theory suggests that both Al 2RE and Al 11RE 3 are thermally stable at temperatures up to 200 C and the precipitation of Mg 17Al 12 is a result of Al supersaturated α-mg matrix. [17]. 8

11 Figure 3 - Microstructure of AE44 [15] Figure 3 shows the microstructure of an AE44 specimen derived from high pressure die cast sample. The microstructure of AE44 is made up of primary α-mg, lamellar eutectic phase of α- Mg and Al xre y precipitations as well as single Al xre y phases. [15] The grain size has a big influence on the properties of the material. Bigger grains normally mean improved creep properties by delaying the grain boundary sliding. Grain size is influenced by the casting process; therefore, the casting process have large influence on the creep properties of the material. [10] 3 Experimental 3.1 Specimens To find out the mechanical properties of the magnesium alloys, experimental tests in form of tensile testing and creep testing will be performed. Sub-size specimens were made according to ASTM E8 standard. The flat AE44 specimens were taken from cast components. From each sample five specimens could be taken. Five positions in the cast components were identified suitable for testing as they were of uniform thickness. a) b) Figure 4 - a) Specimen according to ASTM E8 standard b) specimen taken from component 9 and position 1 in the cast component Figure 4a shows the approximate dimensions of the specimens. The test specimens had a rectangular cross-sectional area ca 2 mm thickness and 6 mm gauge width. 9

12 a) b) Figure 5- a) cast cylindrical AZ91 specimen b) shape of AZ91 specimen after milling process The test specimens of AZ91 were cast and later solidified in a Bridgman-furnace to control the solidification rate. In this case, two solidification speeds of 0.3mm/s and 6 mm/s were chosen. The cast AZ91 specimens were then milled to an approximate thickness of 2 mm and later to shape according to figure 4a. 3.2 Tensile test Tensile tests were performed using a tensile testing machine, a furnace to control the testing temperature and an extensometer to measure the elongation. Figure 6 Zwick Roell z100 tensile testing machine with laser extensometer in front and furnace in the back The specimens were tested under temperatures of 120 C, 150 C and 180 C and the speeds 0.01 s -1 and s -1. When the furnace reached the desired test temperature, the specimen was kept in the furnace for 45 minutes to reach a homogenous temperature before the test was run. The test equipment collects data from the tests and produces graphs of interest; standard force vs elongation, standard force vs time etc. The collected data is then used to create graphs in excel to find out the ultimate tensile strength reached for each test. Each test of same the temperature and loading rate, on AE44 specimens, was conducted twice to reduce the risk of anomalous results. If the results were to deviate from each other more than was determined acceptable, the test would to be performed again. When performing the tests on AE44 specimens, which were taken from the cast components, the positions from which the specimens were taken was of interest in evaluation of the results. 10

13 3.3 Creep tests The creep tests were performed using a creep test rig which was designed at the department of Materials and Manufacturing at Jönköping University. The creep test rig includes a furnace that can be lowered down to cover the specimen. Figure 7 - creep test rig with furnace covering the specimen Creep tests were performed at 180 C and with a constant load. The first set of tests at 180 C was at a constant load of 90MPa for AE44 specimens and two different AZ91 specimens with a solidification temperature of 0.3 mm/s and 6 mm/s respectively. The second set of tests were at a constant load of 70MPa. The steady state rate, which will be studied, depends on the stress and temperature the specimens are subjected to. To investigate the creep mechanism of as-cast AZ91 and AE44, the stress exponent n and activation energy Q must be calculated. To calculate the stress exponent, the specimen needs to undergo changes in load during creep test. This can be done by subjecting the specimen to a load of 70 MPa, after it has creeped significantly, the load can be raised to 80 MPa and later 90 MPa. Stress exponent is then the gradient of the line in a log ε vs. log σ plot at a given temperature. The activation energy is calculated similarly to stress exponent except the temperatures is raised after significant creep. The slope of the line in the ln ε vs. 1/T plot will yield the activation energy Q at a specific load. 3.4 Microstructural analysis The microstructures of the specimens were studied by scanning electron microscope and optical microscope. A small sample of the specimens was ground and polished to achieve a smooth surface using Struers Tegramin-30. For optical microscopy, the AE44 specimens were etched with Nital, a solution of 3 vol% nitric acid and ethanol. Through SEM analysis the grain size of the specimens was calculated as well as by using the line intercept method. An EDAX analysis was also performed to identify the elemental composition of the specimens. 11

14 Figure 8 - Line intercept method Line intercept method is the measurement of the grain size by placing a line on an image and counting the grains the line intersects. The length of the line is divided by the number of grains to calculate the grain size. To ensure validity, ten lines of the same length were placed over the micrograph. The length of the lines was then divided by the number of grains and the average of these was calculated. Figure 8 shows the microstructure of specimen vh4t1 with ten lines randomly placed on the micrograph. 3.5 Validity and reliability Certain measures have been taken in order to ensure validity and reliability. Testing an alloy twice during tensile testing (at same rate and temperature) and making sure the results are similar assures reliability in case a single test would be inaccurate. In case the two test results of the same alloy during the same testing parameters would deviate from each other exceedingly, another similar test would be performed. Tensile tests were performed at temperatures of 120 C-180 C. The furnace was equipped with three thermometers inside which measured the temperature at three levels in the furnace to assure the right temperature is attained thus entailing reliable results. When measuring the grain size diameter using line intercept method, several lines were drawn across the micrograph to calculate a more accurate grain size than if only drawing one or few lines. However, this method is not entirely objective as what is considered a grain or dendrite arm depends on the person doing the measurements. The grain size of some specimens was also measured by SEM analysis and compared to the former method. Measurements of the specimens thickness and width were carefully performed using both digital caliper and micrometer, for the software to calculate cross sectional area required to calculate the load. 4 Results 4.1 Tensile test AE44 Tensile tests were performed at three different temperatures and at two different rates. Tensile testing yields the ultimate tensile strength of the material. The specimens of AE44 were cut from ten cast components of which yielded 5 specimens per component. The components and the specimens have designated names to identify the component of which the specimen was taken from and their position in the cast component itself. E.g. vh4 indicates component nr 4 while vh4t1 indicates specimen taken from position nr 1 in component nr 4. Each test of same parameters was conducted twice to reduce the risk of anomalous results. 12

15 Figure 9 - AE44 specimens tested at 120 C at two speed rates Figure 9 displays the tensile test curves of four specimens of magnesium alloy AE44 that were taken from a cast component. Vh4t4 and vh4t1 (position nr4 from component nr 4 and position nr 1 from component nr4 respectively) were tested at a rate of s -1. Vh4t5 and vh4t3 were tested at a speed of 0.01 s -1. In the graph, S stands for slow ( s -1 ) while F stands for fast (0.01 s -1 ). From figure 10 the ultimate tensile strength can be yielded. Table 2 - UTS of AE44 specimens at 120 C Max Standard Force [MPa] vh4t4 vh4t1 vh4t5 vh4t Figure 10 - AE44 specimens tested 150 C at two loading rates Table 3 - UTS of AE44 specimens at 150 C Max Standard Force [MPa] vh10t4 vh10t2 vh8t4 vh8t

16 Figure 4 - AE44 specimens tested at 180 C at two loading rates Table 4 - UTS of AE44 specimens at 180 C Max Standard Force [MPa] vh10t1 vh8t3 vh10t3 vh4t AZ91 Figure 52 - AZ91 specimens tested at 180 C at different loading rates Table 5 - UTS of AZ91 at 180 C Max Standard Force [MPa] 0.3mm/s S 0.3mm/s F 6mm/s S 6mm/s F

17 4.2 Creep tests AE44 AE44 specimens were creep tested with the temperature 180 C and 90MPa load. The steady state rate was of interest to study the creep behavior of the materials. Table 6 and 7 show the steady state rate of the AE44 specimens and the duration of the entire creep tests. Figure 63 - AE44 specimens, 180 C, 90MPa Table 6 - Steady state rate of AE44 specimens (180 C, 90MPa) Vh8t5 Vh10t5 3.42E E-06 Table 7 - Duration for creep test in seconds, minutes and hours Duration s/min/h Vh8t5 Vh10t

18 4.2.2 AZ91, 0.3 mm/s Figure 74 - creep curve for AZ91 specimen solidified at 0.3mm/s in Bridgman-furnace. 180 C, 90MPa Duration of the creep test as shown in figure 14 lasted approximately 345 seconds and had the steady state rate of 5.19E-05. The secondary creep state is highlighted and the slope of the curve in that area is the steady state rate. Figure 85 - creep curve for AZ91, solidified at 0.3mm/s. Testing parameters: 180 C, 70MPa Duration of the test as shown in figure 15 was approximately 2272 minutes (38 h). Previous test, at 90MPa, only endured ca 5 minutes. 16

19 4.2.3 AZ91, 6 mm/s Figure 96 - AZ91 specimens with solidification temperatures of 6 mm/s Figure 16 displays the creep curves of AZ91 specimens which were cast and later solidified at a rate of 6 mm/s. These specimens were both creep tested at temperature 180 C, the first one at constant load of 90MPa and the second 70MPa. The duration of the first test was approximately 170 minutes and 1962 minutes for the second. We can also note that the steady state rate was 7.38E-06 for specimen tested at a constant load of 90MPa and 5.72E-07 for 70MPa. 4.3 Creep test, activation energy Q To determine the activation energy for creep, the test specimen need to undergo changes in temperature while tested. These creep tests began at 180 C and the temperature escalated 5 C after sufficient creep deformation. When the specimen reached a homogenous temperature of 185 C, it was allowed to deform furthermore before elevating the temperature up to 190 C. Afterwards, the test was aborted and restarted at temperature 170 C. The same procedures were performed (i.e. temperature bumped up by 5 C after sufficient deformation) until the test temperature was 180 C and later aborted AE44 Q Figure 17 - Creep curve for AE44 at 180 C-190 C 17

20 Figure 17 shows creep curve for AE44 measured from after nearly seconds up until the test was aborted. Below is the steady state rate of the AE44 specimen at temperatures 180, 185 and 190 C. Table 8 - steady state rate of graph in figure 18 Temperature 180 C 185 C 190 C Secondary state rate 6.64E E E-06 Figure 18 - Plot of AE44 starting at a test temperature of 180 C, elevating to 185 C and 190 C A plot of ln ε versus 1/T will yield the activation energy Q. The slope of the line equals -Q/R (where R is the gas constant J/mol). This gives an activation energy of 222 kj/mol for AE44 at a constant load of 90 MPa. Figure 19 - creep curve with temperatures 170 C, 175 C and 180 C Above is the creep curve for AE44 specimen vh9t4 tested at 170, 175 and 180 C. The test was aborted after substantial creep at 180 C. 18

21 Table 9 - steady state rate of AE C Temperature 170 C 175 C 180 C Steady state rate 1.67E E E-07 Figure 20 Plot of AE44 at 170 C, 175 C and 180 C Plot in Figure 20 yields an activation energy of 222 kj/mol AZ91 Q Figure 21 - creep curve with a starting test temperature of 180 C before being elevated up to 185 C and 190 C Table 10- steady state rate 180 C, 185 C and 190 C Temperature 180 C 185 C 190 C Steady state rate 8.99E E E-05 Creep test in figure 21 was aborted after substantial creep at 190 C. Table 10 shows the steady state rate of the AZ91 specimen at the corresponding temperatures, it can be noted that the higher the temperature the faster the creep rate. The activation energy can be derived from a ln ε versus 1/T plot where ε is the steady state rate and T is the absolute temperature. 19

22 Figure 22 - First set of tests where each point indicates the test temperature and the corresponding secondary creep rate. Activation energy of AZ91 (6mm/s) is 153 kj/mol at 180 C-190 C and constant load of 90 MPa. Figure 23 - creep curve with starting temperature of 170 C before being elevated to 175 C and lastly 180 C Figure 23 shows the creep curve of the same specimen as previously which was tested at testing temperatures of C. The creep test was aborted after enough information could be gathered from the test. From the creep curve the steady state rate can be acquired at the different temperatures. Table 11 shows the steady state rate of the AZ91 specimen at the corresponding temperatures. Table 11 - steady state rate from 170 C Temperature 170 C 175 C 180 C Steady state rate 4.76E E E-05 20

23 ln ε C /T y = x Figure 24- Arrhenius plot of the second creep test at temperatures 170 C, 175 C and 180 C Slope of line in figure 24 gives an activation energy of 192 kj/mol. 4.4 Microstructural analysis Optical micrographs of the surface of the specimens after tensile testing were taken to study the microstructure. The micrographs were taken of tensile tested specimens since the risk of the microstructure being affected by high temperature weren t as likely as the specimens being creep tested. The samples were etched using Nital, a solution of ethanol and nitric acid, to reveal the microstructure Optical microscope a) b) c) d) Figure 25 a) vh4t4 b) vh10t4 c) vh4t1 d) vh10t2 The micrograph of the surface of vh4t4 was taken after the tensile test at 120 C while the micrograph of vh10t4 was tested at 150 C. These specimens come from the same position in the cast components thus a fair comparison of the microstructure can be made. 21

24 Vh4t1 was tested under the temperature and rate as vh4t4 and exhibited similar behavior as can be seen in figure 1. Vh10t2 was tested under temperature and rate as vh10t4 as can be seen in figure 2. A comparison can be made of the microstructure of vh4t4 and vh4t1, these are taken from the same component (vh4) but from different positions in the component. A comparison between vh10t4 and vh10t2 can also be made since they come from the same component, were tested under same parameters in the tensile test and showed similar results. Figure 26 - average grain size of the specimens according to line intercept method Table 12 - grain size measured using line intercept method Specimen Average dendrite arm size [µm] Standard deviation Vh4t4 (120) Vh10t4 (150) Vh4t1 (120) Vh10t2 (150)

25 4.4.2 Scanning electron microscope Microstructures of two specimens were studied and the grain size measured using a SEM. Figure 27 - A micrograph taken by a SEM and a table displaying the grain size of the specimen followed by a graph with grain diameter on x-axis and number fraction on y-axis Above is a micrograph of specimen vh4t1 which was tensile tested at 120 C. The picture illustrates the grain boundaries the software could detect. From the table in figure 27 we can gather that the average diameter of the grains is approximately 5.30 microns. Figure 28 - Micrograph of vh10t2 with highlighted grain boundaries followed by a table displaying the grain size and a graph with grain diameter on x-axis and number fraction on y-axis According to the table in figure 28, the average grain diameter of the tensile tested specimen vh10t2 is approximately 5.98 microns. 23

26 5 Analysis 5.1 What is the activation energy for AE44 and AZ91? Table 13 - Summary of the activation energy for creep Alloy Stress [MPa] Temperature [ C] Activation Energy Q [kj/mol] AE AE AZ91 6 mm/s AZ91 6 mm/s From the table above, we can gather the activation energy for alloys AE44 and AZ91 (solidified in Bridgman-furnace at 6 mm/s). Performing the same type of test on AZ91, which solidified at 0.3 mm/s in Bridgman- furnace, turned unsuccessful as the material ruptured after a few minutes and there was not enough time for the material to attain the new set temperature in order to compute the activation energy. As can be noted, AE44 had higher activation energy than AZ91 (6mm/s). AZ91 had high activation energy compared to other research papers. As [18] presents, the activation energy for creep and stress exponent increase with increasing temperature and load. Since the test temperatures were higher than those in table 1, the high activation energies are expected. 5.2 What is the main creep mechanism for AE44 and AZ91? It is difficult to determine the main creep mechanism since it is essential to calculate both the stress exponent and activation energy. In this work however, only the activation energy was calculated due to limited time. According to other papers, GBS and dislocation climb are the most common creep mechanisms for magnesium alloys. Creep mechanism of AE44 has been linked to grain boundary sliding for low values of n. This however, could not be confirmed. 24

27 5.3 What impact does the microstructure have on the properties of the alloys? Tensile test Table 14 - UTS of the tensile tested specimens Alloy Temperature [ C] Rate [s -1 ] UTS [MPa] Vh4t Vh4t Vh4t Vh4t Vh10t AE44 Vh10t Vh8t Vh8t Vh10t Vh8t Vh10t Vh4t AZ In table 14, type 2 performed worse than the corresponding specimen undergoing same test (same temperature and rate). We can also see that AZ91 specimens had different results depending on the solidification rate. AZ91 specimen with the higher solidification rate (6 mm/s) had a higher value of ultimate tensile strength compared to 0.3 mm/s at both test rates, this is expected since higher solidification rates leads to smaller grain and better mechanical properties. 25

28 5.3.2 Microstructure A comparison of the microstructure can be made of vh4t4 and vh10t4 since they are taken from the same position in the cast components vh4 and vh10. Vh4t4 was tensile tested at 120 C and vh10t4 was tested at 150 C. When tensile testing the specimens, they were kept in the furnace for about 45 minutes or until a homogenous temperature was attained prior to testing. From what can be seen in the micrographs of type 4, the dendrites seemed to be larger and more connected than type 1 and 2. These micrographs however were taken of one position only and do not represent the entire microstructure of the specimens. Vh4t4 had the average grain size of while vh10t4 had the average grain size of according to the line intercept method. The relatively small difference in the grain size can be due to the specimens coming from the same position in the cast components and the slightly larger grain size of vh10t4 can be attributed to higher testing temperature. Vh4t4 and vh4t1 were both tensile tested at 120 C at a rate of s -1, to ensure that the first test didn t produce inaccurate results. These AE44 specimens performed similarly having the UTS of 172 and 161 MPa respectively. The grain size of these specimens, and microns, were similar in size using line intercept method. Computing the grain size using line intercept method versus SEM analysis, produced two results. Vh10t2 had an average grain size of 6.79 and 5.98 microns using line intercept method versus SEM analysis. Vh4t1 had an average grain size of and 5.30 microns using line intercept method versus SEM analysis. This is a relatively big difference in size and can be explained by the reliability of the grain size measurement methods. Line intercept method was implemented on micrographs of etched samples, the atoms along the grain boundaries are loosely bonded and tend to react more easily with the etchant than the atoms that are a part of the grain structure. When viewed under microscope the grain boundaries appear darker, however some grains may have been affected by the etchant and as a result appear to be part of the grain boundaries. Another reason for this can be that in line intercept method it is recognized that the microstructure is composed of dendrites as opposed to grains and therefore counts a grain, roughly split by a boundary, as one, resulting in a relatively big value of grain size Steady state rate According to literature studies, magnesium alloys with rare earth metals as alloying elements should have better creep properties than AZ91. The β-phase in AZ91 have a low melting temperature which eventually leads to cracking, thus the AE44 should have superior creep properties. Bigger grains are also favorable since they impede grain boundary sliding which is shown to be one of the most occurring creep mechanisms for magnesium alloys. Table 15 - summary of the steady-state rate at constant load and temperature of 180 C. The specimens are listed in numerical order starting with highest steady state rate to lowest Alloy Stress [MPa] Creep rate [s -1 ] AZ91 (0.3 mm/s) E-05 AZ91 (6 mm/s) E-06 AE44 (vh10t5) E-06 AZ91 (6 mm/s) E-07 AZ91 (0.3 mm/s) E-07 AE44 (vh8t5) E-07 From the table above, we can note the minimum creep rate for the different alloys and their positions in the component or solidification speed. Vh8t5 and vh10t5 are taken from the same position in two cast components. As can be seen in figure 13, although the steady state rates of 26

29 vh8t5 and vh10t5 didn t deviate from each other too much, the creep curves didn t have much resemblance. Low solidification rate is linked to bigger grain size which should impede dislocation motion. As can be seen in the table, AZ91 specimen with solidification rate of 0.3 mm/s had, by far, the highest secondary state rate in this set of tests. It is followed by AZ91 solidified at 6 mm/s. At 90 MPa, AZ91 (0.3 mm/s) had the highest creep rate while the two AE44 specimens had the lowest. At 70 MPa however, AZ91(0.3 mm/s) had lower creep rate than AZ91 (6 mm/s) which is according to the theory of bigger grains being beneficial for the creep properties. AE44 was also tested at 70 MPa and the test carried on for several weeks before increasing the temperature to determine the activation energy. The superior creep properties of AE44 over AZ91 were expected based on the papers arguing for the thermally instable β-phase of AZ91. However, more studies on the effect of β-phase and the influence it has on creep properties are needed. Among the different AE44 specimens, no assumptions can be made as to which position in the component have better creep properties. 6 Discussion and conclusion 6.1 Conclusion There seem to be a difference in the performance of the specimens from the AE44 cast component, where type 2 performed the worst compared to specimens undergoing similar tensile test. The grain size of one type 2 specimen revealed to have smaller size compared to the other specimens, however this average size may not be continuous throughout the whole microstructure. There is not enough data to fully determine the performance of each AE44 specimens and the AZ91 specimens. The difference in ultimate tensile strength of the AE44 specimens versus AZ91 specimens at 180 C was considerably small and no certain conclusion can be made. Creep testing revealed AE44 to have superior creep properties compared to AZ91 at MPa and C. AZ91 specimen with solidification rate of 0.3 mm/s had higher creep rate than 6 mm/s at 90 MPa and 180 C but lower creep rate at 70 MPa. Activation energy of AE44 (as cast components) was approximately 222 kj/mol at 90 MPa with temperatures C and C. AZ91, cast and solidified at 6 mm/s and creep tested at 90 MPa, had the activation energy 153 kj/mol at C and 192 kj/mol at C. The dominating creep mechanism is determined by the stress exponent and activation energy by comparing these values to the results of other research papers. Without stress exponent no creep mechanism was determined in this paper. However, many articles concluded grain boundary sliding and dislocation climb to be the dominating creep mechanism. 6.2 Further work For future work, the creep mechanisms can be determined by computing the stress exponent n. Creep testing under different stresses at a set temperature would yield the stress exponent. Many other papers had similar results in that they agreed on certain creep mechanisms being typical for magnesium alloys. It would be interesting to see how the as cast alloy would perform and what the dominating creep mechanisms would be to compare to the results of other research papers. As creep testing is time consuming, not many creep tests could be performed. It was therefore, in this work, difficult to, with certainty, draw any conclusions as to the best performing alloy and best performing specimen of the AE44 alloy. For future work, to see how the different positions in the as cast component performs during creep tests, it would be beneficial to do more testing and gather more data to accurately determine the properties of the as cast material. More in depth analysis of the microstructure, especially the β-phase, is beneficial for understanding the affect it has on the properties. 27

30 References [1] W. Leijon, i Karlebo Materiallära, Stockholm, Liber, 2014, pp. 67, [2] X. Wang, S. Zhu, M. Easton, M. Gibson och G. Savage, Heat Treatment of vacuum high pressure die cast magnesium alloy AZ91, International Journal of Cast Metals Research, vol. 27, nr 3, pp , [3] S. Zhu, M. A. Easton, T. B. Abbott, M. A. Gibson och J.-F. Nie, The Influence of Individual Rare Earth Elements (La, Ce, or Nd) on Creep Resistance of Die- Cast Magnesium Alloy AE44, Advanced Engineering Materials, vol. 18, nr 6, pp , [4] H. Cao och M. Wessén, Effect of microstructure on mechanical properties of ascast Mg-Al alloys, Metallurgical and Materials Transactions A, vol. 35, nr 1, pp , [5] J. T. Black och R. A. Kohser, i DeGarmo's materials and processes in engineering, Wiley, 2008, pp [6] M. E. Kassner, Fundamentals of Creep in Materials, i Fundamentals of creep in metals and alloys, Amsterdam, Elsevier, 2009, pp [7] A. Srinivasan, K. Ajithkumar, J. Swaminathan, U. Pillai och B. Pai, Creep Behavior of AZ91 Magnesium Alloy, Procedia Engineering, vol. 55, pp , [8] B. Jiang, C. Zhang, T. Wang, Z. Qu, R. Wu och M. Zhang, Creep behaviors of Mg 5Li 3Al (0,1)Ca alloys, Materials & Design, vol. 34, pp , [9] A. Luo, Recent magnesium alloy development for elevated temperature applications, International Materials Reviews, vol. 49, nr 1, pp , [10] K. U. Kainer, Y. D. Huang, H. Dieringa och N. Hort, Status of the Development of Creep Resistant Magnesium Materials for Automotive Applications, Materials Science Forum, Vol , pp , [11] A. Kiełbus och T. Rzychoń, Microstructure and Creep Properties of AJ62 and AE44 Die-Casting Magnesium Alloys, Materials Science Forum, Vol. %1 av % , pp , [12] K. R. Athul, U. T. S. Pillai, A. Srinivsan och B. C. Pai, A Review of Different Creep Mechanisms in Mg Alloys Based on Stress Exponent and Activation Energy, Advanced Engineering Materials, vol. 18, nr 5, pp , [13] M. Pekguleryuz och M. Celikin, Creep resistance in magnesium alloys, International Materials Reviews, vol. 55, nr 4, pp ,

31 [14] S. Zhu, M. Gibson, J. Nie, M. Easton och T. Abbott, Microstructural analysis of the creep resistance of die-cast Mg 4Al 2RE alloy, Scripta Materialia, vol. 58, nr 6, p , [15] G. Gerstmayr, H. Leitner och W. Eichlseder, Creep and Relaxation Behaviour of Mg-Al based Die cast Alloys AZ91 and AE44, Proceedings of EMC, Leoben, [16] A. Srinivasan, J. Swaminathan, M. Gunjan, U. Pillai och B. Pai, Effect of intermetallic phases on the creep behavior of AZ91 magnesium alloy, Materials Science and Engineering, vol. 527, nr 6, pp , [17] S. Zhu, J. Nie, M. Gibson, M. Easton och P. Bakke, Microstructure and Creep Behavior of High-Pressure Die-Cast Magnesium Alloy AE44, Metallurgical and Materials Transactions A, vol. 43, nr 11, pp , [18] H. Li, W.-b. Du, J.-h. Li, S.-b. Li och Z.-h. Wang, Creep properties and controlled creep mechanism of as-cast Mg-5Zn-2.5Er alloy, Transactions of nonferrous Metals Society of China, vol. 20, nr 7, pp ,