Study On Micro-mechanical Behavior Of Aluminum Silicon Alloys By In-situ Synchrotron Diffraction

Aluminum silicon alloy is the most widely used casting aluminum alloy, which is mainly used for complex shaped and low-intensity casting manufacture. It has received increasing attention in construction and automotive industries for its light weight, low thermal expansion coefficient and high corrosion resistance properties. Due to the different thermal expansion coefficient and anisotropy, different degree of deformation will take place in materials during thermal or machining process. This process will inevitably introduce residual stresses, which will eventually affect materials processing and application. Residual stress is the main cause of premature failure or even serious accidents for industry components. Therefore, to make clear microscopic stress state and the microscopic mechanical behavior of the material evolution is of great significance.Micro mechanical behavior distribution and evolution of aluminum silicon alloy during deformation process have not been reported. Thus Al-Si eutectic alloy is investigated in this thesis. Three groups of different heat treatment are carried out to obtain different organization and performance for ratio analysis. the first group is heated to300℃then cooling in the air, the second group is kept warm for4h after furnace heated to550℃, the third group is kept warm for12h after furnace heated to550℃. XRD diffraction, metallographic microscope and digital microscope Vivtorinox hardness tester are used to assess the basic properties of the material. In-situ synchrotron radiation of high energy X ray diffraction technique are carried out to research on the evolution of micro mechanical behavior of Al Si eutectic alloy in direction both parallel and perpendicular to the tensile direction during the deformation process. The main conclusions in this dissertation are as follows:It is confirmed that Al-Si eutectic alloy are constituted of FCC Al phase and FCC Si phase in all the three groups of heat treatment conditions using XRD technique.Microstructure observation shows that, Si particles in the samples tend to be more round by kept warm for4h and12h after furnace heated to550℃than by heated to300℃then cooling in the air. Changes in the shape of Si particles between samples kept warm for4hours and12hours after furnace heated to550℃are not notable.The sample kept warm for12h after furnace heated to550℃shows the most optimal mechanical properties in the mechanical performance test. Its yield strength is83MPa, the tensile strength is159MPa and the micro hardness is59.4HV, which is superior to the other two groups of heat treatment of samples. The mechanical properties of the sample kept warm for4h after furnace heated to550℃are as follows, the yield strength is77MPa, the tensile strength is151MPa and the micro hardness is58.8HV. The mechanical properties of the sample heated to300℃then cooling in the air are as follows, the yield strength is76MPa, the tensile strength is149MPa and the micro hardness is55.3HV. Micro hardness test of three groups heat treatment results show that, hardness of Si phase (70HV) is larger than Al phase (45HV).In-situ synchrotron radiation demonstrates that tensile deformation has little effect on the distribution and intensity of diffraction peaks. Lattice planes are relatively uniform parallel to the loading direction and the lattice plane parallel to the surface of the three samples are mainly the Al(111), Al(200) and Si(220) plane. While the lattice planes along the transverse orientation show distinguished distribution, Al (111) and Al (220) are the main lattice planes paralleled to the surface of the three samples and of great intensity. For the Si phase in alloy, due to the lower proportion, the intensity of diffraction peaks are weak.Based on the quantitative description of the diffraction intensity, the distribution of diffraction intensity of Si (111) and Al (200) lattice plane is nearly a horizontal line, indicating little effect of the deformation on the crystal grain orientation. Diffraction intensity of Al (111) and Si (220) lattice plane changes indicating that internal grain rotate then change orientation due to the external force in the loading process. Serrated wave are observed in Al (200) lattice plane, we guess it is the micro twins that lead to structural changes, and changes of diffraction intensity may be due to structural changes. Diffraction intensity of Al (111) in the sample kept warm for12h after furnace heated to550℃increases obviously after the stress exceeds to100MPa, which is consistent with chart of the diffraction peak, and suggests the rotation of grains to the Al (111) lattice plane which is prone to slip when enter plastic deformation stage.Lattice strain evolution demonstrates that heat treatment has influence on elastic diffraction constant. When enter the plastic deformation stage, stress strain curves of Al phase plane yield first and below the yield point in the macro stress-strain curve. With the increase of stress, Al curves bent backwards. Our view is that the bend back is likely representative of an instability caused by dislocations suddenly entering the Al phase. When dislocations finally penetrate through the interface, there can be significant redistribution of stresses among differently oriented [hkl] grains, including possible transverse constraint effect. The net effect would be lower stress or elastic strain in the Al phase. Si phase exhibits a concave-up curvature, has not obvious yield point and the inflection point coincides with the onset of non-linearity exhibited by the macroscopic extensometer curve, indicating that the Si particles bear the main load in plastic deformation process. Al phase are subjected to a compressive residual stress, Si phase are subjected to a tensile residual stress after unloading.FWHM (full-width half-maximum) of each lattice plane shows little change with the increase of external load, but serrated waves are observed. The FWHM of Al (200) lattice plane of the sample kept warm for4h after furnace heated to550℃trends to decline with the increase of stress. Since the plastic deformation of the material unlikely causes a reduction in the dislocation density, the observed decrease in the FWHM of various diffraction peaks with increasing stress can be, therefore, attributed to the stress-induced grain growth. The FWHM of Al (220) lattice plane of the sample kept warm for12h after furnace heated to550℃trends to increase, which is attributed to the reduction in the grain sizes or to the increase in the dislocation density.