Investigations On The Microsturcture Evolution Of Magnesium And Magnesium-aluminum Alloys Under Pressure

There have been increasing uses of magnesium alloys for light-weight structuralmaterials for automobile, mobile communication, electronics and aerospace industries dueto high specific strength and stiffness, excellent electromagnetic shielding effectivenessand easy recycling capability. Magnesium alloy is being famed for "21century greenmaterials". It is well known that microstructure plays a critical role in determining theproperties of materials. The results show that the hexagonal-close-packed (hcp) ofmagnesium is stable at the atmospheric pressure, however, magnesium has other structuresunder high pressure, such as body-centered cubic (bcc), double hexagonal-close-packed(dhcp) and faced-centered cubic (fcc). The plasticity of the fcc structure is the best, the bccstructure takes the second place, the hcp structure is the poorest, by analysis of the numberof slip systems. Therefore, the change of the structure of magnesium by high pressure canimprove the plasticity of magnesium. The main research works and results are listed asfollows.(1) The structural stability and phase transitions of hcp, bcc, dhcp and fcc structuresof magnesium under high pressure have been studied systematically by using the firstprinciples methods. Meanwhile, the structural stability of magnesium has been studied bythe energy, electronic structure and elastic constants. When the applied pressure is0²20GPa, the volume of four structures of magnesium decreases but the energy of the fourstructures increases with the pressure increasing. By comparing of enthalpy differences forthe bcc, dhcp and fcc structures with the hcp structure, it can be seen that when thepressure is0GPa, the hcp structure of magnesium should be the most stable structure, thebcc structure takes the second place, the third is the fcc structure, the bcc is the last one.When the pressure is up to65GPa, the bcc structure becomes more stable and thetransformation from hcp structure to bcc structure may occur. When the pressure is130GPa, the enthalpy for the dhcp structure of magnesium becomes less than that for the hcpstructure. Therefore, the dhcp structure should be more stable relative to the hcp structureunder such high pressure. So the phase transition between the hcp and dhcp structure ofmagnesium may happen. Similarly, the lower enthalpy for the fcc structure of magnesium at190GPa should also be considered as a more stable structure in comparison to the hcpstructure. Therefore, the hcp structure may change to fcc structure. The calculated resultson the density of states and elastic constants for the four structures of magnesium under0GPa and various phase transformation pressures further evidence the possibility of theabove phase transformation.(2) Under high pressure, the effect of added Al atoms on the structural stability, phasetransition sequence and electronic structure of hcp and bcc structures of magnesium hasinvestigated systematically by using first principles methods.The results show that, when the pressure range is0¹00GPa, the volume for hcp andbcc structures of Mg, Mg-4.17at.%Al and Mg-8.33at.%Al decreases, but the enthalpyfor the three Mg-based materials increases. For Mg, when the pressure is about60GPa,the enthalpy of bcc structure is lower than the hcp structure, indicating that the bccstructure of Mg becomes more stable, and the hcp→bcc transition may take place. Thephase transition pressure under which the hcp→bcc transition may occur for Mg-4.17at.%Al and Mg-8.33at.%Al is about70GPa and85GPa, respectively. It can be concludedthat, as the added Al atoms increase, the phase transition pressure under which thehcp→bcc transition may take place becomes higher, implying that the hcp→bcc transitionof Mg under pressures will be more difficult with increasing Al addition.(3) AZ91D alloys treated by solid solution at673K for24h are aged under highpressure. The effect of holding pressure times and pressure on the microstructures andproperties of AZ91D magnesium alloys are investigated respectively.â‘ High pressure can refine the grain significantly. When the pressure is3GPa andholding pressure time is60min, AZ91D magnesium alloy obtains superior refining effect:the grain size is about1/8¹/10of grains size under ambient pressure treatment.â‘¡In the case of holding pressure treatment under high pressure, there occurs a lot oftwining in the grain of AZ91D magnesium alloy, and the number of twining is dependentupon the holding pressure time and the applied pressure.â‘¢High pressure inhibits the precipitation of the β-Mg₁₇Al₁₂phase during holdingpressure treatment and affects the precipitated position of the β-Mg₁₇Al₁₂phase.â‘£Holding pressure treatment under high pressure for AZ91D magnesium alloy leads to higher micro-hardness. When the pressure is3GPa and holding pressure time is60min, the micro-hardness reaches the maximum value (69.033HV).⑤The corrosion resistance of AZ91D magnesium alloys is improved throughholding pressure treatment under high pressure. When the pressure is1GPa and agingtime is60min, the highest corrosion potential is-1.053V.