Study On Microstrucure Modification And Performance Of Magnesium Alloys By Ultrahigh Pressure Treatment

The compelling need for energy conservation,environmental protection and sustainable development are driving the development of a wide range of lightweight structural and functional materials.Magnesium(Mg)alloys are the lightest metallic structure materials.In addition,they have high specific strength,good castability,good electro-magnetic shielding and high damping properties.They are desirable for applications in the automotive,aerospace,defence,biomedical,sporting and electronic goods sectors.However,low strength,poor ductility and thermal instability have limited their industrial applications.Recently,ultrahigh pressure(UHP)technique can synthesize some novel materials not accessible by other conventional techniques.It is one of the most effective methods to modify the microstructure and develop unique properties of materials.Five Mg alloys are chosen,including the most commom commercial Mg-Al alloy,the lightest Mg-Li alloy,the potential heat-resistant Mg-Sn alloy,the commom rare earth containing Mg-Y alloy and Mg-Y-Zn alloy with long period stacking fault phase.The effects of UHP treatment on the microstructure and mechanical properties of these Mg alloys are studied in this thesis.UHP technique remarkably extends the solid solubility limitation of Al alloying element(～24.9%)in Mg-Al alloys,resulting in unique solid-solution strengthening.The microhardness,yield strength and ultimate compressive strength of the saturated UHP Mg-Al alloy are improved simultaneously without degrading plasticity by forming homogeneous and plate-shaped Mg17Al12 precipitates of 10～30 nm combined wtih artificial aging.In addition,the thermal resistance of the peak hardness UHP Mg-Al alloy is enhanced by eliminating the dominant growth of Mg17Al12 precipitates along(101)plane and anchoring dense stacking faults in phase interface.We report a novel strategy for simultaneously achieving high specific yield strength(～160 kN·m·kg-1)and good elongation(～23.6%)in a duplex Mg-8Li(wt.%,all compositions given hereinafter in wt.%)alloy at room temperature,based on the introduction of densely hierarchical {10(?)1}-{10(?)1} double contraction nanotwins(DCTWs)and full-coherent hexagonal close-packed(HCP)particles in twin boundaries by UHP technique.The conventional strengthening methods mainly involve the formation of internal defects,in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably.However,these hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids,hindering possible dislocation motions.Meanwhile,orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process.The processes lead to a concomitant but unusual situation where the double contraction twinning strengthens rather than weakens magnesium alloys.A novel hexagonal Mg2Sn strengthening precipitate has been achieved by high pressure aging method in a Mg-9Sn alloy.The results show the particle is ellipsoid shaped with an average diameter of 25 nm.Differing from the conventional coarse plate-like or rod-like Mg2Sn strengthening particles with face-centered cubic structure after artificial aging at 200 ℃,the formation of novel fine hexagonal Mg2Sn particles significantly improves the strength of Mg-Sn alloys together with the ductility.The high strength and ductility of the alloy are mainly associated with the presence of fine strengthening precipitates and the low lattice misfit(0.025)of hexagon-on-hexagon crystallographic orientation,respectively.In addition,the microstrucrues of the Mg-9Sn alloy resolidified under UHP are also studied.It reveals UHP can remarkably refine the microstructure of the alloy.The refinement of the microstructure is attributed to the increasing melt point,decreased critical radius,increased rate and site of new phase nucleation and suppressed grains growth during the UHP resolidification.Microstructure evolution and mechanical properties of the high heat-resistance Mg-7Y alloy treated by the UHP boriding process were studied.The UHP boriding process and strengthening mechanism were also discussed.A YB1z strengthening precipitate of-9 nm is reported for the first time in a Mg-7 wt.%Y alloy after UHP boriding processing.The B atoms are preferred to form a B12 cluster and then the Y elements occupy lattice vacancies in B12 cluster to form the YB12 compound directly during boriding.The formation of nanoscale Y312 particles with a high melting point improves the strength of the Mg-Y alloy significantly and this is coupled with its thermal stability.In addition,the age-hardening response is weakened because of the decrease of yttrium solubility in the Mg matrix.The microstructure and mechanical properties of the Mg97Zn1Y2(at.%)alloy treated under UHP(6 GPa)in the temperature range of 500-1200 ℃ were investigated.The results indicated that the as-cast sample consists of a-Mg equiaxed dendrites and continuous lamellar long period stacking ordered(LPSO)phase in grain boundaries.After the SHP treatment,the LPSO phase is gradually replaced by eutectic phase(Mg,Zn)3Y with increasing temperature.The microhardness and strength of sample prepared at 1100 ℃ under SHP treatment are significantly improved compared with the as-cast one at room temperature.The improved mechanical behaviors are mainly attributed to LPSO phase kink-banding strengthening at low temperature and the precipitation strengthening of a large amount of fine(Mg,Zn)3Y particles at high temperature after SHP treatment.The UHP technique is an effective method to modify the microstructure and improve the mechanical properties of the Mg alloys.We obtain the high strength and high ductility Mg alloys by controlling their microstructures evlution and the compositions,morphologies and distributions of the phases in the alloys under the pressure and temperature treatment.