Effect Of High Presure On Microstructure And Properties Of AZ91D Alloy

Magnesium alloy becomes an important novel structure material for its advantages such as low density, high specific strength and specific stiffness. However, the application of magnesium is limited by its low plasticity and corrosion resistance.The purpose of this work was to improve the properties of AZ91D alloy. The cubic anvil high pressure apparatus was used to produce high pressure, the AZ91D alloy was treated by changing the pressure and temperature simultaneously. The microstructures and properties of the alloys were studied.The phases and microstructure of AZ91D alloys before and after treated were analyzed by X-ray diffraction and optical micrograph; the mechanics properties of the alloys were analyzed by Gleeble thermal-mechanical simulation tests; the compression fractographs of the alloys were investigated by scanning electron-microscope(SEM); and the corrosion properties of the alloys were analyzed by CHI660A electrochemical workstation.It was shown that there was little change in phases of AZ91D alloys before and after treated under high pressure at room temperature. The compressive strength of the alloys treated under high pressure at room temperature increased, but the maximum plastic strain reduced. The polarization curve testing indicated that the corrosion currents increased and the corrosion resistance reduced.When the AZ91D alloys treated by high pressure, with the temperature increase, the microstructure of the alloys changed. First, recrystallization occurred, subsequently, the second phaseβ-Mg17Al12, which distributed discontinuously along the grain boundaries, fragmentated into chain-shaped, sphericized particles and gradually decomposed to dissolve in matrixα-Mg, and the grain size grew up, the diffraction peak corresponding toβ-Mg17Al12 phase disappeared gradually in the XRD spectrum. The temperature of recrystallization and the dessolution incresed with the pressure increase.β-Mg17Al12 was the strengthen phase of AZ91D alloy, the microstructure of the alloy was the main cause of the changes of its mechanics properties, which were connected with the pressure and the temperature simultaneously. The maximum plastic strain of the alloy increased with the increase of temperature. Considering the compressive strength and maximum plastic strain simultaneously, the alloy treated under 5 GPa high pressure at 800℃showed the best mechanics property: the compressive strength increased from 233.5 MPa to 280.1 MPa and the maximum plastic strain increased from 13.3% to 19.0%. The compression fractures of the alloys were brittle fractures with obvious crystallography faces. Compared with the alloys treated under highpressure at room temperature, the polarization curve testing indicated that the corrosion current of the alloys reduced when teated under high pressure at high temperature, and the corrosion resistance was improved.