Papers by Author: Da Li Mao

Abstract: The effect of second-phase particles on the grain refinement of AZ61 and AZ91 Mg-Al-Zn alloys with different volume fractions of β-Mg17Al12 phase particles during equal channel angular extrusion (ECAE) has been investigated. The microstructure were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which revealed that grain refinement was enhanced by second phase particles at initial stage of ECAE. And finer grains with the high angle grain boundaries (HAGBs) and disperse second-phase particles could be obtained in AZ61 and AZ91 after 8 passes of ECAE.

Abstract: The effect of second phase on the mechanical properties of AZ61 and AZ91 Mg-Al-Zn magnesium alloy processed by equal-channel angular extrusion (ECAE) has been investigated. The results revealed that cracks initiated at and propagated through the α-Mg/β-Mg17Al12 interfaces and grain boundaries of Mg-Al-Zn alloys during tensile deformation. For AZ61, the elongation increased with the increase of ECAE pass number and the decrease of grain size. However, the elongation of AZ91 with more second phase particles decreased with the increase of ECAE pass number and the decrease of grain size.

Abstract: The platelike bainitic ferrite growth rates were calculated by two modified diffusional models. Good agreements between experimental and theoretical results are found in Fe-0.59C wt.%, Fe-0.81C wt.% and Fe-0.478C-4.87Ni wt.% alloys. A slowing down effect due to the alloying element Mo is emphasized in Fe-0.69C-1.8Ni-0.8Mo wt.% alloy. However, the experimental data are lower than theoretical ones about two orders in Fe-C-8.7Ni wt.% alloys. According to the discussion of the results, it is suggested that the bainite transformation mechanism may relates to steel composition and transformation temperature.

Abstract: The maximum number of carbon atoms adsorbed at edge dislocation was calculated using a model based on Boltzmann distribution. The number of carbon atoms adsorbed by edge dislocation increases with bulk concentration increasing and temperature decreasing. When the interfacial energy was chosen as 0.2J/mol, the maximum radius of carbon depleted zone is smaller than the critical radius required for displacive nucleation at 700K, however, larger than the critical radius at 600K. Moreover, the radius of carbon depleted zone around the dislocation at 600K is larger than that at 700K. It seems that bainite may nucleate by a displacive mechanism at lower temperature.

Abstract: Microstructure evolution and mechanical properties of AZ31 Mg alloy during equal channel angular extrusion (ECAE) at the temperature range of 453-498K was investigated. The processing temperature is an important factor to affect the microstructure and mechanical properties of the Mg alloy during ECAE. ECAE processing can be practiced at 498k or upon the temperature for as-received AZ31 alloy. A new two-step ECAEed processing was successfully used with lowing the processing temperature to 453k. The ductility increased but yield stress decreased though the grain refinement after ECAE at 498k because the recrystallization took place and large angle grain boundary formed. However both the ductility and yield stress were increased after two-step ECAE, which was ascribed to grain refinement as well as incomplete dynamic recovery and recrystallization during the processing.