To explore the effect of strain rate ε on the high temperature deformation characteristics of ultrafine-grained materials, the deformation and damage features as well as microstructures of ECAP-treated pure Al at different temperatures T and strain ratesε were systematically studied through compression tests and microscopic observations. The increase in ε eliminates strain softening at T≤473 K, and largely enhances the yield strength and flow stress at 473?573 K. The shear deformation dominates the plastic deformation of ECAP-treated Al. Many cracks along shear bands (SBs) are formed at T≥473 K and secondary SBs basically disappear at 1×10?3 s?1; however, at 1×10?2 s?1, cracks are only observed at temperature below 473 K, and secondary SBs become clearer at T≥473 K. The microstructures of ECAP-treated Al mainly consist of sub-grains (SGs). The increase in ε inhibits the SG growth, thus leading to the increases both in yield strength and flow stress at high temperatures.
To explore the influence of cyclic pre-deformation on the mechanical behavior of ultrafine-grained（UFG）materials with a high stacking fault energy（SFE）,UFG Al processed by equal-channel angular pressing（ECAP）was selected as a target material and its tensile behavior at different pre-cyclic levels D（D=N_i/N_f,where N_i and N_f are the applied cycles and fatigue life at a constant stress amplitude of 50 MPa,respectively）along with the corresponding microstructures and deformation features were systematically studied.The cyclic pre-deformation treatment on the ECAPed UFG Al led to a decrease in flow stress,and a stress quasi-plateau stage was observed after yielding for all of the different-state UFG Al samples.The yield strengths_（YS）,ultimate tensile strengths_（UTS）,and uniform straineexhibited a strong dependence on D when D≤20%;however,when D was in the range from 20%to 50%,no obvious change in mechanical properties was observed.The micro-mechanism for the effect of cyclic pre-deformation on the tensile properties of the ECAPed UFG Al was revealed and compared with that of ECAPed UFG Cu through the observations of deformation features and microstructures.
Ying YanLi-jia ChenGuo-qiang ZhangDong HanXiao-wu Li
By the combination of transmission electron microscope, neutron diffraction and small-angle neutron scattering methods, mechanical fatigue behavior of AL6XN austenitic stainless steel was investigated in the temperature range of 400–600 °C. At 400 °C, in addition to the occurrence of dynamic strain aging, the formation of short-range order was evidenced from the forbidden electron diffraction spot of 1/3 {422} in face-centered cubic(fcc) structure viewed down [111] zone axis, which facilitate the planar slip mode of dislocation and result in the work hardening during the fatigue deformation. The fatigue damage is mainly dominated by the accumulation of planar slip band and the interaction among various slip systems. With increasing temperature, precipitates of chi phase, Laves phase and sigma phase were formed during the fatigue tests at 500 and 600 °C. An increase in precipitation content at 600 °C has also been confirmed by both scanning electron microscope and small-angle neutron scattering analysis. The dislocation pileup originating from the uncoordinated deformation between precipitate and austenitic matrix is an important fatigue damage leading to crack. The continuous cycle softening behavior was also observed on the fatigue curve at 600 °C, which is considered to be caused by dynamic recovery.
Yanyan HongPenglin GaoHongjia LiChangsheng ZhangGuangai Sun
