Improvement of Thermal Stability and Evaluation of Fatigue Behavior of an Ultra fine-Grained Al-Mg-Sc Alloy Using ECAE

研究了在573 K和623 K条件下等径角挤压制备的超细晶Al-3wt%Mg-0.2wt% Sc合金的微观组织演变。623 K短时间退火使超细晶粒完全再结晶。573K时的结构发育可分为三个阶段：(1)初始阶段（~ 30min）。随着退火时间的延长，硬度迅速下降，晶粒尺寸从300 nm增大到600 nm。位错、位错密度随着恢复而降低，晶粒长大和位错密度降低导致硬度降低。(2)中间阶段（30min ~ 11h）。硬度增加，晶粒尺寸基本保持不变。晶粒内部形成细小的Al_3Sc析出颗粒。这些颗粒主要引起硬度的增加。晶界上细小的Al_3Sc颗粒固定晶界，限制晶界的迁移。(3)后期（11h ~）。晶粒生长加快，硬度下降。这是因为随着Al_3Sc晶粒尺寸的增大，固相力和颗粒强化逐渐减小。由晶粒尺寸与Al_3Sc颗粒半径的线性关系可知，晶粒的生长受Al3Sc颗粒粗化速率的控制。对平均直径为4 nm和11nm的Al_3Sc颗粒Al-Mg-Sc合金进行了塑性应变控制的低周疲劳试验。结果表明：(1)直径为4nm的Al_3Sc颗粒在较高的塑性应变幅值（ε_<pl>【大于或等于】1×10^<-3>）下表现出循环软化。其他试样均表现为循环硬化至饱和。(2)当Al_3Sc颗粒直径为11nm时，位错分布非常均匀。在较高应变幅值下，直径为4nm的Al_3Sc颗粒出现了滑移带。(3)含4nm Al_3Sc颗粒试样的疲劳软化与滑移带内Al_3Sc颗粒的溶解有明确的关系。少

The microstructure evolution of an ultrafine grained Al-3wt%Mg-0.2wt% Sc alloy produced by equal channel angular pressing during annealing at 573 K or 623 K has been examined. Annealing at 623 K for a short time caused complete recrystallization of the ultrafine grains. The structure development at 573K can be divided into three stages as follows :(1)Initial stage (〜30min). The hardness decreases rapidly with annealing time and the grain size increases from 300 to 600 nm. The dislocation, density becomes lower by recovery, The grain growth and the reduction in dislocation density bring about the decrease in hardness.(2)Intermediate stage (30min〜11h). The hardness increases and the grain size remains almost unchanged. Fine precipitated Al_3Sc particles are formed in grain interiors. These particles cause primarily the increase of the hardness. The small Al_3Sc particles on grain boundaries pin grain boundaries and restrict their migration.(3)Later stage (11h〜). At this stage, again the … More grain growth occurs and the hardness decreases. This is because the pining force and the particle strengthening gradually decrease with increasing the Al_3Sc particle size. From the linear relationship between the grain size and Al_3Sc particle radius, it is concluded that the grain growth is controlled by the rate of coarsening of the Al3Sc particles.Plastic-strain-controlled low-cycle fatigue tests of an Al-Mg-Sc alloy containing Al_3Sc particles with average diameters 4 nm and 11nm were carried out. The results and conclusions are summarized as follows :(1)Specimens with Al_3Sc particles of 4nm in diameter show cyclic softening at higher plastic-strain amplitudes (ε_<pl>【greater than or equal】1×10^<-3>). All other specimens show cyclic hardening to saturation.(2)Dislocations were very uniformly distributed in specimens with Al_3Sc particle of 11nm in diameter. At higher strain amplitudes, the slip bands were observed in specimens with Al_3Sc particles of 4nm in diameter.(3)The fatigue softening of specimens with 4 nm Al_3Sc particles is unambiguously related to the dissolution of the Al_3Sc particles within the slip bands. Less