退火提高中熵合金CrCoNi中的化学短程有序度和孪晶应力

Aging elevates chemical short-range order and twinning stress in a CrCoNi medium-entropy alloy

纳米压入被广泛应用于研究化学短程有序对高中熵合金(如CrCoNi)强度的影响,尤其是对塑性载体的形核应力的影响.然而,对于这类合金在纳米压入过程中极小位移下的应变突跳事件所对应的变形机制,目前除了基于突跳统计数据进行推测,还未能直接在实验上进行捕捉.在本工作中,通过精细的加工,我们可以对深度小至7 nm的极浅压痕进行提取,并在透射电镜下进行表征.我们发现CrCoNi中熵合金纳米压入过程中的第一个应变突跳来源于纳米孪晶和层错的形成.由此得出结论,沿[001]方向加载时,退火后具有更高化学短程有序度的CrCoNi单晶相比均匀化处理态CrCoNi单晶,其孪晶(形核)应力升高.退火态和均匀化处理态CrCoNi中的化学短程有序程度可通过倒易空间中基体斑点之外的弥散衍射盘来进行探测.

Nanoindentation has been widely used to study the effect of chemical short-range order(CSRO)on the strength of high-and medium-entropy alloys(such as CrCo-Ni),especially the nucleation stress of plasticity carriers.However,apart from the speculation based on pop-in statis-tics,the mechanism responsible for the extremely small pop-in events during nanoindentation has not been experimentally captured.Here,with meticulous manipulation,we were able to lift out very shallow pop-in indentations with depths down to 7 nm for characterization under a transmission electron mi-croscope.We found that the first burst arose from the for-mation of stacking faults and nanotwins.This enabled us to conclude that it is the twinning(nucleation)stress that has been elevated in the[001]-orientated/loaded CrCoNi single crystal with a higher CSRO level after aging when compared with its as-homogenized counterpart.The degree of CSRO can be monitored using extra diffuse reflections in reciprocal space.