CoCrNi多组元合金退火硬化现象的微观机制

Mechanism of anneal hardening in CoCrNi multi⁃principal elemental alloys

冷变形金属或合金在再结晶温度以下退火后,其强度或硬度上升的现象称为退火硬化现象。与传统金属类似,在面心立方结构的多组元合金中也观察到了退火硬化现象。尽管多组元合金有类似的退火硬化行为,但其中退火硬化的机制仍存在争议,这可能是由于不同的冷变形条件产生不同的微观结构导致的。本文以不同变形量的CoCrNi多组元合金为研究对象,首先研究了其在100~800℃的温度区间内退火后维氏硬度和拉伸性能的变化,发现CoCrNi多组元合金的硬度和强度随温度的增加先升高后降低,峰值温度在500℃,同时,随着变形量的增加,退火硬化效应逐渐趋于饱和。然后借助扫描和透射电镜对变形态和峰值温度退火态的CoCrNi合金进行了多尺度的微观组织表征。研究发现在变形量为20%时,退火硬化可能是由于Cr元素向层错处偏析导致的铃木强化;在变形量为60%时,除铃木强化之外,冷变形引入的剪切带在退火后形成的亚晶粒导致了强化;在变形量为90%时,冷变形产生的高密度位错墙在退火后转变为大角度晶界,起了强化作用。本实验对CoCrNi多组元合金在不同变形量时的退火硬化机制提出了新的见解,对设计高强度结构材料有借鉴意义。

The anneal hardening of cold⁃rolled metals or alloys is defined as their increase of strength or hardness after being annealed below their recrystallized temperatures.Similar to conventional metals,the anneal hardening can also be observed in FCC structured multi⁃component alloys.However,the mechanism of the anneal hardening is still unclear in multi⁃component alloys due to very complicated microstructures caused by different cold⁃rolling parameters.Here we focused on CoCrNi multi⁃component alloys deformed under different cold rolling parameters,and studied the variation of their microhardness and yield strength in the temperature range of 100℃-800℃.We found that both the microhardness and the strength of CoCrNi alloys firstly increase and then decrease with the increase of annealing temperature,and the optimized temperature is 500℃.The anneal hardening tends to be saturated as the increase of height reductions.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to check their microstructural evolution.At a height reduction of 20%,the segregation of Cr into the stacking fault leads to the Suzuki hardening,which probably result in the anneal hardening.At a height reduction of 60%,the sub⁃grains in shear bands induced by cold rolling contribute to the additional hardening besides the Suzuki hardening.At a height reduction of 90%,high⁃density dislocation walls generated by cold rolling transfer into large⁃angle grain boundaries after annealing,dominating the hardening effect.This work proposed a new insight into the mechanism of anneal hardening in CoCrNi multi⁃principal element alloys under different deformation reductions,which can shed light on the novel design of high⁃strength materials.