基于原位TEM拉伸的稀土H13钢塑性形变行为和断裂机制

Plastic Deformation Behavior and Fracture Mechanism of Rare Earth H13 Steel Based on In Situ TEM Tensile Study

采用原位TEM拉伸结合离位EBSD分析,研究了稀土H13钢在拉伸过程中的组织演化和裂纹扩展行为。结果表明,稀土H13钢拉伸试样晶界处的应力集中和粗大的颗粒状夹杂物是裂纹萌生的主要来源;拉伸过程中多处裂纹萌生后汇聚成较大的主裂纹,主裂纹沿着与拉伸方向垂直的方向扩展到试样边缘,主裂纹具有"Z"字形的锯齿状特征。裂纹附近区域的应力分布不均匀,与应力相对较低的区域相比,应力相对较高区域的V1/V2变体对的晶界长度分数从56.5%增加到58.8%,V1/V3&V5变体对的晶界长度分数从16.3%增加到21.6%;变体对的晶界长度分数增加表明了孪生马氏体含量的提高,从而有效缓解晶界处的应力集中,有利于减少裂纹萌生并提高塑韧性。拉伸过程中,稀土H13钢试样晶界处的残余奥氏体发生应力诱导相变;马氏体基体内的位错发生大量增殖,并在大角度晶界和碳化物析出处形成明显的位错塞积,其中晶界处的位错塞积促进了残余奥氏体的应力诱导相变。

Fatigue failure caused by crack propagation is one of the main failure modes of H13 die steel.Because H13 die steel is mainly used under operational conditions involving high-pressure cycling or high friction,its crack initiation and propagation behavior play a critical role in fatigue failure.However,to the best of the authors’knowledge,few reports on the direct observation of deformation and crack propagation behavior of H13 die steel exist,which limits the understanding of the fracture mechanism of H13 die steel.In this work,an in situ TEM tensile study combined with post-mortem EBSD analysis was employed to investigate the microstructure evolution and crack propagation behavior of rare earth(RE)H13 steel.Results indicate that the stress concentration at the grain boundaries and coarse granular inclusions in the tensile specimen were the main sources of crack initiation.After crack initiation and during the tensile process,many cracks converged into the main crack.The main crack propagated along the direction perpendicular to the tensile direction,exhibiting zigzag-shaped features.The stress distribution in the area near a crack in a specimen was heterogeneous;the length fraction of V1/V2 inter-variant boundaries in the relatively high-stress area increased from 56.5%to 58.8%compared with the relatively lowstress area,and the length fraction of V1/V3&V5 inter-variant boundaries increased from 16.3%to 21.6%.The increase in the length fraction of V1/V2 inter-variant boundaries indicated an increase in the twin martensite fraction,which effectively relieved the stress concentration at the grain boundaries and reduced crack initiation.During the tensile process,the austenite retained at the grain boundaries underwent stress-induced phase transformation and was partially transformed into V1/V2 and V1/V3&V5 variant pairs.The dislocation propagation in martensite contributed to dislocation pile-up at high-angle grain boundaries and carbide precipitation.Moreover,the dislocation pile-up at the high-angle grain boundaries promoted the stress-induced phase transformation of the retained austenite.