Void Formation and Strain-Induced Martensitic Transformation in TRIP780 Steel Sheet Submitted to Uniaxial Tensile Loading

This work aimed to analyze the damage behavior of cold rolled TRIP780 steel sheet submitted to interrupted uniaxial tensile tests performed along the rolling direction. The formation of voids is investigated as a function of the straining level using digital image analysis of scanning electron micrographs to obtain the measures of void density, void area fraction, void aspect ratio and mean void size. The volume fractions of both ferrite/martensite and retained austenite constituents were obtained from X-ray diffraction measurements. An abruptly decrease of retained austenite was observed at early stages of deformation followed by a slow saturation. The resulting strain-induced martensite is responsible for improving the formability of the TRIP780 as observed by instantaneous strain-hardening exponent. In the lower strain range, growth and coalescence of existing microvoids prevailed at both in-plane directions whereas nucleation of microvoids was also observed along the loading direction. Conversely, nucleation prevailed at the transverse direction in the intermediate strain range whereas growth and coalescence were predominant aligned to the loading direction. At larger strain levels, growth and coalescence of microvoids prevailed at both directions. The microvoids were initially found around inclusions and at the interface of ferrite-martensite phases and lastly also at the ferrite matrix.

本研究旨在分析沿轧制方向（rolling direction）开展中断式单轴拉伸试验（interrupted uniaxial tensile tests）的冷轧TRIP780钢板（cold rolled TRIP780 steel sheet）的损伤行为。通过对扫描电子显微照片（scanning electron micrographs）开展数字图像分析（digital image analysis），探究了微孔（voids）的形成随应变水平（straining level）的变化规律，以此获取微孔密度（void density）、微孔面积分数（void area fraction）、微孔长径比（void aspect ratio）及平均微孔尺寸（mean void size）等量化表征参数。通过X射线衍射（X-ray diffraction）测量，得到了铁素体/马氏体（ferrite/martensite）与残余奥氏体（retained austenite）两相的体积分数。
变形初期可观察到残余奥氏体出现急剧下降，随后逐渐趋于缓慢饱和。所得应变诱导马氏体（strain-induced martensite）可改善TRIP780钢的成形性能，该结论可通过瞬时应变硬化指数（strain-hardening exponent）得到验证。
在低应变区间，两个面内方向（in-plane directions）均以现有微孔的长大与聚合为主，同时在加载方向（loading direction）还观察到了微孔形核现象。反之，在中等应变区间，横向（transverse direction）以微孔形核占主导，而沿加载方向则以微孔的长大与聚合为主。当应变水平进一步提升时，两个方向均以微孔的长大与聚合为主。
微孔最初主要萌生在夹杂物（inclusions）周围及铁素体-马氏体相界面处，后续也会在铁素体基体（ferrite matrix）中形成。