Geometric dynamic recrystallization of austenitic stainless steel through linear plane-strain machining

ABSTRACT Type 316L austenitic stainless steel was severely plastically deformed at room temperature using linear plane-strain machining in a single pass that imparted shear strains up to 2.2 at strain rates up to 2 × 103 s−1. The resulting microstructures exhibited significant grain size refinement and improved mechanical strength where geometric dynamic recrystallization was identified as the primary microstructural recrystallization mechanism active at high strain rates. This mechanism is rarely observed in low to medium stacking fault energy materials. The critical stress required for twin initiation is raised by the combined effects of refined grain size and the increase in stacking fault energy due to the adiabatic heating of the chip, thus permitting geometric dynamic recrystallization. The suppression of martensite formation was observed and is correlated to the significant adiabatic heating and mechanical stabilisation of the austenitic stainless steel. A gradient of the amount of strain induced martensite formed from the surface towards the interior of the chip. As the strain rate is increased from 4 × 102 s−1–2 × 103 s−1, a grain morphology change was observed from a population of grains with a high fraction of irregular shaped grains to one dominated by elongated grain shapes with a microstructure characterised by an enhanced density of intragranular sub-cell structure, serrated grain boundaries, and no observable twins. As strain rates were increased, the combination of reduction in strain induced martensite and non-uniform intragranular strain led to grain softening where a Hall-Petch relationship was observed with a negative strengthening coefficient of −0.08 MPa m1/2.

摘要：316L型奥氏体不锈钢在室温下通过单次线性平面应变加工进行了剧烈塑性变形，在高达2×10³ s⁻¹的应变速率下施加了高达2.2的剪切应变。所得微观结构显示出显著的晶粒细化和力学强度提高，其中几何动态再结晶被确定为在高应变速率下起作用的主要微观结构再结晶机制。这种机制在低到中等层错能材料中很少被观察到。由于切屑的绝热加热，细化的晶粒尺寸和层错能的增加共同作用提高了孪晶形核所需的临界应力，从而允许几何动态再结晶。观察到马氏体形成受到抑制，这与奥氏体不锈钢的显著绝热加热和力学稳定性相关。从切屑表面到内部形成的应变诱发马氏体量存在梯度。当应变速率从4×10² s⁻¹增加到2×10³ s⁻¹时，观察到晶粒形态从具有高比例不规则形状晶粒的群体转变为以拉长晶粒形状为主的群体，其微观结构的特征是晶内亚晶胞结构密度增加、晶界呈锯齿状且没有可观察到的孪晶。随着应变速率的增加，应变诱发马氏体的减少和晶内不均匀应变的共同作用导致晶粒软化，其中观察到霍尔 - 佩奇关系，强化系数为 -0.08 MPa·m¹/²。