Crystal plasticity finite element modeling of mechanically induced martensitic transformation (MIMT) in metastable austenite

A new crystal plasticity model incorporating the mechanically induced martensitic transformation in metastable austenitic steel has been formulated and implemented into the finite element analysis. The kinetics of martensite transformation is modeled by taking into consideration of a nucleation-controlled phenomenon, where each potential martensitic variant based on Kurdjumov–Sachs (KS) relationship has different nucleation probability as a function of the interaction energy between externally applied stress and lattice deformation. Therefore, the transformed volume fractions are determined following selective variants given by the crystallographic orientation of austenitic matrix and applied stress in the frame of the crystal plasticity finite element. The developed finite element program is capable of considering the effect of volume change by the Bain deformation and the lattice-invariant shear during the martensitic transformation by effectively modifying the evolution of plastic deformation gradient of the conventional rate-dependent crystal plasticity finite element. The validation of the proposed model has been carried out by comparing with the experimentally measured data under simple loading conditions. Good agreements with the measurements for the stress–strain responses, transformed martensitic volume fractions and the influence of strain rate on the deformation behavior will enable the model to be promising for the future applications to the real forming process of the TRIP aided steel.     

稀土对SiMnCr和SiMnMo结构钢回火脆性的影响

研究了ＳｉＭｎＣｒ钢和ＳｉＭｎＭｏ钢低温回火过程中机械性能的变化。稀土对低温回火时的材料强度没有明显的影响。ＳｉＭｎＣｒ钢和ＳｉＭｎＭｏ钢分别在３５０和４００℃存在ＴＭＥ谷底值；稀土对谷底值的温度虽然没有影响，却可以在一定程度上改善其数值。稀土改善回火脆性是细化晶粒和抑制奥氏体晶界脆化作用的结果。     

Finite element and experimental method for analyzing the effects of martensite morphologies on the formability of DP steels

Abstract

In this article, we investigated the effect of martensite morphology on the mechanical properties and formability of dual phase steels. At first, three heat treatment cycles were subjected to a low-carbon steel to produce ferrite–martensite microstructure with martensite morphology of blocky-shaped, continuous, and fibrous. Tensile tests were then carried out so as to study mechanical properties, particularly the strength and strain hardening behavior of dual phase steels. In order to study the formability of dual phase samples, Forming Limit Diagram was obtained experimentally and numerically. Experimental forming limit diagram was obtained using Nakazima forming test, while Finite Element Method was utilized to numerically predict the forming limit diagram. The results indicated that the dual phase samples with fibrous martensite morphology had the highest tensile properties and strain rate hardening out of the three different microstructures. Blocky-shaped martensite morphology, on the other hand, had the worst mechanical properties. The study of the strain hardening behavior of dual phase sample by Kocks–Mecking-type plots, evinced two stages of strain hardening for all specimens with different microstructures: stages III and IV. The forming limit diagram of dual phase steels also proved that samples with fibrous martensite morphology had the best formability compared to other two microstructures. The simulated forming limit diagram manifested that there is a good agreement between experimental results and those obtained by FEM.     

GCr15马氏体钢剪切试样断口局部熔融研究

本文采用开45?缺口剪切压缩试样, 比较研究GCr15马氏体钢在准静态压缩和高速冲击下样品断裂面的温升机制. 结果显示两种加载状态的断口上都发现了大量的局部熔融, 说明温升均超过1500℃. GCr15马氏体钢的塑性很低, 然而在剪切应力主导的试样上, 两种加载的剪切面均发生了很大塑性应变. 试样断裂瞬时所释放的能量以及裂纹面间的大滑移摩擦导致局部温升超过熔点. 分析结果表明, 两种加载模式下产生的熔融物均由残余奥氏体和孪晶马氏体组成. 受热的影响, 熔融物下面的基体组织经历了动态再结晶, 从而形成马氏体和奥氏体等轴晶. 因此,在剪应力主导的应力状态下, 马氏体钢的剪切断裂机理与加载速率无关, 高速冲击与准静态加载下的断裂模式和机理没有本质区别. 断裂瞬间产生局部温升促使材料熔融, 这是该材料剪切断裂的特性. 本文结论对GCr15马氏体钢剪切主导断裂机理的认识有重要意义.     

Formation mechanism of the hierarchic structure in the lath martensite phase in steels

Martensitic transformation is the phase transformation accompanying orderly shear deformation without atomic diffusion. The structures made by martensitic transformation are classified as thin plate, lens or lath in steels. The mechanism by which the hierarchic microstructure in the lath martensite phase forms has heretofore not been understood. We have made clear the mechanism by considering, independently, two plastic deformations using the slip deformation model proposed by Khachaturyan, and present herein a deformation matrix for each of the six crystallographic variants in a packet of the hierarchic structure. Our results are quantitatively consistent with experimental results for the Kurdjumov–Sachs (K-S) crystal orientation relationship and habit plane. Furthermore, the important points of our study are as follows: the origin of the sub-block structure and the specific combination of the sub-block structure are clarified; the laths existing in a block can be explained; and deviations between the directional parallel and plane parallel are obtained quantitatively, without any adjustable parameters.     

二次硬化马氏体时效钢中纳米级强化相的分析

本文采用电化学相分析法确定它是一种Ｍ２Ｃ型碳化物，并利用两种溶液的联合处理法克服了在碳化物间相分离时复合合金化的碳化物（Ｆｅ，Ｃｒ，Ｍｏ）２Ｃ中元素间的选择性溶出，提高了相分析的准确度。     

Tempering effects on three martensitic carbon steels studied by mechanical spectroscopy

Tempering effects have been studied in three martensitic carbon steels by mechanical spectroscopy. The mechanical-loss spectra present a relaxation peak similar to the Snoek-Köster peak in ferrite. The peak amplitude decreases upon tempering, indicating a decrease of the dislocation density. Transition carbides start to precipitate at 380 K in all the three grades. This tends to decrease the mechanical loss and to increase the modulus. Retained austenite decomposes around 520 K in two of the grades. In the third grade, the presence of Si delays this decomposition to 670 K. The decomposition of retained austenite leads to a sudden decrease of amplitude of the relaxation peak and a modulus anomaly. Both these effects can be attributed to a decrease of the dislocation density in martensite, probably associated with the depletion of carbon atoms in the dislocation core. At low frequency, a mechanical-loss peak associated with the decomposition of retained austenite is visible.     

New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.     

In situ synchrotron X-ray microdiffraction analysis of thermomechanically induced phase transformations in Cu–Al–Ni shape-memory alloy

Phase transformations in a single-crystal Cu–Al–Ni shape-memory alloy induced by thermomechanical effects were investigated in situ by high-resolution synchrotron X-ray microdiffraction. Contrary to the common belief, austenite texture maps revealed that austenite-to-martensite transformation occurred during heating of the partially transformed material under fixed specimen elongation. Twinned and detwinned types of martensite coexisted during this austenite-to-martensite phase transformation. Twinning and detwinning structures evolved to accommodate changes in stress and strain generated in the temperature-varying environment. Small amounts of austenite exhibiting distorted crystallographic orientation were detected in regions of stress-induced martensite during heating of the partially transformed material. The results of this investigation provide insight into intriguing stress rate-dependent phenomena intrinsic of shape-memory alloys and elucidate complex phase transformations due to thermal and mechanical stress effects.     

Grain boundary engineering: fatigue fracture

Abstract

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.