Reverse transformation mechanism of martensite to austenite and amount of retained austenite after reverse transformation in Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel

Abstract

The reverse transformation mechanism of martensite to austenite and the volume fraction of retained austenite have been studied in an Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel by means of dilatometry, transmission electron microscopy and X-ray diffraction. Below a heating rate of 10 K s^-1, the reverse transformation of α' to γ occurs by diffusion, whereas it occurs by a diffusionless shear mechanism above 10 K s^-1. After reversion treatment at low temperatures, filmlike retained austenite is observed along α' lath boundaries, while reversion treatment at high temperatures produces granular retained austenite inside the α' laths in addition to filmlike retained austenite. The volume fraction of retained austenite at room temperature increases with increasing reversion treatment temperature, exhibiting a maximum at ~625^° C, above which it decreases with increasing reversion temperature.     

Fe-Ni机械合金化过程中马氏体的形成及相变

研究了Ni含量、机械合金化工艺参数对Fe-Ni机械合金化过程中马氏体相变的影响及其机理.结果表明:在Fe-Ni机械合金化过程中存在着马氏体相变,但继续机械合金化马氏体是否会发生逆转变主要由Ni含量决定.当Ni≤30%(质量分数,下同)时,机械合金化引起的材料局部温度未达到形变促使马氏体相变逆转变开始温度,因此继续机械合金化马氏体不转变.对于Fe-35Ni,形变促使逆转变的开始温度低于局部温升,马氏体将向奥氏体转变.当Ni含量为35%时,随着机械合金化时间的延长、球磨速度和球料比的提高,机械合金化可以提供的相变驱动力增大导致奥氏体的量逐渐增多.     

Deformation and martensitic transformation

The influence of applied stresses and imposed plastic deformation on the martensitic transformation of a parent phase is described. Changes in mechanical properties such as flow stress, work hardening rate, fracture toughness, etc brought about by strain-induced martensitic transformation are briefly examined. In the absence of appreciable dislocation glide, atomic displacements associated with glissile boundaries are highly ordered and reversible modes of (plastic or nonlinear pseudoelastic) deformation. Such processes lead to large strains and are encountered in deformation twinning, martensitic transformations and in the reorientation of martensite units. The reversibility leads to phenomena such as elastic twinning, thermoelastic martensites, superelasticity, shape memory and two-way shape memory effects, and rubber-like behaviour. These are discussed using a unified approach based on thermoelastic equilibrium. The shape memory effect suggests several potential applications of the martensitic transformations in non-ferrous alloys in which the effect is most commonly observed. Recent developments in this area are reviewed with special reference to the prerequisites for the effect and the influence of metallurgical processing on the extent of shape recovery.     

Modeling of austenite to ferrite transformation

In this research, an algorithm based on the $\boldsymbol{Q}$ -state Potts model is presented for modeling the austenite to ferrite transformation. In the algorithm, it is possible to exactly track boundary migration of the phase formed during transformation. In the algorithm, effects of changes in chemical free energy, strain free energy and interfacial energies of austenite?Caustenite, ferrite?Cferrite and austenite?Cferrite during transformation are considered. From the algorithm, the kinetics of transformation and mean ferrite grain size for different cooling rates are calculated. It is found that there is a good agreement between the calculated and experimental results.     

马氏体相变的界面吸引子模型

从经典的马氏体相界面运动方程出发,利用相平面分析,证明具有界面摩擦的马氏体相界面是吸引子,与理想的马氏体相界面的孤立子特征不同.通过对非线性方程解的分析,得到发生应力诱发马氏体相变的相界面趋向为焦点型定常吸引子,而热诱发相变的马氏体相界面趋向为结点型定常吸引子,因此考虑界面摩擦后相界面的运动与理想相界面的运动具有不同的非线性特征.     

反铁磁Mn-Fe(Cu)合金在马氏体相变过程中的组织演化

采用电阻-温度曲线、X射线衍射分析仪(XRD)、调制示差扫描热分析仪(DSC)及原位透射电子显微镜(TEM)等,研究了的Mn-Fe(Cu)合金在马氏体相变过程中的组织演化.研究结果表明,Mn-Fe(Cu)合金孪晶面为{011}面的FCT板条状马氏体孪晶组织.原位TEM观察可见,板条状马氏体孪晶具有良好的可动性.在较高温度下降温时,会出现由于反铁磁畸变产生的"花呢"状组织,导致原有孪晶的位置与取向发生变化.Landau自由能方程的解可证明反铁磁转变对马氏体孪晶形貌的影响.     

Dual orientation and variant selection during diffusional transformation of austenite to allotriomorphic ferrite

The crystallographic relationship between austenite and grain boundary nucleated allotriomorphic ferrite has been investigated using electron back-scattered diffraction with a view to establishing a mechanism of variant selection. It is possible in some circumstances for the ferrite to adopt a favoured orientation relationship with both of the austenite grains with which it is in contact. However, the theoretical probability for the development of such a dual orientation has in previous work been shown to be very small, although experiments indicate otherwise. In this work, we have discovered experimentally that the probability of dual orientations is significantly increased when adjacent austenite grains are connected by special high-angle boundaries. Crystallographic calculations validate these observations and lead to the conclusion that simultaneous lattice matching between ferrite and its parent austenite grains is more likely in the presence of certain kinds of microscopic texture in the austenite. The phenomenon of dual orientation provides a criterion for crystallographic variant selection during diffusional transformation.     

Thermoelastic martensitic transformation in Cu-Zn-Al alloy

Thermoelastic martensitic transformation in a Cu-29% Zn-3% Al alloy has been observed with optical and electron microscopy, including 1.0–1.2 MV high-voltage transmission electron microscopy (TEM), and treated with the phenomenological theory suggested before. The observation showed that the transformation was conducted in three stages: parallel plates growing, self-accommodating variants advancing, and plates merging and/or tiny plates forming in carved-up parent phase areas. TEM showed that the martensite consisted of a huge number of packets with constant size and distinct interfaces. By using the phenomenological theory, the free-energy function as well as the friction quasi-enthalpy and friction quasi-entropy are obtained in SI units. Comparing with classical theory, the free energy can be broken into three parts: the chemical free energy, the interfacial energy and the elastic strain energy. In the range of 20–70% martensite in the alloy, the interfacial energy per unit of martensite formation is constant and the corresponding elastic strain energy is a linear function of martensite percentage. Some possible explanations for this energetics relating to the observation are given.     

Formation of the reversed austenite during intercritical tempering in a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Formation of the reversed austenite obtained by intercritical tempering has been studied via transmission electron microscopy (TEM) in a Fe-13%Cr-4%Ni-Mo low carbon martensitic stainless steel. It is found that the precipitation of M₂₃C₆ carbides along the martensite lath boundaries will result in Ni-enrichment in the adjacent region. The reversed austenite forms with the Ni-enrichment region as the nucleation sites, keeps a cube-cube orientation relationship with the M₂₃C₆ carbides and bears the Kurdjumov-Sachs (K-S) relationship with the martensite. Moreover, the reversed austenite formed inside the martensite laths is also confirmed. The mechanism for formation of the reversed austenite is discussed in detail.     

A multi-scale model of martensitic transformation plasticity

The remarkable mechanical engineering properties of many advanced steels, e.g. TRIP steels and metastable austenitic stainless steels, are related to their complex microstructural behaviour, resulting from the interaction between plastic deformation of the phases and the austenite to martensite phase transformation during thermomechanical loading. In this paper, a multi-scale physically-based model is presented for the prediction of such structure–property relations for materials exhibiting the martensite phase transformation during mechanical loading. The model incorporates several spatial levels: a macroscopic or engineering level, a mesoscale level of a single austenite grain and a microscale level of smaller domains within the austenite grain where the martensitic transformation takes place on particular crystallographic transformation systems. The model directly incorporates the coupling between elastic and plastic deformation of the phases and the transformation, as well as the dependence of the transformation on the (hydrostatic) stress state, grain orientation with respect to the loading and the history of deformation and transformation. The performance of the model is evaluated on several examples, illustrating the ability of the model to predict the orientation and stress-state dependence of the transformation. The developed model can be used for the systematic study of structure–property relations of these inherently multi-scale materials.     

非热弹性马氏体相变的细观本构模型

为了更清楚地认识铁基合金经受非热弹性马氏体相变的本征特性,在细观尺度对非热弹性马氏体相变进行了研究.基于马氏体相变晶体学和内变量本构理论建立了非热弹性马氏体相变的细观本构模型.该模型采用微区相变应变、奥氏体及马氏体的塑性应变表征宏观的非弹性响应,把奥氏体和马氏体变体的等效塑性应变率和体积分数变化率作为内变量描述微观结构变化.模型采用J2流动理论描述微区塑性流动,与采用晶体塑性的描述方法相比模型更简单,且更适用于工程计算.单晶奥氏体单变体简单剪切的模拟结果表明:随着应变的增加,先发生奥氏体塑性变形,进而发生相变,马氏体体积分数与应变呈线性关系;温度较低时易发生马氏体相变并使得材料的强度提高.     

Kinetics on the micro- and macro-levels in polycrystalline alloy materials during martensitic transformation

Summary Kinetics is studied during the martensitic transformation on both the micro- and macro-level from the thermomechanical point of view. A variant, a smallest microstructure element in an alloy, is assumed to transform at a burst when a transformation condition expressed by means of the driving force is satisfied. It has a micro-fraction showing 1 (transformed) or 0 (yet untransformed). The macrofraction, which represents a certain extent of transformation in a representative volume composed of a large enough number of variants, is derived by performing an ensemble average of the micro-fraction over the representative volume. The progress of the macro-fraction during thermomechanical loading is shown to be governed by a differential equation, the solution of which could be reduced to the conventional transformation kinetics discussed in the fields of metallurgy and transformation thermomechanics. A linear relation is derived between the increments of the macroscopic transformation strain and of the macro-fraction.     

Internal friction during martensitic transformation in high manganese Mn–Cu alloys

Low-frequency internal friction and elastic modulus were studied for manganese-rich Mn–Cu alloys in the temperature range of martensitic transformation (20–300 °C). It is shown that the some special features of the transformation peak and its temperature are caused by the degree of the spinodal decomposition. The phenomenological model connecting an-elastic effects with the stages of evolution of the structure during martensitic transformation in manganese-rich Mn–Cu alloys (tweed structure–“parquet” structure–classical twinning martensite) is presented.     

Reformed austenite transformation during fatigue crack propagation of 13%Cr-4%Ni stainless steel

In the as-quenched state, 13%Cr-4%Ni martensitic stainless steels are essentially 100% martensitic. However, a certain amount of austenite is formed during the tempering of this alloy. This reformed austenite is thermally stable at room temperature but can transform to martensite under stress. This transformation is known to happen during impact testing but it has never been established if it occurs during fatigue crack propagation. This study presents the results of X-ray diffraction measurements of reformed austenite before and after crack growth testing. It has been found that reformed austenite does transform to martensite at the crack tip and that this transformation occurs even at a low stress intensity factor. Low-cycle fatigue tests were conducted to verify austenite transformation under cyclic straining. It was found that reformed austenite transforms only partially during the first strain reversal but that essentially all austenite has disappeared after 100 cycles. The relation between austenite transformation under low-cycle fatigue and its transformation during crack growth is also discussed.