Cr5钢马氏体的相变塑性和应力对其相变动力学的影响

对大型支承辊用Cr5钢进行不同拉/压载荷作用下的膨胀实验,研究了Cr5钢马氏体的相变塑性和应力对马氏体相变动力学的影响。结果表明,Cr5钢在不同应力作用下马氏体相变系数α基本相同,近似为随温度变化的三次多项式,且随着温度的增加而减小,而应力对α的影响基本上可以忽略;马氏体转变的起始点(Ms)随着应力值的增大而升高;结合Greenwood-Johnson方程求出在不同应力作用下Cr5钢的马氏体相变塑性系数k值,近似约为4.32×10-5的常数。     

钢的形变诱发马氏体相变和相变诱发塑性

在介稳奥氏体钢中,通过形变诱发马氏体相变,这被称之谓形变诱发马氏体相变。为了了解形变诱发马氏体相变的一般特征,就必须清楚外加应力和塑性应变对马氏体相变的两种作用。由于马氏体相变是通过原子的协同切变位移实现的,所以很容易明白:外加应力有助于相变。但是,关于外加应力是如何促使相变的,仍存在一些问题。此外,塑性应变对马氏体相变的作用也复杂得多。笔者认为,只有依据外加应力的影响而不是应变的影响方可以搞清形变诱发马氏体相变。当在形变过程中形成马氏体时,伸长率显著提高,这种现象被称为相变诱发塑性(TRIP)。本文讨论了 TRIP 的起因和关于 TRIP 的各种控制因素。此外,简要地介绍了 TRIP 现象在工业上的应用。     

基于不可逆热力学的Ni-Ti合金动态本构模型及其有限元实现

为了准确描述Ni-Ti形状记忆合金在高应变率下的动态压缩力学行为,基于不可逆热力学理论框架假定了两个内变量表征Ni-Ti合金应力诱发马氏体相变与塑性屈服的不可逆变形过程,分别推导了马氏体相变与塑性屈服演化规律的主控方程,构建了Ni-Ti合金的三维动态本构模型。根据材料单轴动态压缩实验的应力-应变曲线并采用最小二乘法对本构参数进行了优化识别,然后采用应力补偿更新算法,通过隐式用户子程序接口UMAT将动态本构模型嵌入ABAQUS有限元软件,实现了Ni-Ti合金在高应变率下动态压缩力学行为的数值模拟。通过比对发现,模拟结果与实验数据吻合良好,验证了动态本构模型与UMAT子程序的准确性。本工作为Ni-Ti合金在高速冲击、切削等极端条件下的工程应用奠定了基础。     

S34MnV钢的连续冷却转变行为及相变动力学研究

本工作利用DIL-805ADT动态热膨胀相变仪测定了S34MnV钢不同冷却速率下的相变膨胀量,通过光学显微组织观察、SEM组织观察、能谱成分扫描、XRD物相检测等手段对S34MnV钢在连续冷却过程中的组织转变规律进行了深入分析,运用切线法对相变拐点前后的膨胀曲线进行线性拟合,得到了S34MnV钢在加热和不同速度冷却下的相变点,根据所得到的相变点和对应的相变时间结合微观组织分析绘制了S34MnV钢的连续冷却转变(CCT)曲线.同时为更好地描述S34MnV钢在连续冷却过程中的扩散型相变,本工作采用了针对低合金钢更加准确实用的Li模型,根据绘制出的CCT曲线修正了描述铁素体、珠光体、贝氏体相变的Li模型参数,使其可以用于热处理冷却过程中复杂的奥氏体分解,并拟合出马氏体相变Koistinen-Marburger(K-M)方程的参数,较为完整地建立了S34MnV钢连续冷却转变过程中的铁素体、珠光体、贝氏体和马氏体相变的动力学模型.结果表明,本研究所述模型计算所得的转变量与实验所得结果一致,说明该模型可用于预测S34MnV钢热处理冷却过程中的相变,也为后续大型船用曲轴的数值模拟提供了准确的数学模型.     

SDC99钢淬火过程中应力和组织演变的有限元模拟

为了研究钢铁材料在淬火过程中内部组织和应力的变化,以自主研发的SDC99钢为研究对象,考虑相变潜热的影响,采用有限元方法对偏心圆环的淬火过程进行模拟仿真,并对淬火过程中模型的温度场、应力场和组织场的变化进行分析和研究.结果表明:经实验测定淬火过程中温度场及残余应力的分布与模拟结果吻合较好,偏心圆环上最大残余应力出现在45°及315°位置;模型硬度的分布与其马氏体含量分布趋势一致,模拟的硬度值略小于实测值.     

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.     

马氏体相变诱发塑性量化表征及合金元素的影响

利用亚稳奥氏体不锈钢低温拉伸曲线的特点,提出了表征相变诱发塑性增量的量化指标：单位体积分数的马氏体诱发的平均塑性增量D及本征塑性量D1,且D近似等于D1,对三种材料的相变诱发塑性的研究表明：钢中碳含量的增加使D值降低,降低层错能的合金元素有利于D的增加,而应变速率对D不产生明显的影响。     

超高强度钢板热冲压同步淬火阶段材料模型的构建与实验分析

实验材料为超高强度钢板BR1500HS,利用Gleeble-1500D热模拟实验机,对该材料在300~600℃温度区间内分别以0.03,0.3,0.6和1 s-1的应变速度进行高温拉伸变形实验,获得了该实验条件下流变应力的变化规律。结果表明,变形温度的降低和应变速率的增大都会使流变应力增大,但流变应力随变形量的增加达到峰值后逐渐趋于稳定。基于应力-应变数据构建BR1500HS同步淬火阶段Johson-Cook材料模型,依据此模型对热成形同步淬火阶段进行数值模拟,分析成形件及模具温度场的变化。在实验生产中,模具冷却系统使成形件在保压的同时温度迅速下降,实现其淬火过程,使材料发生马氏体相变,提高成形件的强度。为了实现超高强度钢的热成形同步淬火过程,采用同步冷却热成形系统,通过水流速度及保压时间,使零件马氏体分布均匀化。     

外加载荷对热弹性马氏体正-逆相变影响机制的相场模拟研究

本工作建立了一种新的马氏体逆相变的相场模型,以Cu-Al-Ni合金为例,研究了热弹性马氏体正相变和逆相变的演化规律,揭示了热弹性马氏体的形状记忆效应。同时模拟了拉伸释放弹性应变能这种机制对热弹性马氏体相变和热弹性马氏体逆相变的作用,研究了外加载荷对马氏体逆相变温度As的影响。模拟结果表明：应变能是形状记忆合金马氏体相变的阻力,是其逆相变的驱动力。在马氏体正相变过程中,拉伸载荷释放了应变能,降低了相变阻力,从而对马氏体相变起促进作用;在马氏体的逆相变过程中,由于拉伸载荷降低了马氏体所储存的应变能,因而降低了逆相变过程的驱动力,使合金逆相变As温度升高,进而提高了热弹性马氏体的低温稳定性。模拟结果与实验结果相一致。     

超弹性NiTi合金循环相变诱发塑性本构模型

为定量描述镍钛形状记忆合金循环相变诱发塑性导致的超弹性退化行为,在广义粘塑性框架下对Graesser模型进行了拓展,考虑了奥氏体和马氏体弹性模量的差异以及马氏体非线性硬化行为,引入循环相变过程中相变应力和残余应变的演化方程,建立了超弹性Ni Ti合金循环相变诱发塑性本构模型,总结了模型参数确定方法.通过镍钛形状记忆合金微管的循环相变试验结果和模拟结果的对比表明,提出的模型能够很好地预测镍钛形状记忆合金的循环相变诱发塑性行为。     

Finite element simulation of the residual stresses in high strength carbon steel butt weld incorporating solid-state phase transformation

This paper presents a sequentially coupled three-dimensional (3-D) thermal, metallurgical and mechanical finite element (FE) model to simulate welding residual stresses in high strength carbon steel butt weld considering solid-state phase transformation effects. The effects of phase transformation during welding on residual stress evolution are modeled by allowing for volumetric changes and the associated changes in yield stress due to austenitic and martensitic transformations. In the FE model, phase transformation plasticity is also taken into account. Moreover, preheat and inter-pass temperature are included in the modeling process. Based on the FE model, the effects of solid-state phase transformation on welding residual stresses are investigated. The results indicate the importance of incorporating solid-state phase transformation in the simulation of welding residual stresses in high strength carbon steel butt weld.     

Study of the effects of stress and strain on martensite transformation: Kinetics and transformation plasticity*

In this paper, the effects of stress and strain on the kinetics and plasticity during martensitic transformation are studied. The mathematical models of transformation kinetics and plasticity under stress are developed. According to experimental results, the transformation plasticity parameter k is concluded not to be a constant, but it varies with the stresses.     

Simulation of mechanical behavior of multiphase TRIP steel taking account of transformation-induced plasticity

Due to the effect of transformation-induced plasticity, multiphase low alloy TRIP steel exhibits an enhanced combination of strength and ductility. Moreover, volume fractions of the constituents in TRIP steel vary during its plastic deformation. In this paper, a constitutive model for mechanical behavior of multiphase TRIP steel is presented. In the model, TRIP steel microstructure is decomposed into four individual constituents: austenite, martensite, bainitic ferrite and ferrite. Mechanical behavior of each individual phase is described by using physically-based model. On the basis of introducing transformation-induced plasticity (TRIP) strain into decomposition of total strain, stress–strain relation of multiphase composite is obtained by using mixture rule and Iso-W hypothesis. Kinetics of strain-induced martensitic transformation is described by a generalized form of Olson–Cohen model, which takes into account temperature and stress state. Moreover, a new method to describe evolving grain size of retained austenite due to martensitic transformation is developed. On the basis of presented model, mechanical behavior of multiphase steel is simulated by using finite element method. Parameters of the model are calibrated for different mixture laws used in calculation. The simulated results have a good agreement with those observed in experiments.     

Models for transformation plasticity in loaded steels subjected to bainitic and martensitic transformation

Abstract

The parameter k in the transformation plasticity model for bainitic and martensitic transformations is determined by experiments on four types of steel. The relationships between k and stress and chemical composition of the steels subjected to bainitic and martensitic transformations are obtained. In addition, based on experiments using 26Cr2Ni4MoV steel, the value of k is found first to increase with the increment of stress and then to remain unchanged when predeformation before transformation occurs.     

A statistical mechanics approach describing martensitic phase transformation

A novel theoretical approach modeling martensitic phase transformation is demonstrated in the present study. The generally formulated model is based on the block-spin-approach and on renormalization in statistical mechanics and is applied to a representative volume element which is assumed to be in a local thermodynamic equilibrium. Using fundamental physical properties of a shape memory alloy (SMA) single crystal as input data the model predicts the order parameter “total strain”, the martensitic phase fraction and the stress assisted transformation accompanied by pseudoelasticity without the requirement of evolution equations for internal variables and assumptions on the mathematical structure of the classical free energy. In order to demonstrate the novel approach the first computations are carried out for a simple one-dimensional case. Results for total strain and martensitic phase fraction are in good qualitative agreement with well known experimental data according to their macroscopic strain rearrangement when phase transformation occurs. Further a material softening effect during phase transformation in SMAs is predicted by the statistical physics approach.     

The effects of solid-state phase transformation upon stress evolution in laser metal powder deposition

To investigate the influences of solid-state phase transformation on stress evolution during multi-pass laser metal powder deposition (LMPD) process, a 3D finite-element (FE) thermo-mechanical model considering phase transformation has been established. The influences of phase transformation such as mechanical properties changes, volume change and transformation induced plasticity (TRIP) are taken into account. Furthermore, the influences of high magnitude stress upon martensitic transformation characteristic temperature and TRIP are considered. The temperature and history (microstructure) dependent material properties used in the present research are obtained by experiments. The stress field during LMPD process is analyzed with and without solid-state phase transformation, respectively. Stress measurement of X-ray diffraction (XRD) method is applied to deposited samples, and the measuring data are compared with the computational predictions. The results show that phase transformation has a dominant effect on the stress evolution, longitudinal residual stresses significantly reduced as a result of solid-state phase transformation. In addition, the effect of stresses on martensitic transformation temperature is important for accurate prediction of residual stresses (stress state after cooling of the clad to ambient temperature). Residual stresses are lower when the phase transformation temperature is reduced.     

A model for determining initial dislocation-densities associated with martensitic transformations

Abstract

A three-dimensional multiple-slip dislocation-density-based crystal plasticity formulation, and specialised finite element formulations were used to determine the initial dislocation-densities associated with martensitic transformations in steel alloys. The analysis is based on modelling the shear part from the phenomenological theory of martensitic transformation to obtain both the transformation mobile and immobile dislocation-densities. The model was validated with experiments related to the transformation of lath martensite in high-strength low-alloying steels.     

Crystal plasticity analysis of stress–assisted martensitic transformation in Ti–10V–2Fe–3Al (wt.%)

A new model based on crystal–plasticity, crystallography, thermodynamics, kinetics and statistics is developed for stress–assisted martensitic transformation. The model includes the essential features of the stress–assisted martensitic transformation, such as: nuclei of progressively lower potency are activated in the course of transformation, the martensite phase appears in the form of thin plates, the parent phase exerts a higher resistance toward the growth of a plate in the thickness than in the radial direction, the average plate size decreases while the average plate aspect ratio increases with the extent of transformation, etc. The model is implemented in the commercial finite element code ABAQUS/Standard to analyze the evolution of martensite, materials texture and the resulting equivalent stress–equivalent strain curve during the stress–assisted martensitic transformation under different stress and strain states in a polycrystalline Ti–10V–2Fe–3Al (wt.%) alloy. The equivalent stress–equivalent strain curves and the volume fraction of martensite–equivalent strain curves are found to be mainly controlled by the applied stress state. Conversely, the texture observed in the transformed Ti–10V–2Fe–3Al is found to be primarily controlled by the imposed macroscopic strain state. The validity of the proposed materials constitutive model has been established by demonstrating a reasonable agreement between the model predictions and the available experimental data.