【作者】 王丽娜；

【导师】 毛卫民；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2019， 博士

【摘要】 高锰TRIP(transformation-induced plasticity)钢在变形过程中能够发生马氏体相变,因而具有优异的强度、塑性和加工硬化行为。在高应变速率条件下高锰TRIP钢的塑性不减,因而有着广泛的应用背景。本文对高速变形条件(ε>103s-1)下的马氏体相变取向依赖性、变体选择、动力学特征以及裂纹扩展规律进行了系统研究,同时利用马氏体相变模型对TRIP过程的晶体学特征进行理论计算。此外,对高锰TRIP钢在冷轧过程(1 s-1<ε<100s-1)中的形变和相变行为进行研究。在此基础上,还研究了形变诱发α’-M的逆转变(α’-M→γ)机制和奥氏体的再结晶行为,为形变α’-M逆转变在组织调控中的应用提供理论基础。获得的主要结论如下:应变速率对γ→ε-M和ε-M→α’-M两阶段相变具有不同的影响。高应变速率使得γ→ε-M相变被抑制,ε-M→α’-M相变被促进。高速变形条件下,马氏体相变仍然具有与静态变形时相似的取向依赖性。但应变速率的提高使得α’-M变体选择减弱,形变诱发α’-M变体之间的取向差表现出新的特点,满足<111>60°取向差的α’-M变体对可优先出现。应变速率对压缩变形的动力学过程具有显著影响,变形前期的马氏体相变尤其是ε-M→α’-M相变速率显著提高,变形中、后期的TRIP效应被抑制。样品形状显著影响高速压缩时绝热剪切带附近的裂纹扩展。在相同的压缩条件下,柱形样的裂纹总是沿绝热剪切带内细小的等轴奥氏体晶粒扩展。帽形样的裂纹可在基体中的α’-M附近扩展,扩展方式可以是穿晶也可以是沿晶。基于马氏体相变的晶体学计算表明,ε-M和α’-M变体选择的判据并不相同。ε-M变体选择的判据为γ→ε-M相变过程的机械功。由于ε-M→α’-M相变需克服更大的弹性应变能,α’-M的变体选择受相变机械功和相变应变能的共同影响。满足<111>60°和<110>50°取向差的α’-M变体成对出现,可有效减少相变应变能,从而使得上述α’-M变体对具有优先形核的优势。在锯齿形分布的α’-M变体对中,具有最大机械功的α’-M变体可以优先形核,随后出现的α’-M变体受机械功和应变能共同影响,较高的机械功和较低的应变能均是其形核的必要条件。拉伸变形条件下,<110>γ晶粒中ε-M逆相变(ε-M→γ)的发生受两个因素影响。一方面相变晶体学计算表明,<110>γ晶粒内的γ→ε-M相变被促进、ε-M-→α’-M相变被抑制,这促进了ε-M的孪生,同时ε-M孪晶的取向利于逆相变的发生。另一方面ε-M板条的分布对相变行为具有重要影响,<110>γ晶粒内的ε-M板条多数孤立的分布、交叉处很少,这减少了α’-M的有利形核地点,从而促进了 ε-M孪晶及其逆相变的发生。对冷轧高锰TRIP钢的组织和织构研究表明,中等变形量下发生γ→ε-M和ε-M→α’-M相变,此时残余的奥氏体和ε-M已经稳定化。变形量进一步增加,主要发生奥氏体、ε-M和α’-M的变形。中等变形量下,α’-M形成典型的相变织构,以{113}<110>、{112}<110>为主。大变形量下,α’-M在形变的作用下向稳定取向发生偏转,最终形成<110>//RD的线织构并有形成<111>//ND线织构的趋势,以形变织构为主。在退火条件下,α’-M的逆转变以扩散方式进行,存在Mn、Al元素在奥氏体和α’-M中的再分配。{111}<112>取向的α’-M由于具有较大的形变储存能优先发生逆转变。α’-M的逆转变是通过残余奥氏体直接吞并临近的形变α’-M完成的,形成的奥氏体晶粒为长条状且存在较多的亚晶,长条状奥氏体晶粒又通过亚晶合并的方式发生再结晶而被等轴奥氏体晶粒取代。逆转变形成的奥氏体与形变奥氏体的织构类型相同,这是由残余奥氏体直接长大产生的。更多还原

【Abstract】 Due to the strain induced martensitic transformation,superior mechanical properties such as high strength,large elongation and better work hardening rate are maintained in high manganese TRIP(transformation-induced plasticity)steel.The excellent plasticity of high manganese TRIP steel is not reduced during high speed deformation,which expands its application areas.This study investigates the orientation dependence,variant selection,and kinetics of martensitic transformation at high strain rate(ε>103 s-1),as well as the propagation of cracks.Further more,the corresponding crystallographic characteristics are calculated based on the crystallography of martensitic transformation.Additionaly,the deformation and phase transformation during cold rolling(10 s-1<ε<102s-1)are analyzed.Then,the reversion of deformed α’-M(α’-M→γ)and the recrystallization of γ are studied.These can provide theoretica basis for the industrial application of α’-M reversion.The results are shown as follows:The strain rate has different influences on the transfomation of γ→ε-M andε-M→α’-M.γ→ε-M transformation is inhibited while the ε-M→α’-M transformation is promoted at high strain rate.Orientation dependence of martensitic transformation exists even at high strain rate.α’-M variant selection becomes weaker and a pair of α’-M variants with<111>60° misorientation is preferentially selected during high speed deformation.The kinetics of martensitic transformation is significantly affected by strain rate during compression.The TRIP effect especially ε-M→α’-M transformation is accelerated at the early stage of high speed compression,then the martensitic transformation is restrained.The behaviors of crack propagation in cylindrical and hat shaped specimens during high speed compression are different.For the cylindrical specimens,the cracks always propagate along the fine y austenite grains in the adiabatic shear band.For the hat shaped specimens,the cracks are observed near α’-M in matrix and the propagation can be transgranular or intergranular.Calculations based on the crystallography of martensitic transformation indicate that the criteria for variant selection during γ→ε-M and ε-M→α’-M transformation are different.The variant selection of γ→ε-M transformation is determined by the mechanical work induced by external force and transformation strain during γ→ε-M.α’-M variant selection is affected by the mechanical work and the strain energy during ε-M→α’-M transformation because higher strain energy must be overcome.The occurrence of ’α-M variant pairs with misorientations of<111>60° and<110>50° reduces the strain energy effectively,thus these α’-M variant pairs are selected preferentially.In α’-M variant pair with zigzag morphology,the α’-M variant with the highest mechanical work shows nucleation superiority;while higher mechanical work and strain energy reduction are both essential for the nucleation of the other variant in the pairs.The occurrence of ε-M reversion(ε-M→γ)in<110>γ grains during tension is affected by two factors.First,the γ→ε-M transformation is promoted and the ε-M→α’-M transformation is inhibited,thus the twinning of ε-M is promoted and its orientation favors ε-M reversion.Second,the distribution of ε-M plates also affects the phase transformation,less collisions between ε-M plates inhibit the nucleation of α’-M and lead to the twinning of ε-M and its reversion.Investigations of microstructure and texture evolution during cold rolling shows that y phase is almost completely transformed into α’-M phase at medium reduction;and a higher rolling reduction results in the dominant deformation of γ,ε-M and α’-M.The main texture components in α’-M are {113}<110>,{554}<225>and rotated cube({001}<110>)at medium rolling reduction,which are the typical phase transformation textures.The {113}<110>texture rotates toward a more stable orientation {223}<110>and leads to a strong cold rolling texture(<110>//RD)with the increase of reduction.The reversion of α’-M occurs by diffusional mechanism,accompanying with the diffusion of Mn and Al elements.And α’-M grains with {111}<112>orientation,which have high stored energy,’show the advantage of reversion.Deformed α’-M is merged by the adjacent γ,and elongated γ grains with a large amount of subgrains are obtained.Subsequently,recrystallization of γ grains occurrs by sub-grain coalescence and equiaxed γ grains are obtained.The textures of equaxied γ grains are approximately same as that of the deformed γ grains,which is generated by the growth of residual γ grains.更多还原