TWIP钢晶体组织调控及其力学行为

Tailoring the crystal microstructures has been the predominant way to improve the mechanical properties of TWIP steel due to limited effect of chemical composition design. Since the TWIP steel usually consists of equiaxed austenite grains with random orientations, the tailoring on the microstructures is realized through changing the grain size, which leads to a difficulty in improving the both strength and ductility at the same time because of the limitations by the inherent characteristics of twinning deformation and TWIP effect. In order to overcome this difficulty, a new tailoring route is suggested in the present proposal. A sample with columnar grains is prepared first by directional solidification, and its orientation and size are controlled by adjusting the solidification conditions, and then partial plastic deformation is imposed on the sample to produce localized deformation structures. Finally, the sample is subjected to recrystallization annealing to allow the deformed microstructures to be transformed to fine equiaxed grains and to result in the columnar and equiaxed grain coexisted microstructures. This composite crystal microstructure would make it possible to improve the both strength and ductility at the same time, because it utilizes the contribution of coarse and unidirectional columnar grains to the ductility as well as that of fine and randomly oriented equiaxed ones to the strength of TWIP steel．The proposal is aimed to disclose the effect of composed grains on the mechanical behavior of TWIP steel and related mechanisms through observing the changes in the morphologies of different grains and investigating into the evolution of twins, dislocations and grain boundaries in the different grains as well as their interactions during the plastic deformation. It is expected that a guideline for designing the crystal microstructure of TWIP steel will be obtained based on the study on the relationship of composed grains with the overall mechanical properties of TWIP steel, including the morphology, constitute and distribution of composed grains, to give a basis for further improving the mechanical properties of TWIP steel.

通过化学成分调控TWIP钢的力学性能其作用十分有限，因此微观组织控制便成为主要的调控途径。由于其晶体组织一般为取向随机的等轴晶，故对组织的调控实际上只能改变晶粒尺寸，而因孪生变形及TWIP效应的固有特性，又使得晶粒尺寸的改变难以取得强度和塑性同时增加的效果。本项目拟通过定向凝固首先获得取向和尺寸可控的柱状晶，再通过局部塑变和再结晶退火使变形组织转变为等轴晶，从而形成柱状晶、等轴晶共存且各自组成可调的复合晶体组织。利用粗大、单向柱状晶对塑性及细小、随机取向等轴晶对强度的贡献，实现TWIP钢强韧性的综合提高。通过对塑性形变过程中不同晶体组织形貌的观察，对组织中孪晶、位错、晶界演变过程及其交互作用规律的研究，揭示复合晶组织对TWIP钢力学行为的影响规律及物理机制；通过对复合晶形貌、组成及分布与材料整体力学性能之间关系的分析，建立复合晶TWIP钢晶体组织设计依据，为进一步改善其性能奠定一定基础。

孪生诱发塑性（TWIP）钢是一种新型高强韧合金钢，在交通、国防、航天等领域有广泛的应用前景。目前仅通过化学成分及晶粒尺寸调整难以进一步提升TWIP钢的强韧性，因此以晶粒形貌调整为主的晶体组织调控成为新的调控手段。本项目以Fe-30Mn-3Al-3Si（wt.%）为对象，采用光学浮区法，高速水冷法及液态金属冷却法（LMC）制备了不同TWIP钢试样。以LMC法中抽拉速率为60μms-1（DS60）及120μms-1（DS120）制备的试样为基体，制备复合晶TWIP钢，并对各试样微观组织及力学行为进行了表征分析。.结果表明，同一预变形量，退火温度升高，等轴晶平均晶粒尺寸增大，复合晶试样的塑性增强，抗拉强度有所下降。同一退火温度，预变形量增加，等轴晶再结晶数目增多、占比增大，复合晶试样的塑性下降，抗拉强度提升。其中，与传统的等轴晶试样相比，DS120复合晶试样有效地实现了强度、塑性同步提升，其强塑积高达49.3～56.7GPa•%，综合力学性能，尤其吸能性显著提高。 .研究发现，复合晶TWIP钢中，柱状晶会制约相邻的等轴晶内的孪生激发。相较于等轴晶，柱状晶更利于孪生的激发及孪晶的形成，其晶粒内部孪晶增加速度及密度也更高、更大，促进动态Hall-Petch效应，提高材料加工硬化能力，增加材料的均匀塑变，极大提升材料塑性。而等轴晶的引入，最大的贡献是保证了材料能够维持一定的屈服和抗拉强度，弥补了柱状晶晶粒组态对材料强度的弱化效应。这表明，等轴晶及柱状晶分别主导了复合晶TWIP钢的强度和塑性。.此外，研究表明，复合晶TWIP钢的塑变及加工硬化行为，尤其是均匀塑变能力介于柱状晶和传统等轴晶TWIP钢之间，其塑变机制决定于柱状晶及等轴晶各自塑变机制之间的竞争与协作。随着等轴晶占比的增大，试样塑变机制逐渐由柱状晶主导转变为等轴晶主导，相应的力学性能也会由高塑性、低强度逐渐转向低塑性、高强度，意味着存在中间值，使复合晶TWIP钢能够获得最佳的综合性能。.综上，本项目采用的新型调控手段，一定程度上打破了传统等轴晶TWIP钢强韧性无法同时进一步提升的瓶颈，有效实现了材料综合性能的显著提升，同时明确了等轴晶及柱状晶分别主导了复合晶TWIP钢的强度和塑性，此外也弄清了复合晶TWIP钢的塑变机制依赖于两种组织各自的占比，以上研究为TWIP钢晶体组织设计依据，为进一步改善其性能奠定一定基础。