晶界特征对氢致延迟开裂过程的影响

Two kinds of steels with typical grain boundaries characterics were used to study the effect of grain boundary on hydrogen-induced delayed cracking. One is interstitial free (IF) steel with only ferrite grain and no transformation will occur after cold rolling. And the other is twinning-induced plasticity (TWIP) steel with typical compositions of Fe-Mn-C based austenitic steel, in which amount of twins easily formed during tensile process, thereby numerous twins with special coincidence site lattice (CSL) grain boundaries such as sigma 3, sigma 7 and sigma 13 etc appears. On the basis of measuring and understanding their hydrogen-induced cracking (HIC) behavior and hydrogen diffusion parameters and hydrogen concentration, the relationship among some special sigma CSL boundaries especially some low angle boundaries, local hydrogen concentration distribution and hydrogen-induced cracking will be established through combining and analysis all the information including the grain boundaries character distribution measured by electron backscatter diffraction (EBSD), the hydrogen concentration distribution in situ examined by scanning Kelvin probe force microscopy (SKPFM) and/or hydrogen microprint method, hydrogen-induced microcracks three-dimension tomography by diffraction contrast tomography (DCT) and other techniques. Finally, the influence of different sigma CSL boundaries during HIC process and each corresponding resistance to HIC were revealed, to contribution further deep understanding and the improvement of the HIC mechanism.

选用两种具有典型特征的结构材料：具有单一等轴铁素体晶粒、冷轧变形不出现相变的无间隙原子（IF）钢和因变形容易产生大量孪晶而具有丰富的特殊相符点阵（CSL）的Fe-Mn-C系孪晶诱发塑性（TWIP）钢，在研究其宏观氢致延迟开裂性能和氢扩散参数的基础上，通过电子背散射（EBSD）技术表征其晶界面特征；利用扫描开尔文力显微镜（SKPFM）以及氢微印法等技术和手段原位研究氢的浓度分布，特别关注氢在各种晶界上的富集行为；利用衍射衬度层析成像（DCT）技术对氢致微裂纹进行三维成像。通过综合分析上述研究结果，建立晶界面特征——氢浓度局部分布——氢致裂纹三者之间的对应性，从而确定各种不同sigma-CSL晶界的抗氢致开裂能力，为进一步完善氢致延迟开裂机理提供基础。

对具有fcc结构的孪晶诱导塑性（TWIP）钢和具有bcc结构的无间隙原子（IF）钢两种典型材料，采用慢应变速率拉伸试验（SSRT），结合电子背散射（EBSD）技术、扫描开尔文力显微镜（SKPFM）和氢微印技术（HMT）等手段，研究了它们的氢致延迟性能，表征了不同的晶界特征对材料的氢致延迟开裂性能的影响，最后结合断口的分形特征，确定了不同重合点阵（CSL）晶界的抗氢致开裂能力，丰富了氢致延迟开裂机理。具体研究结果如下：1）TWIP钢试样在动态充氢慢拉伸后均出现严重的塑性损失，脆性区断口呈现沿晶断裂特征；对于含有少量ε马氏体（10%）的试样，在1×10-6/s应变速率下，位错滑移对氢致纹的形核起着主导作用；在1×10-5/s应变速率下，形变孪晶对氢致裂纹的形核起主导作用；对于ε马氏体多的试样(45%)，在两种应变速率下，ε马氏体变形的不协调性形成的应力集中导致氢致裂纹的形核；氢致裂纹在扩展中，退火孪晶的Σ3晶界和西山关系的γ/ε相界面对氢致裂纹的扩展起着阻碍作用。2）多重分形方法表征结果表明，多重分形谱的宽度Δα和广义分形维数的参数Dmin/Dmax和氢致塑性损失率Eloss满足良好的负线性相关性，即Δα=-0.3433Eloss+2.202467和Dmin/Dmax=-0.33448Eloss+ 2.02147，表明多重分形方法可以定量表征氢脆断口的粗糙度和均匀性。进一步研究表明，TWIP钢的晶粒尺寸分布满足双重分形特征。随机晶界，Σ3晶界和整个晶界网络满足多重分形特征。具有高Σ3晶界比例的样品显示出高的抗晶间腐蚀性能；建立了TWIP钢晶间腐蚀厚度损失与晶粒和晶界分形参数之间的定量关系，即h=0.27t-37.5Δα-8.88Dg1+24.33。3）通过对IF钢预充氢试样的SSRT实验和HMT技术表征，发现强度较低的IF钢也表现出一定的氢脆敏感性，主要表现为延伸率和韧性下降。IF钢中的氢扩散具有晶体取向依赖性，即H优先聚集在(100)表面，使其成为易开裂的脆性取向；而(111)取向对H不敏感。Σ3晶界处氢致裂纹可被阻止，50-60°取向差的大角度晶界很容易发生开裂。这与(100)表面能较低而(111)表面能较高有关。