利用飞行时间法中子衍射对比研究了充氢与未充氢1250 MPa超高强度钢的拉伸变形行为与轴向晶格变形特征, 并观察了断口区组织形貌与晶粒取向特征. 在无加载条件下, 充氢试样的轴向(110)与(200)面间距分别大于和小于未充氢试样的对应面间距, 显示出四面体间隙中H原子的进入使轴向(110)面间距有所增加, 同时内部应力的平衡作用使轴向(200)面间距有所减少. 未充氢试样达到1250 MPa抗拉强度发生颈缩塑性断裂, 而含有8.0×10^-6可扩散氢的试样在分步加载至500 MPa时发生脆性断裂. 中子衍射分析表明, 未充氢试样在拉应力加载至500 MPa时均基本符合线弹性变形, 但至700 MPa时, 轴向{200}晶粒比其余取向晶粒优先显示非线弹性变形, 至800 MPa时轴向{110}晶粒也出现非线弹性变形, 轴向{200}晶粒优先产生微屈服现象, 而轴向{211}晶粒仍然处于线弹性阶段; 充氢试样在拉伸至300 MPa时, 轴向{110}晶粒出现非线弹性变形, 至400 MPa时轴向{200}晶粒也出现非线弹性变形, 轴向{110}晶粒优先产生微屈服现象, 轴向{211}晶粒仍处于线弹性阶段. 断口剖面观察显示未充氢试样内形成明显的轴向<110>拉伸纤维织构, 而氢脆试样内除了明显的晶界裂纹萌生, 还有晶内裂纹扩展与局部晶体转动特征. 基于不同取向晶粒的微屈服概念, 解释了充氢导致轴向{110}晶粒优先微屈服而不是轴向{200}晶粒优先微屈服, 同时以氢伴随微区塑性变形的方式发生脆性断裂.

The tensile deformation behavior and the axial lattice strain response of 1250 MPa ultra-high strength steels with and without hydrogen charging were comparably investigated using time-of-flight neutron diffraction together with the fracture morphology and microstructure observation. Before tensile loading, the axial (110) lattice plane spacing of hydrogen charged specimen was found larger than that of non-charged specimen while the axial (200) lattice plane spacing of the former was smaller than that of the latter, suggesting that the hydrogen atoms occupied the tetrahedral site promoted the increment of axial (110) lattice plane spacing while the balanced internal stress resulted in the proper decrement of axial (200) lattice plane spacing. The necking and ductile fracture after approaching the 1250 MPa tensile strength occurred in the non-charged specimen, while the brittle fracture occurred in the 8.0×10^-6 hydrogen charged specimen at 500 MPa holding during step-by-step loading. The neutron diffraction analysis showed that in the non-charged specimen, the linear elastic deformation was kept up to 500 MPa loading, the nonlinear elastic deformation was observed preferably on the axial (200) reflection at 700 MPa, and then on the axial (110) reflection at 800 MPa; the axial {200} (i.e. <200>//TD, TD—tensile direction) grain orientation-dependent microyielding was observed preferably at 800 MPa while the (211) reflection was still under linear elastic deformation. Comparably, in the hydrogen charged specimen, the nonlinear elastic deformation was observed preferably on the axial (110) reflection at 300 MPa, and then on the axial (200) reflection at 400 MPa; the axial {110} grain orientation-dependent microyielding was observed preferably at 400 MPa while the axial (211) reflection was still under linear elastic deformation. The longitudinally sectioned microstructure observation under fracture surface confirmed the typical <110>-oriented tensile fiber texture in the non-charged specimen while the intergranular cracks along grain boundaries, quasi-cleavage/cleavage cracks and local crystal rotation in various grains of the hydrogen charged specimen. A concept about crystallographic orientation dependent microyielding was employed here to explain the above results, i.e. the hydrogen charging promoted the axial {110} grain orientation-dependent microyielding rather than axial {200} grain orientation-dependent microyielding, and the diffusible hydrogen embrittled the matrix microstructure, accompanying with local plastic deformation.