超细晶D6A钢动态拉伸力学特性实验研究

为了推进超细晶D6A钢在半穿甲战斗部壳体上的应用，研究了动态加载下其宏观力学行为和细观变形机理。运用旋转盘式Hopkinson拉杆技术，开展了超细晶D6A低合金钢（平均晶粒尺寸为510 nm）的动态拉伸实验，获得了不同应变率（500～1000 s?1）下超细晶钢的应力-应变曲线。运用TEM观测微观形貌，从细观层次研究了高应变率拉伸作用下超细晶钢的动态力学特性。结果表明，超细晶D6A钢具有较高的动态拉伸强度和良好的延展性。并且，晶粒细化和纳米析出相（渗碳体）是超细晶钢同时拥有高强度和较好韧性的重要因素；在动态拉伸过程中析出的大量纳米级渗碳体，与高密度晶界共同作用限制了位错运动，从而产生额外的塑性变形抗力，有效提升了超细晶钢的强度；在塑性变形阶段超细晶钢出现的明显应力下降现象，是可动位错密度增高的结果。

Investigation on dynamic tensile properties of an ultrafine grained D6A steel

In order to promote the application process of an ultrafine grained (UFG) D6A low-alloy medium-carbon steel in semi-armor-piercing warhead shells, mechanical behaviors and microscopic deformation mechanism of the UFG D6A steel under dynamic loading were studied. The UFG D6A steel (d = 510 nm) was prepared by using inter-critical rolling and low temperature annealing process, whose microstructure features show that nanoscale spherical cementite grains are uniformly distributed in the equiaxed ferrite matrix. Dynamic tensile experiments were performed with a rotating Hopkinson bar apparatus at strain rates ranging from 500 s?1 to 1000 s?1. Micromorphology of specimens before and after tensile loading was observed by transmission electron microscopy. Combined with these observations, the dynamic mechanical properties of the UFG steel under high strain rates were extensively studied. The results reveal that the UFG D6A steel achieves both excellent strength and well toughness simultaneously with a dynamic tensile strength of 2 200 MPa and an average dynamic fracture elongation of 13%. The dynamic tensile strength is obviously higher than the quasi-static tensile strength (approximately 2 times), while the toughness is lower than that under quasi-static conditions. It is observed that the cementite content increases dramatically during the dynamic tensile experiment process, which can effectively restrict the movement of dislocations to produce additional plastic deformation resistance. Consequently, grain refinement and the precipitation of nanosized carbides are considered to play key roles for strengthening the steel. The severe plastic deformation reduces the average grain size and increases the density of grain boundaries within the material, which is considered to eventually lead to the decrease of dynamic fracture elongation of the UFG D6A steel. The drops of yield stress were observed apparently during dynamic tensile process, which is mainly due to the increase of the mobile dislocation density. These research results may give deeper insights into the relationship between material microstructure and mechanical behavior of UFG steels, and provide a significantly experimental and theoretical basis for the application of UFG D6A steels in the military equipment field.