Neutron diffraction and surface morhology observation of ultra-fine grained materials

采用热机械热处理法制备了超细晶粒材料，利用中子衍射和激光共焦显微镜对材料的显微组织和拉伸性能进行了研究。研究的材料包括(1)珠光体，(2)铁素体和(3)严重拉丝的铁素体或珠光体钢丝。塑性流动的多尺度不均匀性产生了内应力，即相应力、块应力和层状应力。导致高加工硬化。当珠光体钢承受1.6GPa的外加应力时，嵌入铁素体基体中的渗碳体板的应力约为5GPa。由亚微米铁素体和渗碳体颗粒组成的铁素体钢具有高强度和低加工硬化的特点。通过拉伸、压缩试验和表面观察，明确了拉伸变形初期的塑性失稳。重拉伸铁素体钢在塑性变形过程中表现出连续的再结晶，因此退火过程中只观察到晶粒长大。纳米级试样的强度为1.4GPa。在珍珠岩钢中，渗碳体板在拉伸过程中溶解，抗拉强度高达4GPa。原位中子衍射结果表明，在高应力状态下，材料的弹性行为是非线性的。

Ultra-fine grained materials were prepared through themo-mechanical heat treatment and their microstructures and tensile behavior were examined by using neutron diffraction and laser conforcal microscopy. The materials studied include (1)pearlite, (2)ferrite and (3)severely drawn ferrite or pearlite steel wires.The internal stresses, i.e., phase stress, block stress and lamellar stress are generated by multi-scale inhomogeneity in plastic flow. Leading to high work-hardening. The stress in cementite plates embedded into the ferrite matrix was determined approximately 5GPa when a pearlite steel is subjected to the applied stress of 1.6GPa.A ferrite steel consisting of submicron meters ferrite and cementite particles exhibits a high strength but little work-hardening. Using tensile and compression tests and surface observations, the plastic instability in the early stage of tensile deformation was made clear.Heavily drawn ferrite steel was found to show continuous recrystalisation during plastic deformation and hence only grain growth was observed by annealing. The strength of such nano-grain sized specimen show 1.4GPa. In case of peralite steel, cementite plates dissolve during drawing and exhibited the tensile strength as high as 4GPa. The elastic behavior was revealed nonlinear at the higher stress regime by insitu neutron diffraction during tensile loading.