具有双相组织结构(FCC奥氏体相和B2型金属间化合物FeAl相)的超细晶(Ultra-fine grain, UFG)高比强度钢(High Specific Strength Steel, HSSS)在准静态条件下表现出优异的比强度(强度/密度)塑性匹配，其应变硬化机理包括背应力硬化和林位错硬化。本文主要研究高比强度钢的应变率效应、超塑性行为、层裂行为及其相对应的微观机理。

1)本文首先研究了双相高比强度钢拉伸行为的应变率效应。在0.0006-56 /s的应变率范围内，随着应变率的升高，屈服强度，抗拉强度和拉伸韧性均增大，这使得高比强度钢有望成为汽车工业中吸能器的理想候补材料，因为汽车碰撞时的应变率经常发生在中等应变率。由于两相间的载荷传递和应变分配，背应力硬化对高比强度钢起着重要作用，应变率越高，软相奥氏体晶粒的应变分配越大，起到了延缓变形失稳的作用。在所有应变率条件下，奥氏体晶粒内均观察到变形孪晶，变形孪晶有利于均匀拉伸变形。B2相在越高的应变率下越不容易变形，从而导致B2粒子越容易脆性断裂，在最终断裂面上的韧窝尺寸越小和相界面密度越高。因此，在越高的应变率下进行实验，最终的断裂过程需要消耗的能量越多，从而导致拉伸韧性越大。

2)本文在873-973 K的温度范围内和10^-4-10^-1 /s的应变率范围内研究超细晶双相高比强度钢的超塑性行为。高比强度钢表现出了优异的超塑性性能。在973 K的温度和10^-3 /s的应变率条件下进行不同应变的中断实验并通过微结构观察表明了变形机制可以分为两个阶段。在第一阶段(应变范围为0-400%)，超塑性流变归因于奥氏体相转变为B2相的扩散相变协调晶界滑动。然而晶内的位错激活是第二阶段(应变范围为400-629%)的主要变形机制，这是由于扩散颈缩导致了真实应变率增高。在高温拉伸变形过程中，两相晶粒尺寸相对稳定并始终保持在亚微米级，有利于超塑性流变。

3)本文进行了一系列平板撞击实验研究了冲击应力和微结构对高比强度钢冲击和层裂行为的影响。高比强度钢在屈服强度上表现出了很强的正应变率敏感性。当冲击应力增大到6 GPa时，层裂强度显著下降，然后随着冲击应力的进一步增大，层裂强度趋于平稳。这一趋势被认为归因于初始冲击压缩波在高比强度钢靶板内部传播时造成的累积损伤。在层裂过程中，可以明显观察到微裂纹在奥氏体相和B2相界面上形核并沿着相界面或者剪断穿过B2相传播。结果表明，雨贡纽弹性极限(Hugoniot Elastic Limit, HEL)以及层裂强度与微结构密切相关。孔洞形核位置的密度越低，层裂强度越大，说明层裂强度应该是与微结构参数相界面密度相关的一个力学性能。研究结果也发现高比强度钢的屈服强度和层裂强度呈倒置关系。因此，目前的研究结果可以通过调整微结构为获得一个最佳的力学性能匹配从而实现抗撞击结构应用提供依据。

A high specific strength steel (HSSS) with dual-phase microstructure (FCC austenite phase and B2 type intermetallic compound FeAl phase) and ultra-fine grains (UFG) exhibits excellent combination of specific strength (strength divided by density) and elongation under quasi-static conditions, its strain hardening mechanism includes back stress hardening and forest dislocation hardening. In the present study, the strain rate effect, superplastic behaviors, spall behaviors and their corresponding microstructural mechanism of the HSSS have been investigated.

1) The strain rate effect on the tensile behaviors of the HSSS with dual-phase microstructure has been investigated firstly in the present study. The yield strength, the ultimate strength and the tensile toughness were all observed to increase with increasing strain rates at the range of 0.0006 to 56 /s, rendering this HSSS as an excellent candidate for an energy absorber in the automobile industry, since vehicle crushing often happens at intermediate strain rates. Back stress hardening has been found to play an important role for this HSSS due to load transfer and strain partitioning between two phases, and a higher strain rate could cause even higher strain partitioning in the softer austenite grains, delaying the deformation instability. Deformation twins are observed in the austenite grains at all strain rates to facilitate the uniform tensile deformation. The B2 phase is less deformable at higher strain rates, resulting in easier brittle fracture in B2 particles, smaller dimple size and a higher density of phase interfaces in final fracture surfaces. Thus, more energy need be consumed during the final fracture for the experiments conducted at higher strain rates, resulting in better tensile toughness.

2) The superplastic behaviors of the HSSS with dual-phase microstructure and ultrafine grains have been investigated in the present study under a temperature range of 873–973 K and at a wide strain rate range of 10⁻⁴–10⁻¹ /s. The HSSS exhibits excellent superplastic properties. The microstructure observations at interrupted strains for tests under temperature of 973 K and at strain rate of 10⁻³ /s have provided evidences of different mechanisms for two stages. At the first stage (strain range from 0% to 400%), the superplastic flow is attributed to the diffusional transformation from austenite phase to B2 phase coupled with grain boundary sliding. While intragranular dislocation activities should be the dominant mechanism for the second stage (strain range from 400% to 629%) due to the increased realistic strain rate by diffusive necking. The grain sizes of both phases are observed to be relatively stable and remain always submicron level during the high temperature tensile deformation, facilitating the superplastic flow.

3) A series of plate-impact experiments were conducted in the present study to investigate the influences of impact stress and microstructure on the shock and spall behaviors of the HSSS. The HSSS shows a strong positive strain rate sensitivity on the yield strength. With increasing impact stress up to about 6 GPa, the spall strength is found to decrease significantly and then levels off with further increasing impact stress. This trend is proposed to be attributed to the accumulation damage within the target as the initial shock-induced compression wave propagates through the target. The microcracks are clearly observed to nucleate from the interfaces between austenite and B2 phase and propagate along the interfaces or cut through the B2 phase in the HSSS during the spalling process. The Hugoniot elastic limit and the spall strength were found to be highly dependent on the microstructure. The spall strength was found to be higher when the density of the void nucleation sites is lower, indicating that the spall strength should be a mechanical property related to the microstructure parameter depending on the density of phase interfaces. It was also found that there is a tradeoff between the specific yield strength and the spall strength for this HSSS; thus, the current findings should provide insights for achieving an optimal combination of both mechanical properties for impact-resistant applications by tailoring the microstructure.