金属材料的强度与塑性是工业应用中最重要的两个性能指标。根据Hall-Petch关系可知，通过减小晶粒尺寸获得纳米晶体材料能够显著提升材料的强度。纳米晶体材料因为存在大量的晶界，严重阻碍位错的运动，提高强度的同时牺牲了材料的应变硬化能力，易发生非均匀变形，引起应变局部化。局部化变形容易造成材料的破坏，如样品发生“颈缩”、材料的绝热剪切带失效、裂纹尖端的应变集中等都与材料的非均匀变形密切相关。高强度纳米金属在准静态拉伸时，通常伴随着Lüders带式的非均匀变形，而目前关于高强度纳米金属的非均匀变形机理缺乏深入研究。本工作通过严重塑性变形和时效处理获得纳米层片双相结构FeNiAlC合金，系统地研究了双相异构金属在准静态载荷和动态载荷下的非均匀变形行为，以及纳米析出强化机制，主要结果如下：

(1) 利用轧制变形制备的纳米层片结构双相钢，屈服强度达到2.1 GPa，延伸率约28%，具有优异的力学性能。微结构结果表明：双相钢由奥氏体和BCC相构成，轧制后奥氏体层片厚度约20 nm，BCC相保持等轴晶形貌，晶粒尺寸约3-5 mm。DIC及X射线衍射结果表明：样品在拉伸过程中，首先在局部萌生Lüders带，Lüders带表面形成大量滑移带与褶皱，同时带内发生马氏体相变，产生硬化效果，促进Lüders带向未变形区域稳态扩展；当Lüders带扩展至整个标距段后，样品进行均匀变形，直至断裂。研究表明Lüders带扩展速度与Lüders带前沿应变梯度呈负相关，随着轧制量的增大，Lüders带萌生时的应力集中程度增大，带前沿应变梯度增大，Lüders带扩展速度随之减小，所以Lüders应变相应增大。原子探针分析元素分布结果表明：BCC相中Fe、Ni、Al元素等比例均匀分布；C元素主要分布在奥氏体中，同时在奥氏体界面处形成偏聚。奥氏体中的C元素偏聚在位错附近形成“Cottrell”气团，对位错起钉扎作用，造成局部应力增大，促进了Lüders带的萌生；C元素在晶界处的偏聚能够降低晶界自由能，抑制晶粒粗化，发挥稳定奥氏体的作用。当变形温度降低至200 K以下时，奥氏体稳定性降低，屈服强度随之降低，同时Lüders变形消失，变形由马氏体相变主导。研究表明相变和C元素的偏聚共同促进纳米层片双相钢中Lüders带的萌生和稳态扩展，使材料在高强度下能够获得稳定的塑性变形能力。

(2) 利用霍普金森压杆和帽形样品研究双相钢在高应变速率载荷下的剪切性能及绝热剪切带失效模式，结果表明：层片结构双相钢剪切强度达1.2 GPa，名义剪切应变达1.3，具有优异的动态剪切韧性。粗晶态双相组织变形过程中，剪切区发生大量的孪生变形，孪晶界与位错的交互作用提供材料的应变硬化机制；轧制态层片双相组织变形过程中，相界处位错密度较高，应变梯度增大，造成较大的应力集中，从而形成了大量的纳米晶粒。纳米晶粒的形成实现了材料强化，从而抑制变形局部化，提高材料的硬化能力。高应变速率载荷抑制马氏体相变的发生，位错及动态晶粒细化提供主要的硬化机制，实现了优异的动态剪切性能。两种组织均形成绝热剪切带，粗晶态的剪切带宽度约15.8 mm，轧制态的剪切带宽度约12.3 mm。利用功-热转化计算变形温升得粗晶态温升达720 K，轧制态约为190 K。相比两种微结构样品，轧制态层状结构样品名义塑性变形较小，温升低，依然形成与粗晶态样品相似的绝热剪切带，说明温升不是形成绝热剪切带的主要原因，而非均匀的塑性变形是直接原因。

(3) 对FeNiAlC合金不同微观组织的样品进行时效处理，研究微观结构对时效析出的影响，结果表明：热轧态等轴晶双相组织，时效处理后奥氏体机械稳定性较差，在约500 MPa的应力水平即发生马氏体相变，材料屈服强度较低，拉伸曲线没有明显的屈服现象。抗拉强度约1.7 GPa，强屈比接近4，拥有极高的应变硬化能力。微结构结果表明：析出相为B2有序结构，硬度极高，几乎不发生塑性变形。奥氏体相变过程中形成的纳米层片马氏体孪晶与位错的交互作用提供主要的应变硬化机制。冷轧态层片状双相组织时效过程中C元素几乎全部偏聚到晶界处，浓度高达1.5 at.%，抑制析出相在晶界处形核和长大，促进晶内析出，从而避免晶界脆性的发生。在400 ℃时效2 h，析出相体积分数约为60%，平均晶粒尺寸约500 nm，拉伸屈服强度达2.3 GPa，均匀塑性应变约8%。成份分析及微结构表明，析出相为B2-Fe(Ni, Al)有序结构。两相之间的非均匀变形诱导相界面附近产生大量的几何必需位错协调应变，随着外加应变的增大，非均匀变形产生的迟滞环宽度逐渐增大，表明非均匀变形产生的硬化能力提供主要的硬化机制。

本文获得的纳米层片双相结构和时效析出双相结构是典型的异构金属。异构金属通过非均匀变形诱导应变硬化，既提高了材料的强度，同时保持一定的塑性变形能力，极大的改善了材料强度与塑性之间的制约关系。

The strength and ductility of metals and alloys are the two highly desirable mechanical properties in industrial applications. According to the Hall-Petch relationship, the strength of materials can be significantly elevated by grain refinement to nanoscale. The slip of dislocation can be strongly impeded for enhanced strength due to the high density of grain boundaries in nanocrystal, while the strain hardening ability and the ductility are typically sacrificed due to rapid strain localization. Localized deformation plays a critical role in the failure process of materials, such as "necking", failure of the adiabatic shear band, and strain concentration at the crack tip in materials. High strength nanostructured metals usually exhibit Lüders band (LB) propagation during quasi-static tensile loading, nevertheless, there were few previous investigations on the non-uniform deformation mechanisms of high strength nanostructured metals. In the present study, the nano-lamellar dual-phase FeNiAlC alloy was obtained through severe plastic deformation (SPD) and aging treatment, and the non-uniform deformation behaviors under quasi-static and dynamic loads, as well as the nano-precipitation strengthening and toughening mechanisms by nano-precipitates were investigated systematically. The main results are as follows:

(1) The dual-phase steel with nano-lamellar structure prepared by rolling displays excellent mechanical properties with the yield strength of 2.1 GPa and tensile ductility of 26%. The microstructure characterization results show that the dual-phase steel is composed of austenite and BCC phases, the thickness of the austenite layer is about 20 nm, the BCC phase shows equiaxed grains with grain size of about 3-5 mm. The results of digital image correlation (DIC) and X-ray diffraction show that: during tensile deformation, LB initiates locally, and a great number of slip bands and folds are formed in LB; martensitic transformation occurs within LB, resulting strain hardening. Phase transformation promotes the stable expansion of the LB to the undeformed area, and the sample starts to deform uniformly until fracture when the LB extends to the entire gauge length. The expansion speed of the LB is relevant to strain gradient at the front of the LB. For the samples with larger rolling strains, the strain concentration in the LB and the strain gradient at the front of the LB are higher, and the expansion speed of the LB is slower, resulting in larger Lüders strain. The Atom Probe Tomography (APT) element distribution results reveal that: BCC phase contains equal atomic fraction of Fe, Ni, Al; Carbon is mainly distributed in austenite, at the same time, segregating at the GB of austenitic lamella. The segregation of carbon near the dislocation forms "Cottrell atmosphere", which pins the dislocations and causes the stress concentration, promoting the initiation of LB. The segregation of carbon at grain boundaries (GBs) can reduce the energy of GBs, inhibit grain coarsening, and stabilize austenite. When the deformation temperature is lowered than 200 K, the stability of austenite decreases, the yield strength decreases, and LB disappears, and the deformation is dominated by martensitic phase transformation. The above results indicate that the phase transformation and segregation of carbon promote the initiation and stable expansion of the LB in the nano-lamellar dual-phase steel, obtaining stable plastic deformation at high strength level.

(2) Dynamic shear properties and the failure mode by adiabatic shear band of the dual-phase steel under high strain rate impact loading were investigated by using spilt Hopkinson pressure bar. The results show that the dual-phase steel displays excellent dynamic shear properties: the dynamic shear strength of dual-phase steel with lamellar structure is 1.3 GPa, and the dynamic uniform shear strain is 1.3. During the shearing deformation of the coarse-grained (CG) samples, lots of deformation twins are formed in the shear zone, and the interaction between twin boundaries and dislocations provides the strain hardening mechanism of the material. During the shear deformation of the cold-rolled (CR) samples, numerous nano grains are formed at the phase boundaries. The high strain rates inhibit the phase transformation. Dislocations and grain refinement provide the dominant hardening mechanism. Both samples form adiabatic shear bands, the width of shear band for CG samples is about 15.8 mm, and the width of shear band for the CR samples is about 12.3 mm. The temperature rise during deformation was calculated using work-heat conversion. The temperature rises in the CG samples and CR samples are 720 K, and 190 K respectively. This indicates that the temperature rise is not the main factor for the initiation of adiabatic shear bands. The adiabatic shear zone is a strain-concentrated zone, which produces a relatively high temperature rise. Thus, equiaxed grains with grain size of about 300 nm are formed in the shear band by dynamic recrystallization.

(3) The FeNiAlC samples with different microstructures were obtained by aging treatment, succeeding the effect of microstructure on aging precipitation was investigated. The stability of austenite in the hot-rolled equiaxed dual-phase structure after aging treatment is poor. The martensitic transformation occurs under the stress of 500 MPa, and there is no obvious yielding phenomenon in tensile curve. The tensile strength is about 1.7 GPa, the ratio between tensile strength and yield strength is close to 4, exhibiting strong strain hardening. The microstructure characterization results show that a heterogeneous microstructure is formed after aging, consisting of retained austenite, martensite and precipitated phases. The precipitated phase is B2 phase with super lattice structure, which is extremely hard and nearly un-deformable. The interaction between nano-lamellar martensite twins by phase transformation and dislocations provides the dominant strain hardening mechanism. Most carbon atoms segregate to the GBs during the aging process in CR samples, which greatly decrease the energy of GBs. The segregation of carbon inhibits the nucleation and growth of the precipitated phase at the GBs, and promotes intragranular precipitation, thereby avoiding the occurrence of GB brittleness. After aging at 400 ℃ for 2 h, the volume fraction of precipitate is about 60%, the average grain size is about 500 nm. The aging for CR samples produces a tensile yield strength of 2.3 GPa, and a uniform elongation of 8%. According to the APT and TEM results, precipitates is B2-Fe(Ni, Al) phase with super lattice structure. The non-uniform deformation between the two phases induces a large amount of geometrically necessary dislocation for accommodating plastic strain near the phase interface. With the increase of the applied strain, the width of hysteresis loops caused by the non-uniform deformation gradually increases, resulting in heterogeneous deformation induced hardening as the dominant hardening mechanism.

The dual-phase microstructure and precipitation obtained in this study are typical heterogeneous microstructures. The heterogeneous deformation induce(HDI)-strain hardening can not only improve the strength of the material, but also maintain plastic deformation ability.