本文以热轧得到的304 ss（304 不锈钢）/ low C steel（低碳钢）/304 ss三明治结构的多层板作为模型材料研究层片材料的力学行为及其强韧化机理。文中所用的多层板通过焊接→热轧→退火→酸洗等工艺制备而成。随后对多层板进行轧制和热处理改变界面区域（IZ）所占比例和层间力学性能的差异，研究内容主要包括以下四个方面：

    （1）异构多层板金属中的界面效应对材料的力学行为有重要的影响。通过轧制改变多层板中的界面区域所占比例可以优化其力学性能。多层板界面区域中的化学成分、相成分、晶粒尺寸和硬度是不均匀分布的，界面区域所占比例改变时，多层板的屈服强度不发生改变，但抗拉强度和均匀塑性随着界面区域所占比例的提高而增大。研究表明，由于变形过程中层间的应变不协调，在界面处形成了高密度的几何必须位错。随着应变的增加，界面附近的几何必须位错密度迅速增加，从而促进软硬层的协调变形，提高材料的均匀塑性。在多层板金属的变形过程中，尤其是变形的弹塑性阶段，软硬层间的内应力（背应力）起到了加工硬化的作用。界面区域所占比例越高，背应力硬化越强，多层板的拉伸塑性越好。

    （2）与室温下的力学性能相比，在低温（77K）下，异构多层板金属强度和塑性都有很大程度的提高。使用低温下先进的拉伸测试方法结合原位的数字图像相关技术（DIC）以及一系列的微结构表征研究其变形机制。结果表明，低温下，多层板应变局部化先从较硬的低碳钢层中开始并且向表层的304 ss扩展，已经形成的应变局部化区域（LSZ）沿着标距段向未变形区域扩展，在扩展的过程中，应变局部化程度逐渐降低。马氏体相变集中发生在304 ss的LSZ内，随着LSZ的扩展，LSZ前端发生大量马氏体相变，这个过程使多层板材料的应变局部化程度减弱，重新获得了加工硬化能力从而提高了多层板的塑性。

    （3）绝热剪切带的形成是金属材料在高应变率下常见的一种失效形式。通过霍普金森杆帽型剪切实验表征了low C steel/304 ss多层板在高应变率下的力学行为。结果表明，与单独的低碳钢层和304 ss层相比，多层板有更优异的动态剪切力学性能：多层板硬层中绝热剪切带（ASB）的形成滞后；ASB向软层中的扩展速度缓慢。ASB在均质材料中形成的最大应力准则在多层板金属中不再适用。多层板中软硬层之间的硬度差异程度对ASB的形成和扩展产生了很大的影响。软硬层之间力学性能的差异导致了多层板金属在动态剪切加载下界面附近的应变梯度和几何必须位错的聚集。ASB扩展过程中，多层板金属中发生了额外的加工硬化从而使其获得了更好的塑性。

   （4）动态加载过程中，温度的改变对材料的力学性能有着重要的影响。本文在霍普金森杆动态加载实验的同时对剪切区进行了高速同步测温和拍照，并对多层板金属变形过程中形成的ASB进行了微结构表征。实验结果表明：在多层板金属和组成多层板金属的基体材料中，应力到达最大值之后才会到达最大温升；与单独的低碳钢层相比，多层板中的最大温升相对于最大应力的滞后程度更大；多层板中低碳钢的温升比单独的低碳钢层小；多层板中的低碳钢层和304 ss层变形形成的ASB内均发生了动态再结晶。

In this dissertation, the 304 ss / low carbon (C) steel/ 304 ss sandwiched structural multilayer steel obtained by hot rolling is used as a model material. The mechanical behavior of the multilayer metal and its strengthening and toughening mechanism were systematically studied.This kind of multilayer metal was fabricated by hot-rolled bonding, annealing and pickling.In this dissertation, it was further processed by cold rolling, annealing, and heat treatment followed by water quenching to change the fraction of interface zone and produce various differences in microstructure between layers，then the mechanical behavior and strengthening and toughing mechanism of heterogengous multilayer metal were studied from the following four aspects:

   （1）The interface plays an important role in mechanical behaviors of metals with heterogenous microstructures. The mechanical properties of 304 ss / low C steel / 304 ss sandwiched structural multilayer metal are optimized by adjusting the fraction of interface zone. Heterogenous distribution in chemical composition, grain size, phase and hardness were observed in the interface zone. The yield strength is almost a constant when changing the fractions of interface zone, while the ultimate strength and the uniform elongation were found to increase with the increasing fraction of interface zone.High density of geometrically necessary dislocations(GNDs) were found to distribute in the interface zone and show a peak at the interface due to the mechanical incompatibility across the interface. Moreover, the density of GNDs in the interface zone is increasing with increasing tensile strain. Back stress hardening plays an important role in the multilayer metals, especially at the elasto-plastic transition stage. Higher fraction of interface zone can induce stronger back stress hardening and higher density of GNDs in the samples, thus resulting in better tensile ductility.

 （2）Considering the application of 304 ss / low C steel / 304 ss sandwiched structural multilayer steel at low temperature, the underlying deformation mechanisms have been revealed by a novel tensile testing method coupled with in-situ digital image correlation (DIC) method under cryogenic environment. Strain localization was found to initiate from low C steel, which is a hard layer, propagate across the interface and then towards 304 ss side. While the formed localized strain zone (LSZ) was observed to be delocalized at larger tensile strain due to the propagation of LSZ towards the un-deformed region along the gauge length. Martensite transformation was found to be concentrated in the LSZ of 304 ss to regain strain hardening ability and reduce severity of strain concentration. Strain partitioning between 304 ss and low C steel was found to be more significant in the LSZ than that out of LSZ. The non-uniform martensite transformation along the gauge length should be the origin for the strain delocalization in the LSZ, resulting in large ductility in the multilayer metals under cryogenic temperature.

    （3）The formation of adiabatic shear band is a typical failure mode of matals under high strain rate. The high-strain rate response of 304 ss/low C steel/304 ss multilayer steels have been characterized by set-up of hat-shaped specimen in Hopkinson-bar experiments. The multilayer metal has a better dynamic shear property compared to the standalone low C steel plate and the standalone 304 ss plate. The multilayer metals were found to postpone the nucleation of adiabatic shear band (ASB) in the hard zone of the multilayer metals and to delay the propagation of ASB from the hard zone to the soft zone. The well-known maximum stress criterion on ASB nucleation for homogeneous materials is not valid in the multilayer metals according to the experimental results. The hardness difference between the hard zone and the soft zone in the multilayer metals was observed to have great influence on the pattern of ASB nucleation and propagation. The mechanical incompatibility between the hard zone and the soft zone results in strain gradient and high density geometrically necessary dislocations at the interfaces under dynamic shear loading, contributing to extra strain hardening. The extra hardening was also found to be triggered at the propagation tip of ASB, which helps for achieving better dynamic ductility in the multilayer metals.

    （4）Temperature rise plays an important role in metals and alloys under dynamic loading. Synchronous high speed temperature measurement and high speed photography were conducted during Hopkinson experiment，and the microstructure of the ASB formed during the dynamic load was characterized. It’s shown that the maximum temperature rise is achieved after maximum stress in multilayer metals and its component metals. Greater time lag was found in multilayer metals compared with that in standalone low C steel and temperature rise in low C steel of multilayer metals is lower than that in the standalone low C steel. According to the microstructure characterization of ASB, we found that the dynamic recrystallization occurred in ASB in both the low C steel layer and the 304 ss layer in multilayer metals.