【作者】 李建生；

【导师】 朱运田； 李玉胜；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2020， 博士

【摘要】 层状异构金属材料是一种新型结构的金属材料。典型的层状异构组织是指金属材料内部由软相微米再结晶组织和硬相超细晶/纳米晶/纳米孪晶组织共同构成,软相微米再结晶一般呈现层状聚集且被硬相组织牢牢地包夹着,整个结构在空间尺度上展现出层状分布的特征。层状异构组织包含单金属层状异构结构和多金属复合层状结构。通过对金属材料进行层状异构化处理后,材料的强度和拉伸塑性匹配通常会得到了显著优化。本文首先分别选用两种层错能相差较大的316L和304L不锈钢,利用传统的冷轧+退火工艺成功调控制备出层状异构组织,论文着重讨论316L不锈钢中层状异构组织的形成机制,并且分别对这两种层状异构不锈钢的拉伸变形机制进行探究,同时还进一步研究了层状异构316L不锈钢的冲击行为;另外,本文提出了采用新颖的扩散焊+轧制+退火工艺制备了层状异构铜/黄铜复合金属材料,并结合异构变形诱导强化理论揭示了其优异的力学性能的本质。主要研究成果如下:(1)结合85%压下量轧制和750℃-10 min退火工艺调控出具有优异强-塑性匹配的层状异构316L不锈钢。层状异构组织主要由残留拉长变形粗晶、层状分布的微米再结晶、纳米孪晶束以及纳米晶互相混合、交替层状堆叠而成。该不锈钢具有1 GPa的屈服强度、～10%的均匀延伸率以及～20%的断裂延伸率。层状异构316L不锈钢优异的强-塑性匹配主要归结于:不同微结构各自对强度和塑性的贡献是优异强-塑性匹配的根本,其中包含纳米晶对强度的贡献,再结晶晶粒对拉伸塑性的贡献以及纳米孪晶束的同时增强增韧作用;另外,层状异构组织中软/硬结构界面在协调变形过程中产生大量的几何必须位错,几何必须位错一方面提升了材料的加工硬化能力,有益于拉伸塑性的改善,另一方面大量在界面堆积的几何必须位错提供了额外的异构变形诱导强化作用,同时提高材料的屈服强度和拉伸塑性。(2)层状异构316L不锈钢,在保持1 GPa屈服强度的同时具有良好的抗冲击性能,其冲击吸收功(～1.75 J)可以达到原始粗晶态样品的65%。良好抗冲击抵抗性是纳米孪晶束、层状再结晶和应力诱导马氏体相变共同决定的。特别指出的是纳米孪晶束在断裂过程中产生的超深韧窝可以很好地吸收能量,在很大程度起到了提高冲击功的作用。(3)结合88%压下量轧制和700℃-15 min退火工艺调控出具有优异强-塑性匹配的层状异构304L不锈钢。层状异构组织主要由体积分数为～32%层状微米再结晶和体积分数为～68%的超细晶构成,它们呈现“三明治”状相互堆叠特征,微米再结晶和超细晶的平均晶粒尺寸分别为～3.2μm和～0.6μm。该层状异构不锈钢具有～820 MPa的屈服强度以及～32%的均匀延伸率。层状异构304L不锈钢优异的强-塑性匹配主要归结于:高的屈服强度取决于变形过程中软/硬界面处产生较高的异构变形诱导力;良好的拉伸塑性主要取决于软相和硬相中活跃的协调变形行为,即软相中的林位错、几何必须位错不断增殖以及硬相中持续的应力诱导马氏体相变行为。(4)通过扩散焊+轧制+退火工艺调控出具有优异强-塑性匹配的软/硬层状异构铜/黄铜复合金属。该材料中铜和黄铜组织存在着硬度的差异以及晶粒尺寸的差异。300℃退火处理层状异构铜/黄铜具有～390 MPa的抗拉强度以及～24%的均匀延伸率,400℃退火处理层状异构铜/黄铜复合金属具有～335 MPa的抗拉强度以及～36%的均匀延伸率,它们都显现出优异的综合强度和拉伸塑性。高的强度取决于硬相黄铜结构本身的高强度以及在拉伸变形过程中软/硬界面处产生了很宽的应变梯度(～10-20μm),宽应变梯度有利于产生很高的异构变形诱导力,从而提高材料的强度;良好的拉伸塑性主要取决于界面附近软相中产生大量的几何必须位错塞积,额外的几何必须位错和粗晶铜中林位错增殖共同维持着高的加工硬化能力,从而增强了材料的拉伸塑性。更多还原

【Abstract】 Lamellar heterostructured metals are novel metallic materials in modern industry.A typical lamellar heterostructure(LHS)has a characteristic of layered structure distribution that soft lamellar recrystallized grains(RGs)are surrounded by hard structures(ultrafine/nano grains(UFG/NGs))or nano-twins(NTs)).Generally speaking,the lamellar heterostructured metals are mainly divided into two categories that are lamellar heterostructured single metal and lamellar heterostructured composite metals,respectively.Fabricating metallic materials with lamellar heterostructures is an effectively route to achieve good combination of strength and ductility,which can satisfy the requirements for industrial applications.In this paper,two kinds of typical stainless steels(316L,304L)with various stacking fault energy were used to obtain LHSs prepared by conventional cold rolling(CR)and annealing treatment.The formation mechanism of LHS 316 L sample and the deformation mechanisms of LHS 316/304 L samples were systematacially revealed.This paper has also explored the impact behavior of LHS 316 L sample.In addition,a novel diffusion welding+CR+annealing technique was firstly used to fabricate LHS copper/bronze composite.The mechanical properties of LHS copper/bronze composite were thoroughly studied,which was based on the latest hetero-deformation induced(HDI)hardening theory.The main results of present work are concluded as below.(1)A LHS 316 L stainless steel with superior combination of strength and ductility was produced by 85% CR and subsequent partial recrystallization(750℃-10 min).The LHS was initially generated from 85% CR structure,which was characterized with lamellar coarse grains(LCGs)and lamellar RGs sandwiched by the mixtures of NGs and NT bundles.It was specially noted that a high yield strength of ～1 GPa,combined with an uniform elongation and elongation-to-failure of ～10% and ～20%,can be obtained for the LHS sample.The enhanced strength was obtained from ultra-hard NGs and NT bundles,whereas the high ductility was ascribed to the soft RGs,LCGs and NT bundles.Large amount of geometrically necessary dislocations(GNDs)were generated near the interfaces to coordinate the deformation between hard domains(NGs and NTs)and soft domains(RGs and LCGs),resulting in high HDI hardening to simultaneously increase the yield strength and ductility for LHS 316 L sample.(2)The above-mentioned high-strength LHS 316 L stainless steel had a satisfied impact energy with the value of ～1.75 J.This impact energy was identical with the ～65% value(～2.75 J)of that of low-strength coarse-grained sample,which may infer a good prospect of industrial application(satisfied toughness/ductility with high yield strength of ～1 GPa).The good impact resistance was attributed to NT bundles,lamellar RGs and strain-induced martensitic transformation.Specially,the unique deep dimples caused by NT bundles suppressed crack propagation and consumed much higher fracture energy.(3)A LHS 304 L stainless steel with superior combination of strength and ductility was produced by 88% CR and subsequent partial recrystallization(700℃-15 min).The LHS sample exhibited a typical lamellar character that the lamellar micro-RGs(volume fraction of～32%)sandwiched by equiaxed UFGs(volume fraction of ～68%).The mean grain sizes of micro-RGs and UFGs were ～3.2 μm and ～0.6 μm,respectively.The yield strength and uniform elongation of LHS 304 L sample were 820 MPa and ～32%,which showed a superior combination of strength and ductility.The high yield strength was ascribed to the HDI hardening between the soft/hard interfaces.Good ductility was resulted from the active deformation behavior both in micro-RGs and UFGs that forest dislocations and GNDs accumulated in the micro-RGs and strain-induced martensitic transformation appeared in UFGs.(4)LHS copper/bronze composite with various soft/hard interfaces and superior mechanical properties were fabricated by three primary processing steps: diffusion welding,cold rolling and annealing.The hardness and grain size were different in copper and bronze layers in LHS sample.300℃-annealed and 400℃-annealed LHS copper/bronze composite both had high ultimate strength and uniform elongation(～390 MPa,～24%;～335 MPa,～36%).High strength was attributed to the strong bronze layer and the extra HDI hardening between soft/hard interfaces.HDI hardening was caused by the wide broad strain gradient(～10-20 μm)in the soft copper layer around soft/hard interface.The good ductility was determined by the high work-hardening ability,which was come from the GNDs accumulation around soft/hard interfaces and general forest dislocation accumulation in soft copper layers.更多还原