Preparation And Mechanical Properties Of Metallic Materials With Lamellar Heterostructures

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 ?m and ?0.6 ?m,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 ?m)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.