Combining gradient structure and TRIP effect to produce austenite stainless steel with high strength and ductility

Abstract

a b s t r a c tWe report a design strategy to combine the benefits from both gradient structure and transformationinduced plasticity (TRIP). The resultant TRIP-gradient steel takes advantage of both mechanisms, allowing strain hardening to last to a larger plastic strain. 304 stainless steel sheets were treated by surface mechanical attrition to synthesize gradient structure with a central coarse-grained layer sandwiched between two grain-size gradient layers. The gradient layer is composed of submicron-sized parallelepiped austenite domains separated by intersecting ε-martensite plates, with increasing domain size along the depth. Significant microhardness heterogeneity exists not only macroscopically between the soft coarse-grained core and the hard gradient layers, but also microscopically between the austenite domain and ε-martensite walls. During tensile testing, the gradient structure causes strain partitioning, which evolves with applied strain, and lasts to large strains. The g / a 0 martensitic transformation is triggered successively with an increase of the applied strain and flow stress. Importantly, the gradient structure prolongs the TRIP effect to large plastic strains. As a result, the gradient structure in the 304 stainless steel provides a new route towards a good combination of high strength and ductility, via the co-operation of both the dynamic strain partitioning and TRIP effect.

Enhanced quasi-static and dynamic shear properties by heterogeneous gradient and lamella structures in 301 stainless steels

“…In these structures, high ductility can be attributed to the extra strain hardening due to the change of stress state and the presence of strain gradient, which generates geometrically necessary dislocations and back stress hardening [6]. In our recent research [23], high strength and ductility have been also achieved in a 304 SS by combining the benefits from both gradient structure and TRIP effect. The resultant TRIP-gradient steel takes advantage of both mechanisms, allowing the TRIP effect to large plastic strain by triggering martensitic transformation successively along the depth through strain partitioning in the gradient structure [23].…”

“…In our recent research [23], high strength and ductility have been also achieved in a 304 SS by combining the benefits from both gradient structure and TRIP effect. The resultant TRIP-gradient steel takes advantage of both mechanisms, allowing the TRIP effect to large plastic strain by triggering martensitic transformation successively along the depth through strain partitioning in the gradient structure [23]. So one of objectives in the present study is to produce high strength and ductility in 301 SS by combining several strain hardening mechanisms together, such as TRIP effect, strain partitioning and back stress hardening through GS and HLS.…”

“…Similarly, heterogeneous UFG SS combining with TRIP effect and Lüders bands also exhibited both high strength and good ductility [16][17][18][19]. Recently, several strategies employing hierarchical structures, such as gradient structure (GS) and heterogeneous lamella structure (HLS) [6,[20][21][22][23], have been reported to produce a superior synergy of yield strength and ductility. In these structures, high ductility can be attributed to the extra strain hardening due to the change of stress state and the presence of strain gradient, which generates geometrically necessary dislocations and back stress hardening [6].…”

The Evolution of Strain Gradient and Anisotropy in Gradient-Structured Metal

“…The tensile properties and underlying deformation mechanisms of the gradient-structured metals have been reported recently. [5,20,21,[23][24][25][26] For example, gradient-structured Cu was reported 100 pct stronger than CG Cu, while retaining its ductility, [20] which was attributed to the growth of nanocrystalline layer due to the low thermal stability of Cu. In gradient-structured metals with stable microstructures, the high ductility was attributed to the presence of strain gradient together with stress-state change, which promotes dislocation interactions and generation of geometrically necessary dislocations (GNDs).…”

“…[6,7,12,13] Recently, several promising strategies for achieving simultaneous high strength and high ductility have been proposed by tailoring microstructures through heterogeneous and/or hierarchical structures. [2][3][4][5][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Among them, the gradient structure, where the grain size or the substructure size changes gradually along the depth, [5,[20][21][22][23][24][25][26] has great potential in engineering applications due to their superior combinations of strength and ductility. The tensile properties and underlying deformation mechanisms of the gradient-structured metals have been reported recently.…”