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

Abstract

A B S T R A C TIn the present study, quasi-static and dynamic shear response of 301 stainless steel (SS) with gradient structure (GS) and heterogeneous lamella structure (HLS) were investigated by uniaxial tensile tests and Hopkinson-bar tests with hat-shaped specimens. The 301 SS with GS and HLS show a good combination of strength and ductility under quasi-static tensile tests, which is due to the back stress hardening for heterogeneous structures. Before the formation of adiabatic shear band (ASB), the dynamic shear response of 301 SS with coarse-grained (CG) austenitic structure shows a strong linear hardening stage and a plateau stage, which are due to the martensite transformation and the continuous deformation through strain partitioning between different phases, respectively. For 301 SS with CG austenitic structure, the grain size was observed to significantly refined in the ASB, while the reverse phase transformation occurs and the austenite phase increases significantly again in the ASB with increasing shear displacement, resulting in a hardness valley in the ASB at the shear displacement of 2.0 mm. The GS and HLS show excellent dynamic shear properties, this could be due to the back stress hardening for either macroscopically or microscopically heterogeneous structures. The HLS seems to have better impact shear properties than the GS, which indicates that the HLS with microscopically heterogeneous structures could delay the formation of ASB in a better way than the GS with macroscopically heterogeneous structures. The results in the current study could provide insights for obtaining better mechanical properties under dynamic conditions.

Dynamic shear deformation of a CrCoNi medium-entropy alloy with heterogeneous grain structures

“…Based on the misorientation angle distributions, the strong preference for 60 0 in the undeformed microstructure is weakened during the homogenous shear deformation, while a less obvious preference of 60 0 is still observed at the shear displacements of 1.02 and 1.28 mm for the dynamic shear experiments at room temperature and cryogenic temperature, respectively. These observations are in contrast with the random misorientations observed within ASB, which is induced by grain refinement through dynamic recrystallization (DRX) at high temperature[35,[40][41][42]. The lowering fraction of 60 0 misorientation angle during the shear deformation could be due to the interaction between the dislocations and the coherent TBs.…”

“…The cylindrical high strength maraging steel specimen holder was employed for two purposes: (1) it ensures pure shear deformation by constraining the lateral expansion of two legs for the hat-shaped specimens; (2) the shear displacements for interrupted "frozen" experiments are controlled by changing the height of the specimen holders. The hat-shaped set-up can facilitate the formation of ASB in the narrow concentrated shear zone, and the experimental details for the dynamic shear experiments by Hopkinson-bar technique can be found elsewhere [35,36,[39][40][41][42][43][44]. The concentrated shear zone is marked by two white lines in Fig.…”

“…4a and 4b display the shear stress-shear displacement curves of the samples annealed at various temperatures for the dynamic shear experiments conducted at room temperature (298 K) and cryogenic temperature (77 K), respectively. It is well known that the point at peak stress can be considered as the initiation point of the ASB according to the widely accepted maximum stress criterion for hat-shaped specimens [35,41,44]. Thus, the uniform shear strain can be estimated through localization increases with decreasing annealing temperature.…”

“…During the dynamic shear process, the shear zone (200 µm width) can be considered as a homogeneous shear deformation zone before the onset of ASB, and the deformation can be approximately considered as an adiabatic process. Thus, the temperature rise during the homogeneous dynamic shear deformation due to the plastic dissipation work within the homogeneous shear zone can be estimated as [39,41]:…”

“…The thermal softening due to the adiabatic heating can facilitate strain localization, such as adiabatic shear band (ASB) [35][36][37], which is a precursor of the final failure. Thus, the microstructure evolution before and after the formation of ASB is critical for understanding the mechanical properties and the corresponding deformation mechanisms of metals and alloys subjected to impact loading [38][39][40][41][42]. The dynamic shear behaviors in homogeneous metal and alloys with CG, ultra-fine grains (UFG) or NG have been investigated extensively in the literature [35][36][37][38][39][40].…”