High Tensile Ductility and Strength in Dual-phase Bimodal Steel through Stationary Friction Stir Processing

H. S. Arora,M. Pole,S. Riyadh,J. Saini,A. Ayyagari,S. Mukherjee,K. Selvam,H. S. Grewal

doi:10.1038/s41598-019-38707-3

H. S. Arora, M. Pole + Show 6 more

Open Access

https://doi.org/10.1038/s41598-019-38707-3

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Abstract

The combination of high strength and good ductility are very desirable for advanced structural and functional applications. However, measures to enhance strength typically lead to ductility reduction due to their inverse correlation, nano-grained structures for an instance. Bi-modal grain structure is promising in this regard, but its realization is limited by multiple complex processing steps. Here, we demonstrate a facile single-step processing route for the development of bimodal grain structure in austenitic stainless steel, SS316L. The bimodal structure comprised of fine martensite grains (<500 nm) sandwiched between coarse austenite grains (~10 µm). The dual-phase bimodal structure demonstrated higher yield strength (~620 MPa) compared to ultra-fine grain structure (~450 MPa) concurrent with high uniform tensile ductility (~35%). These exceptional properties are attributed to unique dual-phase, bimodal grain structure which delayed the onset of plastic instability resulting in higher strength as well as larger uniform elongation and work-hardening rate. Our approach may be easily extended to a wide range of material systems to engineer superior performance.

Highlights

The trade-off between strength and ductility remains one of the major bottlenecks in the development of advanced structural materials
friction stir processing (FSP) was performed with the following processing parameters: (1) rotational speed of 1800 rpm and 0.4 mm plunge depth, tool was traversed along the longitudinal direction at 20 mm/min on the workpiece while submerged in a pool of liquid at 0 °C; (2) tool was rotated at 1800 rpm at a particular location of the workpiece for a period of 15 minutes while submerged in a pool of liquid, resulting in localized straining
Rotating the tool at a specific location on the workpiece resulted in bimodal grain structure comprising of fine grains in a matrix of coarse grains

Summary

Introduction

The trade-off between strength and ductility remains one of the major bottlenecks in the development of advanced structural materials. The bimodal grain steel showed extremely fine grains (

Results

Conclusion