Significant dislocation strengthening of stainless steel 316L via co-directed energy deposition of silica

Abstract

This study aims at fully utilizing dislocation strengthening for higher hardness of stainless steel 316 L through two methods: 1) Multi-layer directed energy deposition (DED) is utilized to intensify dislocation generation induced by thermal cycling compared to single-layer DED. 2) Ultra-low thermal expansion coefficient silica particles are incorporated into a 316 L substrate, utilizing thermal misfit stresses between particles and the matrix. Silica particles, dispersed within the substrate, enable both multi-layer DED and surface DED. Nine samples (labeled as 1 L–9 L, respectively) with varying deposition layers from 1 to 9 are prepared. As deposition layers increase, silica particles agglomerate and elongate, promoting micro-crack formation in the matrix. Increasing deposition layers is found to alter gradually the solidification structure from planar to cellular and columnar dendrites, and transform the primary austenite to martensite (approximately 100 % at 9 L). This phase transformation is driven by oxygen-affine Cr and Mn elements diffusing into silica particles. Surprisingly, the hardness is significantly increased from about 170 HV at the base metal, 180–220 HV at 1 L–5 L, and 270 HV at 6 L, to maximum 300–330 HV at 8 L. This improvement is theoretically explained by significant dislocation strengthening mechanism (Δρ ∼ 6.70 × 1014 m−2) amplified by the multi-layer silica DED process. Furthermore, distinct dislocation structures including dislocation entanglement around silica particles, patterned dislocation walls, and nano-lamellae pile-ups of dislocations are found at 8 L from transmission electron microscope analysis. The strengthening origins and the potential of SiO2/316 L bulk composites are briefly explored.