Junliang Liu

304 stainless steel is known to be metastable as the austenite phase can transform into martensite under deformation. In this work, both ex-situ and in-situ transmission electron microscopy (TEM) characterization were used to investigate the mechanisms of the deformation-induced transformation at room temperature. The ex-situ tensile tests were conducted at a strain rate of 10-3 s-1 until rupture, followed by TEM and X-Ray Diffraction (XRD). Samples were also interrupted at strains of 7%, 18%, and 30% with the goal of investigating the intermediate microstructure. In addition, tensile tests were conducted in-situ in a TEM at 25 °C using a special straining-stage with the goal of capturing the nucleation and growth of the martensitic phase as it develops under deformation. The formation of stacking faults and the subsequent formation of e-martensite (hcp) through their overlapping/bundling was captured in-situ, confirming the role played by Stacking Faults (SFs) as intermediate step during the transformation from ?-austenite to e-martensite. Direct transformation of ?-austenite (fcc) to a’-martensite (bcc) was also captured upon straining and characterized. Such unique in-situ observations showcase how in-situ straining in a TEM, as a small scale tensile technique, is a powerful technique to visualize and investigate the mechanisms of deformation induced phase transformations.

The stability of the β-Nb Second Phase Particles (SPPs) in two types of Zr–Nb alloys (recrystallised Zr-1.0Nb and Zr-2.5Nb) was studied by in-situ heavy ion irradiation in a transmission electron microscope (TEM), combined with ex-situ analysis by energy dispersive x-ray spectroscopy (EDX). TEM thin foils were irradiated by 1 MeV Kr+ ions at four different temperatures from 50 K to 873 K, and by 350 keV Kr+ ions at different doses up to 39dpa. The change in size of individual β-Nb SPPs has been measured quantitatively, and the degradation mechanisms under irradiation at different temperatures discussed. It has been shown that the Nb redistribution between the SPPs and the Zr matrix is governed both by radiation induced mixing and local diffusion in the surrounding Zr matrix. Under the radiation conditions reported in this study, the β-Nb SPPs have shown remarkably stability against irradiation, and the extent of Nb redistribution between the SPPs and Zr matrix is very limited under all experimental conditions.

Tensile tests are conducted in-situ in a TEM at cryogenic temperatures (from - 100 °C to 0 °C) using a cooling TEM straining-stage with the goal of capturing the growth of the martensitic phase as it develops under stress in the material. The in-situ technique is used to explore the mechanism of deformation induced martensitic transformation in 304 and 304L austenitic stainless steels. The formation of stacking faults is captured, as well as the subsequent formation of e-martensite, confirming the role played by Stacking faults (SFs) as intermediate step during the transformation from ?-austenite to e-martensite. In addition, direct transformation from ?-austenite to a'-martensite is captured (i) upon straining at a fixed temperature and (ii) upon cooling after pulling on the sample, indicating how straining and temperature are both effective on the transformation.