Phase evolution characteristics, thermal stability, and strengthening processes of Fe–Ni based ODS invar steel produced by mechanical alloying and spark plasma sintering

Abstract

In the present study, the base composition of Fe-42 wt% Ni invar was used to prepare three different oxide dispersion strengthened (ODS) steel compositions with addition of 2 wt% Y2O3 and a varying amount of Ti (0.3%, 1%, and 2% wt.%) by mechanical alloying (MA) using SPEX8000 M high energy ball mill. The mechanically alloyed samples were compacted first to make pellets, and batch-annealed at 400, 600, 800, and 1000 °C to verify its thermal stability. The hardness of the as-milled and annealed samples was estimated to apprehend the effect of Ti content and temperature on the mechanical properties. Phase evolution was studied by using the x-ray diffraction (XRD) technique of the as-milled and annealed samples of all the compositions. Then, all the milled samples were spark plasma sintered (SPSed) at 1000 °C for further investigation of sintering characteristic, microstructure, and mechanical properties. The SPSed samples were subjected to compression and Vickers hardness testing to study the change in mechanical properties of the different compositions after sintering. Yield strength values of the three compositions were theoretically calculated through strengthening mechanisms analysis using XRD data and microstructural information of the SPSed samples and correlated with the experimentally determined compressive yield strength. The microstructural study of the SPSed samples was performed using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analysis. The relationship between microstructure and mechanical properties has been examined in the context of various strengthening processes such as dislocation strengthening, solid solution strengthening, grain size strengthening, and precipitation/dispersoid strengthening in the SPSed alloys.