Thermal analysis and microstructure of oxide dispersion strengthened ferritic steels produced by ball milling with different amounts of process control agent

Abstract

Fe–14Cr–3 W–0.4Ti–0.25Y2O3 ferritic steels were produced by ball milling of initial mixture of elemental powders with various amounts of process control agent (PCA), ethanol (0.25 mass%, 2.5 mass%, 4 mass% and 20 mass%) under an Ar atmosphere and spark plasma sintering (SPS) consolidation at 1070 °C. The influence of the quantity of PCA on the properties (microstructure, density and Vickers hardness) of the as-milled powders and of the consolidated steels was investigated. X-ray diffraction shows a bcc–α-phase with fine crystallite size, 6.7–11 nm, for all powders. The particle size and the lattice constant of α-ferrite of the as-milled powders decrease as the amount of PCA increases. The powder milled with the highest amount of PCA, 20 mass%, contains carbides (M3C) and oxides ((Fe,Cr)2O3). The thermal analysis shows that as the amount of PCA increases, (1) the Curie temperature, Tc, increases, (2) the temperature of α → γ transition, Tα→γ, decreases, (3) the mass loss with CO/CO2 degassing increases, and (4) the milling with PCA hinders the nitrogen incorporation from milling media and air. The density and Vickers hardness of the SPS-consolidated and post-SPS annealed steels show an increasing trend with the increase in the amount of PCA. Carbides and oxides were detected in the post-SPS annealed steels derived from the powders milled with 2.5 mass% PCA and 4 mass% PCA (M23C6 and (Fe,Cr)2O3), and 20 mass% PCA (M3C and (Fe,Cr)2O3). The obtained results were discussed in terms of: (1) dissolution into the alloy matrix of carbon and oxygen released after the disintegration of PCA, (2) carbides and oxides precipitation during milling and/or upon heating and (3) promotion of thermally activated processes (such as carbothermal reaction) upon heating which can develop as well at the contact points/particle surfaces during SPS consolidation and can influence the process of densification.