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Structural evolution of mechanically alloyed ODS steel powders during ball milling and subsequent annealing treatment

In the present work, a novel 9Cr oxide-dispersion strengthened (ODS) steel powders with Y2O3 (0.5 wt%) dispersoids were synthesized by high planetary ball milling at different time intervals (2, 8, and 16 hours). The structural and crystallographical evolution of the produced powders during the ball milling and post-annealing treatment were evaluated by SEM, XRD, and micro-Vickers hardness analyses. The SEM results showed that the fine dispersions of powders were achieved with the extending milling time. When milling time was 8h, it was observed that the mean size of powders increased maximum level of 101 μm and then dramatically reduced to 5 μm at latest milling time (16h). The XRD data revealed that the crystallite size of ODS powders diminished gradually with increasing milling time. Plus, all reflection peaks of the Fe, Cr, W, Mo expanded and the diffraction peaks of the Y2O3, W progressively disappeared with the increasing milling time. The hardness results revealed that the increasing milling time was beneficial for hardness improvement, due to dominant strain hardening mechanism and it developed from 160 to 334 Hv after 16h of milling protocol. To understand high temperatures characteristics such as grain growth, phase transformation, and hardness of produced powders, 16h milled powders subjected to post-annealing treatments at 700 oC and 900 oC for 1 h. When pure Fe and Cr peaks were observed in the non-annealed powders, no evident reflection peak of Y2O3 was observed. However, all pure Fe and Cr reflection peaks became narrower and Y2O3 reflection exhibited more sharper tendency with increased annealing temperatures, which resulted in increased grain growth and formation of Fe-based oxide structures.