Effect of Ti addition on thermal stability and phase evolution of super-invar based yttria added ODS alloys developed by mechanical alloying and spark plasma sintering
By Arora, Arpan; Mula, Suhrit
Published in Journal of Alloys and Compounds
2022
Abstract
In the present study, Ti and Y2O3 nano-powders were introduced in a base composition of Fe-42 wt% Ni invar alloy to investigate their effects on phase evolution and its thermal stability. Three compositions, namely, Fe-42Ni-2Y2O3-0.3Ti, Fe-42Ni-2Y2O3-1Ti, Fe-42Ni-2Y2O3-2Ti (all in wt%), were mechanically alloyed through high energy SPEX8000M ball mill with a varying amount of Ti (0.3, 1, and 2 wt%). The milled compositions were consolidated by spark plasma sintering at 1000 °C at 60 MPa pressure with a holding time of 5 min. The addition of Ti and Y2O3 nano-powders in the Fe-Ni metal matrix is expected to develop nanoclusters precipitates, thereby increasing its strength by Orowan strengthening. Consequently, submicron size grain was found to form in the case of 2 wt% Ti alloy (205 nm) as compared to 348 nm size grains in 0.3 wt% Ti alloy. The corresponding composition dictated higher nanoindentation hardness values (4.4 GPa and 5.2 GPa, respectively), which also validated Hall-Petch relationship. The Y2O3 nanoparticles break down into Y and O atoms through mechanical alloying and consequently dissolve into Fe-Ni metal matrix to form a supersaturated solid solution first. The addition of Ti atoms consolidates with Y and O and re-precipitates as Y-Ti-O based nanosize complex clusters within the matrix. XRD phase analysis and TEM-SAED study confirmed the formation of complex dispersoids (Y2Ti2O7 and TiO2), which played an important role in hindering matrix grain coarsening. The grain size evolution was confirmed by EBSD analysis in addition to TEM. The relative sintered density of 2Ti added alloy was found to be low (96.5%) compared to 1Ti (97.7%) and 0.3Ti (98.2%) added alloys, respectively. A ball on disk wear test showed a lower wear rate for the 2Ti added alloy due to the uniformly distributed dispersoids present within the metal matrix. Corrosion-resistant of the alloys (conducted in an electrolyte of 3.5% NaCl) were found to have insignificant effect of Ti variation in the said compositions. Orowan strengthening and Zenner pinning played pivotal roles to yield high thermal stability and ultrafine structure.