Fe-42Ni INVAR-BASED ODS STEELS DEVELOPED BY MECHANICAL ALLOYING + SPARK PLASMA SINTERING

Abstract

In our past study, Fe-42Ni, Fe-2Y2O3, Fe-42Ni-2Y2O3 alloys were developed by mechanical alloying (MA) using a high energy ball mill (SPEX8000M) followed by spark plasma sintering (SPS), and a detailed investigation was performed to enlighten the effects of nanosize Y2O3 addition on their mechanical behavior, wear resistance and corrosion resistance. The maximum density of ~98% was achieved for the 25 h milled compositions during consolidation through SPS at 1000 °C. Overall, the addition of 2wt.% Y2O3 in Fe-42wt.% Ni alloy resulted in better mechanical properties, and superior wear resistance due to the evolution of complex, hard and nanoscale oxide particles in a relatively finer grained matrix of the SPSed samples. Also, it showed reasonably good corrosion resistance. Based on this study, Fe-42wt.% Ni-2wt.%Y2O3 alloy is chosen as the base composition for further investigation in the present study. To explore the effect of Ti in Fe-42wt.% Ni-2wt.%Y2O3 alloy, three compositions, namely, Fe-42Ni-2Y2O3-0.3Ti, Fe-42Ni-2Y2O3-1Ti, Fe-42Ni-2Y2O3-2Ti (all in wt.%) were mechanically alloyed using the same ball mill with a varying amount of Ti (0.3, 1, and 2 wt.%). 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 temperatures on the mechanical properties. Phase evolution was studied by using x-ray diffraction (XRD) technique of the as milled and annealed samples. The milled compositions were consolidated by SPS 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 case of the 2%Ti alloy (205 nm) as compared to 348 nm size grains in 0.3% Ti alloy. The corresponding composition dictated higher nanoindentation hardness values (5.2 GPa and 4.4 GPa, respectively), which also validated Hall-Petch relationship. The Y2O3 nanoparticles break down into Y and O atoms through MA and consequently dissolved into Fe-Ni metal matrix to form a supersaturated solid solution first. The addition of Ti atoms associates with the dissolved Y and O and re-precipitates as Y-Ti-O based nanosize complex clusters within the matrix. The XRD as well as TEM analysis confirmed the formation of complex dispersoids (Y2Ti2O7 and Y2TiO5), 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 fine-grained metal matrix. Corrosion resistant of the alloys (at 3.5% NaCl) was found to have insignificant effect of Ti variation in the said compositions. Yield strength (YS) values of the SPSed samples were assessed through compression testing and correlated with the theoretically calculated YS analyzing various strengthening mechanisms and microstructures.