Mechanical properties of pulsed electric current sintered CrFeNiMn equiatomic alloy

Abstract

An equiatomic CrFeNiMn alloy was consolidated using pulsed electric current sintering (PECS) from gas atomized (GA) powder. A range of sintering temperatures was applied to determine its impact on the received microstructure and material properties. It was found that the phase structure of the disc shape sintered samples varied greatly depending on the sintering temperature. According to X-ray diffraction (XRD) the surface of the samples contained a noticeable amount of BCC phase while in the middle of the sample cross sections no BCC phase was found. Ball milling of the gas atomized powder prior to sintering for 25 h increased the density and hardness of the sintered sample from 98.3% to 99% and from 200 HV to 300 HV, respectively. At the same time, the ultimate tensile strength increased from 700 MPa to 1000 MPa and the elongation at fracture decreased from 40% to 25%. The enhanced hardness and tensile strength were attributed to the grain refinement caused by milling the powder prior to sintering. Milling of the powder resulted in a reduction in grain size of the sintered material from 5.7 μm to 1.9 μm when sintered at 1100 °C for 5 min. The grain refinement was also found to affect the deformation mechanisms. While the deformation of the samples prepared directly from GA powder was accommodated by the cell-forming process and deformation twinning, milling refined the initial microstructure to such an extent that deformation twinning was suppressed.In summary, a single phase equiatomic CrFeNiMn alloy can be achieved by pulsed electric current sintering of gas atomized powder. Sintering at temperatures above 1100 °C results in a single-phase FCC structure excluding the surface of the samples. Milling of the starting powder increases the density and strength and decreases the grain size of the sintered material. Decrease in grain size also suppresses deformation twinning and promotes dislocation cell formation. As the CrFeNiMn alloy system does not contain cobalt as an alloying element it shows great promise to be used in nuclear applications. The CrFeNiMn alloy showed good phase stability during tensile testing by not undergoing phase transformation contrary to metastable austenitic stainless steel where martensite transformations take place.