Bulk synthesis of Ti28Zr40Al20Nb12 high entropy alloy: a critical evaluation of processing conditions

The bulk Ti28Zr40Al20Nb12 refractory high entropy alloy (RHEA) ingot with total mass of 20 kg was produced by vacuum induction melting followed by mould casting. The bulk ingot shows dual-phase structure: the soft (Ti, Nb)-rich bcc matrix and the hard Al3Zr5-type hcp phase. The variation of cooling rate of the bulk ingot affects both microstructure and compression properties. Phase morphology changes from smaller grains at the edge to coarse grains at the center of the ingot. The total volume fraction of hcp phase at the edge is above 40%, which decreased to around 30% in the intermediate region and the center of the ingot. Whereas, the fraction of hcp phase inside the grains inversely increased to be larger than 10% at the center. Crushing destruction happened for all samples during compression. The compression strength for samples prepared from the edge of the ingot ranges from 640 to 944 MPa, which dramatically increases to over 1500 MPa for the intermediate regions. The highest strength of 1950 MPa was obtained in the specimen prepared from the center of the middle area of the ingot. When comparing the sample properties along the axial direction, it is found that the samples prepared from the bottom areas were of higher embrittlement than those from the top or middle areas due to larger cooling rate. The findings indicate that more efforts should be devoted to investigate the processing of bulk RHEAs ingot with respect to their potential application.