A novel ultra-high temperature Pt–Rh alloy with balanced tensile strength and creep resistance achieved by rare-earth intermetallics

Abstract

A novel ultra-high temperature Pt–25Rh (wt.%) alloy with a minor addition of rare-earth (RE) elements (0.2 wt% La and 0.2 wt% Ce) was developed for the first time which exhibits the capability to withstand aggressive environment with a high operating temperature of >1500 °C. Tensile creep tests of the alloy were carried out at 1500 °C under a load of 5 MPa, the high tensile strength (σb) was up to 47.8 MPa, the creep rate (ε) as low as 0.008%/h and a rupture time (τ) more than 53 h, respectively. Detailed microstructure characterization revealed that there exist two balanced phases in the alloy, a ductile (Pt, Rh) solid solution phase with a face-centered cubic structure and a strengthening Pt5RE (RE = La, Ce) intermetallic phase with a hexagonal structure. The Pt5RE phases present three types of forms in the alloy, precipitates in the (Pt, Rh)ss matrix, an intergranular phase along the grain boundaries and a eutectic mixture in the matrix. Composition analysis showed that RE elements almost completely exist in the Pt5RE intermetallic phase but rarely in the (Pt, Rh)ss phase. It is interesting to note that the Pt5RE intermetallic phase is a RE-poor phase in which the content of RE elements is less than 5 wt%, much lower than the stoichiometric ratio (12.46 wt%). Compared with the reported Pt–Rh binary alloys, a minor additions of La and Ce in the new alloy effectively improves its high-temperature mechanical properties, which can be attributed to the formation of the strengthening Pt5RE intermetallic phase. By the way, an orientation relationship was identified between the Pt5RE and (Pt, Rh)ss phases in the eutectic structure for the first time.