The reduction of the interfacial segregation of phosphorus and its embrittlement effect by lanthanum addition in a W-Ni-Fe heavy alloy

Abstract

The addition of La to sintered 93W-4.9Ni-2.1Fe (weight percent) heavy alloy substantially reduces the embrittlement effect of P, because it forms a stable compound with P of possibly the form LaPO4. When 150 ppm of P is added to the powder mixture with about 19 ppm of P and 1 ppm of S as trace elements, the impact energy is very low, even after the heat treatment at 1150°C in N2 and water quenching due to the segregation of P at the interfaces. With the addition of La to this alloy (with 150 ppm of P added), its impact energy increases until, at the La content of 0.3 wt pct, it becomes equal to that of the alloy without any P added. When 0.2 wt pct of La is added to the alloy (without doping with P), the impact energy does not vary with the cooling rate after the heat treatment at 1150°C and is unaffected by the additional heat treatments at 500°C or 600°C. The structure of the fracture surfaces and the concentration of P measured at the interfaces also show that the La addition of a sufficient amount almost eliminates the P segregation at the grain-matrix interface. It appears that a stable compound of La, P, and O is formed at relatively high temperatures, above the heat-treatment temperature of 1150°C. The results show that by adding La, the variation of the impact property of this alloy with the amount of P in raw material powder or with the heat-treatment condition after sintering can be substantially reduced.