Precipitation in a liquid phase sintered WNiFe alloy

Abstract

The precipitate phases formed within samples of a W-5 wt% Ni-5 wt% Fe alloy following sintering and controlled, post-sintering heat treatments have been characterized using techniques of transmission electron microscopy. Interphase boundary precipitation of a complex eta carbide has been confirmed in an as-sintered, commercial alloy of high impurity carbon content, and reproduced on a reduced scale in an experimental alloy of reduced carbon content by isothermal ageing (10 h, 900–950°C of solution treated (1 h, 1350°C) and quenched specimens. The experimental alloy contained no evidence of third phase precipitation in the as-sintered condition, but fine particles were noted along tungsten grain boundaries in solution treated and quenched specimens and were found to have a structure similar to that of the matrix γ phase. Isothermal ageing of solution treated specimens at temperatures of 750–850°C produced local colonies of cellular precipitation within the matrix phase, the discontinuous phase of essentially pure tungsten arising in response to a retained supersaturation of tungsten in matrix solid solution in the quenched material. A competitive precipitation of the eta carbide, (Ni, Fe) 6W 6C, was observed in the matrix phase under similar heat treatment conditions, with the most common form of the precipitate being Widmanstätten laths of (111) 7 habit plane. The present study produced no evidence for the formation of an intermetallic phase in the WNiFe alloy containing a weight ratio of Ni:Fe of 1:1.