Microstructure, Texture, and Mechanical Behavior of As-cast Ni-Fe-W Matrix Alloy
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I.
INTRODUCTION
THE application of Ni-based alloys as performance materials has taken a quantum jump during last four decades due to a large number of industrial applications.[1-3] These materials are being used in the form of cast, wrought, and powder products. The wrought forms of these alloys are plate, sheet, strip, bar, wire, pipe, flanges, etc.[4,5] On the other hand, the cast and powder products are related to high-temperature applications wherein conventional thermomechanical processing is difficult. A large number of ternary and quaternary alloys were derived, due to excellent deforming capability, having moderate to high work-hardening rate, corrosion resistance, and weldability. These alloys consist of single-phase solid solution at all temperatures below the solidus.[5,6] Ni-base solid solution alloys are one of the constituent phases of tungsten heavy alloys (WHAs) prepared by the typical powder metallurgy technique. The
A. SAMBASIVA RAO, PREMKUMAR MANDA, T.K. NANDY, and A.K. SINGH are with the Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad 500 058, India. Contact e-mail: singh_ashok3@rediffmail.com M.K. MOHAN is with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal, Telangana 506 004, India. Manuscript submitted July 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
WHAs are two-phase composites consisting of nearly pure bcc tungsten grains embedded in a relatively soft and ductile matrix phase such as W-Ni-Fe or W-Ni-Cu or even W-Ni-Cu-Fe solid solutions.[7,8] The WHAs consist of 90 to 98 wt pct W along with nickel, iron, cobalt, and copper (minimum of two transition elements among these). These elements are used with different combinations to form liquid phase through eutectic reaction with tungsten.[9] It is known that both the Fe and W elements act as solid solution strengtheners in the Ni matrix. The Fe and W have negligible and strong effects, respectively, on the stacking fault energy (SFE) values of the Ni-based solid solution alloys.[10] The WHAs are usually processed by the powder metallurgy route using liquid-phase sintering due to the high melting point of tungsten.[11] The other alloying elements (Ni, Fe, and Co) melt first during liquid-phase sintering due to the eutectic reaction. It is known that the eutectic reaction brings down the melting temperature significantly during sintering. As per the W-Ni-Fe ternary phase diagram, the eutectic liquid phase forms at 1465 C.[12] The liquid forms by eutectic reaction and subsequently wets the tungsten particles by spreading; it also dissolves some of the tungsten into the liquid phase.[13-15] The resultant microstructure, therefore, consists of nearly globular tungsten grains in the size range of 20 to 40 lm and a continuous Ni-based solid solution matrix phase dispersed throughout the alloy.[8,16-19] The mechanical behavior of WHAs
depends on the size, shape, and distribution of tungsten grains as well as the amount and extent of solid solution of the matrix phase.[2
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