Effect of sintering temperature on microstructure and mechanical properties of 95W-HEA alloys

Abstract

The use of high entropy alloy as a binder for tungsten heavy alloys offers potential advantages. The 95W-5CoCrFeMnNi alloys (95W-HEAs) were prepared via powder metallurgy at sintering temperatures of 1400−1550 °C. The microstructure analysis revealed that the tungsten phase in 95W-HEAs exhibited a nearly spherical morphology in the HEA binder matrix and the formation of a Cr−Mn oxide mixed phase was observed. The sintering temperature exerted a significant influence on the relative density, grain size, W−W contiguity, and mechanical properties of the alloys. The optimal performance was achieved when sintering at 1450 °C, yielding a relative density of 96.61%, a W−W contiguity of 0.528, an average grain size of 18.97 µm, a compressive strength of 2234.82 MPa, and a hardness of HV 400.6. The activation energy for the diffusion of tungsten in the liquid phase formed by HEA binder was calculated to be 354.514 kJ/mol, highlighting its role in controlling grain growth.

1. Introduction

Tungsten heavy alloys (WHAs) are widely used in aerospace, military weapons, and medical devices due to their high density, superior strength and plasticity, excellent corrosion resistance, and good radiation shielding properties [1−3]. WHAs are typically fabricated by liquid-phase sintering (LPS) of tungsten with a blend of transition metals, resulting in tungsten grains with a body-centered cubic (BCC) structure dispersed in a face-centered cubic (FCC) binder network [4]. Conventional binders including Ni−Fe, Ni−Cu, Ni−Fe−Co, and Ni−Fe−Mo are commonly used in WHAs [5]. However, these binders exhibit limitations such as poor corrosion resistance and self-sharpening [6,7]. To solve these problems and enhance the overall performance of WHAs, alloying elements like Mn and Cr are often introduced. Nevertheless, this can lead to oxidation issues and render the sintering process control more intricate [8,9]. Therefore, the development of a novel binder phase for WHAs is imperative.

Recently, there has been a growing interest in high entropy alloys (HEA) due to their unique phenomena, such as high entropy effect, lattice distortion effect, sluggish diffusion effect, and cocktail effect [10]. As a result, they exhibit a good balance between strength and plasticity, high temperature stability, and thermal shock resistance, which have generated increasing scholarly attention and a constant expansion of their application fields [11]. Currently, HEA has been gradually applied as a binder or reinforcement phase in copper-based, magnesium-based, WC, and other materials [12]. And, they also show promise as potential binder materials for WHAs.

JIAN et al [13] and ZHOU et al [14] developed an Al0.5Cr0.9FeNi2.5V0.2 HEA as a binder phase for WHAs. They investigated the wettability between two different phases and the effect of HEA on tungsten grain growth behavior. The system exhibited a low contact angle during liquid-phase sintering, and tungsten had a high solubility in HEA, which was beneficial for the densification process in LPS and contributed to high HEA/W interface bonding strength. Due to the low diffusion performance of high entropy alloys, the calculated grain growth constant was significantly lower than that of traditional binder phases, and an ultimate tensile strength up to 1267.3 MPa can be achieved when sintered at 1500 °C. A series of subsequent treatments such as solid solution strengthening, forging, and aging performed on sintered W-HEA alloys resulted in the formation of unique precipitates within the alloy system. These microstructural modifications improved the dynamic yield strength of the alloy and generated a narrow adiab...