Investigation on the Thermodynamic Stability of Nanocrystalline W-Based Alloys: A Combined Theoretical and Experimental Approach.

Francesco Torre,Claudio Mingazzini,Teodor Huminiuc,Daniele Mirabile Gattia,Tomas Polcar,Antonio Mario Locci,Antonio Rinaldi,Francesco Delogu

doi:10.3390/ma14237179

Abstract

The stability of nanostructured metal alloys is currently being extensively investigated, and several mathematical models have been developed to describe the thermodynamics of these systems. However, model capability in terms of thermal stability predictions strongly relies on grain boundary-related parameters that are difficult to measure or estimate accurately. To overcome this limitation, a novel theoretical approach is proposed and adopted in this work to identify W-based nanocrystalline alloys which are potentially able to show thermodynamic stability. A comparison between model outcomes and experimental findings is reported for two selected alloys, namely W-Ag and W-Al. Experimental results clearly highlight that W-Ag mixtures retain a segregated structure on relatively coarse length scales even after prolonged mechanical treatments. Moreover, annealing at moderate temperatures readily induces demixing of the constituent elements. In contrast, homogeneous nanostructured W-Al solid solutions are obtained by ball milling of elemental powders. These alloys show enhanced thermal stability with respect to pure W even at high homologous temperatures. Experimental evidences agree with model predictions for both the investigated systems.

Highlights

Nanocrystalline (NC) metals exhibit enhanced physical and chemical properties with respect to their bulk counterparts [1,2,3,4,5]. This makes NC metals attractive in many areas of science and engineering. Their reduced grain size and the consequent highvolume fraction of grain boundaries (GBs) renders NC metals intrinsically unstable and prone to coarsening, such that the properties related to the NC status of the material are eventually lost during fabrication or in-service stages [6,7,8,9,10]
According to the theoretical procedure adopted in this work and described in Section 2.1, the coefficient β defined by Equation (1) is considered a model free parameter [46]. This allows to highlight the central role played by Ω( gb) in determining the thermodynamic character of polycrystalline materials stability while avoiding the effects of inaccuracy in the estimation of this parameter
A theoretical approach of general application is adopted in this work to ide nanocrystalline metal alloys potentially able to show thermodynamic stability. This stability against grain growth, phase separation or intermetallic phase formation ca According to Figure 9, the thermodynamically stable polycrystalline structure is ( g) composed of a grain interior and a GB phase with a composition of about xW = 0.83 and xW = 0.52, respectively

Summary

Introduction

Nanocrystalline (NC) metals exhibit enhanced physical and chemical properties with respect to their bulk counterparts [1,2,3,4,5] This makes NC metals attractive in many areas of science and engineering. In the light of the great promises shown by NC metallic materials in terms of possible applications, research on coarsening-resistant NC metal alloys has been increasingly pursued. This is since alloying has been proved to be the key to a significant improvement of thermal stability of these materials by increasing the temperature range where the as-produced nanostructure can be retained [11,12]. Binary [13,14,15,16,17,18,19,20,21,22,23], ternary [24,25,26,27,28,29], and higher order systems have been investigated [30,31,32,33,34]

Methods

Results

Conclusion