High-throughput additive manufacturing and characterization of refractory high entropy alloys

Highlights

• Rapid screening of HEAs/CCAs was demonstrated via additive manufacturing and high-throughput mechanical testing.

• A comprehensive analysis was conducted on an exemplar BCC RHEA/CCA system of interest for harsh operating environments.

• The hardness of the additively manufactured samples significantly increased with increasing configurational entropy.

• Strain rate sensitivity of the alloys aligned with the expectations for conventional BCC metals and alloys.

• Implications suggest that the high-throughput approach could be implemented to accelerate HEA/CCA discovery.

Abstract

Refractory High Entropy Alloys (RHEAs) and Refractory Complex Concentrated Alloys (RCCAs) are high-temperature structural alloys ideally suited for use in harsh environments. While these alloys have shown promising structural properties at high temperatures that exceed the practical limits of conventional alloys, such as Ni-based superalloys, exploration of the complex phase-space of these materials remains a significant challenge. We report on a high-throughput alloy processing and characterization methodology, leveraging laser-based metal additive manufacturing (AM) and mechanical testing techniques, to enable rapid exploration of RHEAs/RCCAs. We utilized in situ alloying and compositional grading, unique to AM processing, to rapidly-produce RHEAs/RCCAs using readily available and inexpensive commercial elemental powders. We demonstrate this approach with the MoNbTaW alloy system, as a model material known for having exceptionally high strength at elevated temperature when processed using conventional methods (e.g., casting). Microstructure analysis, chemical composition, and strain rate dependent hardness of AM-processed material are presented and discussed in the context of understanding the structure-properties relationships of RHEAs/RCCAs.

Introduction

Refractory High Entropy Alloys (RHEAs) and Refractory Complex Concentrated Alloys (RCCAs) are a promising class of high-temperature structural alloys originally developed with the intent of replacing conventional materials such as Ni-based superalloys, targeting harsh operating environments. Examples of applications where these alloys present an opportunity for significant improvements in performance and efficiency include aerospace propulsion systems, gas turbines, nuclear reactors, heat exchangers, and rocket engine nozzles [1]. Most literature on RHEAs/RCCAs explores the process-structure-properties relationships by extensively characterizing specific alloy compositions that are typically processed via conventional techniques, such as powder metallurgy, casting, or thin coatings [[1], [2], [3], [4]]. While conventional manufacturing techniques have proven useful to develop a growing library of properties for high entropy alloys (HEAs), RHEAs/HEAs present a substantial and unique challenge for rapidly assessing process-structure-properties relationships. Reports have suggested that many millions of unique alloy combinations are available and a shift in the development of RHEAs via high-throughput processing and characterization is imperative [5,6].

While rapid screening and production of alloys is far from a novel concept [[7], [8], [9]], there has been limited progress with high-throughput processing and characterization of RHEA/RCCAs. Only a few reports have focused on applying rapid screening techniques with a small subset of the expansive RHEA space via thin-film coatings [10], time-consuming multistage rapid alloy prototyping [11,12], in situ synthesis via diffusion couples [13], and laser-based additive manufacturing/cladding [14]. Additive manufacturing (AM) in particular provides unique opportunities for rapidly constructing bulk near-net-shape geometries, including compositionally graded ones, from difficult-to-process alloy compositions, such as those consisting of refractory elements (e.g., Ta, W, Mo, Nb, etc.). These benefits, which are not possible with other high-throughput processing techniques, represent a potential paradigm shift in the approach for rapidly screening alloys. A majority of the AM studies have focused on processing and characterization of single sets of transition metal HEAs [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]] and RHEAs [[31], [32], [33], [34], [35], [36], [37], [38]] without demonstrating the benefits of high-throughput AM. In this study, a Directed Energy Deposition (DED) process, Laser Engineered Net Shaping (LENS™) [39], was used as a high-throughput AM methodology to systematically and efficiently investigate the quaternary MoNbTaW RHEA system. This model material system was selected based on reports that show exceptional high-temperature mechanical properties, far exceeding those of Ni-based superalloys. It also serves as a useful means of validating the usefulness of the proposed approach by enabling evaluation of the impact that rapid layerwise solidification can have on a predominately single-phase body-centered cubic (BCC) RHEA.

Dobbelstein, et al., explored pulsed-laser DED processing of the MoNbTaW RHEA using blended elemental powders [32]. These authors identified a notable difference between the chemistry of bulk consolidated specimens and the blended powder feedstock [32]. Initially blended as an equimolar elemental blend, consolidated specimens exhibited notable compositional heterogeneity along the build axis, resulting in a non-equimolar RHEA composition with a significant enrichment above 25 at. % for the Nb and Ta species. This compositional heterogeneity occurred despite attempts to iteratively re-melt layers to promote homogeneous mixing of the blended elemental powders. The authors proposed that the chemical heterogeneity was the result of a higher probability of preferential melting of the lower melting temperature Nb species, exacerbated by differences in the constituent element melting points. The authors also processed an additional set of specimens with an adjusted powder blend used to achieve the equiatomic composition in bulk specimens by accounting for the observed Nb enrichment. The ability to achieve an equimolar/equiatomic composition was discussed in the context of two primary considerations: 1) developing wider melt pools by increasing power density and processing at higher temperatures, and 2) calibrating powder feedstock chemistry based on relative flowability, morphology, mass, and powder utilization efficiency.

Zhang, et al., published on the use of Laser Powder Bed Fusion (LPBF) with blended elemental powder feedstock [38,40]. Compositional characterization of bulk specimens also deviated from the initial powder feedstock chemistry. Interestingly, and in disagreement with Dobbelstein, et al. [32], consolidated specimens were depleted, rather than enriched, of the lower melting temperature species, having less than the expected 25 at. % concentrations of both Nb and Mo. The authors proposed that the deviation in composition might be the result of selective vaporization of the lower melting point elements that segregated to the upper surface of the melt pool due to density-driven fluid flow. These authors also characterized microstructure, mechanical properties (hardness), and corrosion resistance. The grain structure was significantly refined for as-built specimens relative to a conventionally cast coupons, with grain sizes of 13.4 μm and 200 μm, respectively. Vickers microhardness values varied from approximately 475–830 MPa for as-built specimens, and hardness was shown to increase with the number of deposited layers. Corrosion resistance was shown to be significantly higher for the RHEA relative to a benchmark stainless steel 316 L alloy, highlighting another important benefit of these alloys.

In this study, specimens from the MoNbTaW RHEA system were processed via AM and characterized in terms of microstructure, chemical composition, and strain rate dependent hardness. Structure-properties relationships of the RHEAs/RCCAs are discussed.