Strength enhancement and slip behaviour of high-entropy carbide grains during micro-compression

Tamás Csanádi,Ján Dusza,Michael J Reece,Elinor Castle

doi:10.1038/s41598-019-46614-w

Abstract

Bulk polycrystalline high-entropy carbides are a newly developed group of materials that increase the limited compositional space of ultra-high temperature ceramics, which can withstand extreme environments exceeding 2000 °C in oxidizing atmospheres. Since the deformability of grains plays an important role in macromechanical performance, in this work we studied the strength and slip behaviour of grains of a spark-plasma sintered (Hf-Ta-Zr-Nb)C high-entropy carbide in a specific orientation during micropillar compression. For comparison, identical measurements were carried out on the monocarbides HfC and TaC. It was revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of {boldsymbol{{}}{bf{110}}{boldsymbol{}}}{boldsymbol{langle }}{bf{1}}bar{{bf{1}}}{bf{0}}{boldsymbol{rangle }} slip systems. Additionally, it was concluded that the crystal orientation and stress conditions determine the operation of slip systems in mono- and high-entropy carbides at room temperature.

Highlights

The development of materials for engineering applications exceeding 2000 °C in oxidizing atmospheres, such as hypersonic vehicles and spacecraft, is a great challenge for material scientists
Effort should be devoted to understanding the deformation behaviour of high-entropy carbides, including the identification of the slip systems operating within grains at the micro-scale; which is rather limited in the base monocarbides, and can only be tested practically using indentation techniques
We look at the results obtained on the structure of (Hf-Ta-Zr-Nb)C at the macro, micro and atomic levels, which is necessary to understand the micropillar compression results described later; information on the structures of HfC and the least brittle monocarbide (TaC) can be found in the Supplementary data (Supplementary Fig. 1)

Summary

Introduction

The development of materials for engineering applications exceeding 2000 °C in oxidizing atmospheres, such as hypersonic vehicles and spacecraft, is a great challenge for material scientists. Ultra-high temperature ceramics (UHTCs) are the only, and limited, group of materials that can withstand such extreme environments They are based on the refractory borides, carbides and nitrides of the group of IV and V transition metals and are typically defined as having melting temperatures higher than 3000 °C1, with HfC exhibiting the highest melting point of all materials (4232 ± 84 K) known to man. A promising way to overcome this problem is the exploration of a new class of materials, motivated by the discovery of high-entropy alloys and pioneering work on entropy-stabilized oxides8 These so-called, bulk ‘high-entropy UHTCs’, which are composed of four or five or more different transition metal elements of equimolar proportions and boron or carbon atoms, form hexagonal or cubic solid solution structures, respectively. In addition to the chemical composition, the orientation of the grains and the testing method have a significant influence on slip operation as shown for TaC and ZrC during micropillar compression

Methods

Results

Conclusion