Microstructure and mechanical properties of sintered and heat-treated HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) have attracted researchers' interest in recent years. The aim of this work was to prepare the HfNbTaTiZr high entropy alloy via the powder metallurgy process and characterize its properties. The powder metallurgy process is a prospective solution for the synthesis of various alloys and has several advantages over arc melting (e.g., no dendritic structure, near net-shape, etc.). Cold isostatic pressing of blended elemental powders and subsequent sintering at 1400 degrees C for various time periods up to 64 h was used. Certain residual porosity, as well as bcc2 (Nb- and Ta-rich) and hcp (Zr- and Hf-rich) phases, remained in the bcc microstructure after sintering. The bcc2 phase was completely eliminated during annealing (1200 degrees C/1h) and subsequent water quenching. The hardness values of the sintered specimens ranged from 300 to 400 HV10. The grain coarsening during sintering was significantly limited and the maximum average grain diameter after 64 h of sintering was approximately 60 mu m. The compression strength at 800 degrees C was 370 MPa and decreased to 47 MPa at 1200 degrees C. Porosity can be removed during the hot deformation process, leading to an increase in hardness to similar to 450 HV10.Web of Science912art. no. 132

Nanomechanical characterization of high pressure torsion processed HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) are a new material class in which the configurational entropy of a multicomponent solid solution phase is maximized so that the entropy of mixing stabilizes disordered solid solution phases against the possible intermetallic phases development. Generally, to achieve high entropy of mixing, the alloys contain typically five or more major elements in equimolar concentrations. The composition of HEAs is generally based on 3d transition metals, refractory metals, light metals, lanthanide transition metals, precious metals, brasses and bronzes. The HEAs exhibit promising structural and mechanical properties in wide range of applications. Mechanical properties of such alloys can further be improved by grain refinement especially by severe plastic deformation. However, studies of ultrafine grained HEAs are rather scarce in the literature. An increase of strength with decreasing grain size was achieved in the probably most investigated HEA i.e. Cantor alloy (equiatomic CoCrFeMnNi with fcc structure) processed by high pressure torsion (HPT) [1]. Much less attempts were made to process in such a way HEAs with bcc structure. Recently HfNbTaTiZr bcc HEA was successfully nanostructured by HPT straining [2]. It was reported that grain refinement by HPT resulted in a significant enhancement of the strength of this bcc HEA, keeping excellent ductility during room temperature straining. Nevertheless, there is still a lack of information about the development of microstructure and physical properties of this refractory metal HEA subjected to severe plastic deformation processing. Recent investigations [3] revealed that thermodynamically stable system of HfNbTaTiZr alloy at room temperature is a mechanical mixture of Zr, Hf rich hcp phase and Ta, Nb rich bcc phase. The decomposition of the solid solution after long-term annealing obviously leads to the deterioration of mechanical properties (loss of ductility and decrease of strength). The difference in hardness of both phases is relatively small and both are softer than the random solid solution. On the other hand, considerable contribution to the solid solution strengthening can arise from atomic size misfit (phase separation on the nano-meter scale) which is provoked by the high density of vacancies introduced by HPT. This work thus aims on the relationship between phase (de)composition, microstructure, lattice defects, and length-scale-dependent material response of HfNbTaTiZr HEA after different thermal treatment and HPT straining. The microstructure and phase composition evolution were characterized by the electron microscopy and X-ray diffraction. The length-scale-dependent material response was characterized by indentation at various indentation depths. The contributions of different hardening mechanisms were separated and attributed to distance between dislocation pinning defects so that the differences between thermal treatment (diffusion) and HPT (straining) ‑induced hardening could be explained. Acknowledgement: This research was carried out in frame of the project CZ.02.1.01/0.0/0.0/15_003/0000485 (European Regional Development Fund). [1] A. Heczel et al. Defect structure and hardness in nanocrystalline CoCrFeMnNi high-entropy alloy processed by high-pressure torsion, J. Alloy. Comp. 711 (2017) 143-154. [2] J. Čížek et al. Strength enhancement of high entropy alloy HfNbTaTiZr by severe plastic deformation J. Alloy. Comp. 768 (2018) 924-937. [3] B. Schuh et al. Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties, Acta Mater. 142 (2018) 201-212

INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density

The effect of processing route on properties of HfNbTaTiZr high entropy alloy

High entropy alloys (HEA) have been one of the most attractive groups of materials for researchers in the last several years. Since HEAs are potential candidates for many (e.g., refractory, cryogenic, medical) applications, their properties are studied intensively. The most frequent method of HEA synthesis is arc or induction melting. Powder metallurgy is a perspective technique of alloy synthesis and therefore in this work the possibilities of synthesis of HfNbTaTiZr HEA from powders were studied. Blended elemental powders were sintered, hot isostatically pressed, and subsequently swaged using a special technique of swaging where the sample is enveloped by a titanium alloy. This method does not result in a full density alloy due to cracking during swaging. Spark plasma sintering (SPS) of mechanically alloyed powders resulted in a fully dense but brittle specimen. The most promising result was obtained by SPS treatment of gas atomized powder with low oxygen content. The microstructure of HfNbTaTiZr specimen prepared this way can be refined by high pressure torsion deformation resulting in a high hardness of 410 HV10 and very fine microstructure with grain size well below 500 nm.Web of Science1223art. no. 402

Alloy design for intrinsically ductile refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials. However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5 Nb 0.5 Ta 0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries

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Mechanical properties and microstructure of large IN713LC nickel superalloy castings

When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test bodies often exhibit lower values than required, in particular ultimate tensile strength and elongation. The presence of fragile niobium carbides in the structure of the alloy was identified as the main cause of this situation, which originated as a result of the slow speed cooling. Because of large grain size compared to gauge length diameter, crystal orientation of individual grains also plays an important role. Interdendritic carbides are oriented parallel with dendrites thus parallel with directions in Ni superalloy crystal lattice. Grain oriented with axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis. In that position carbides crack easily at low grain deformation. Longitudinal metallographic cuts of selected tested specimen gauge lengths were prepared and grain orientation of individual grains was investigated by EBSD, Specimens, where grain oriented with axis parallel to tensile direction were found, performed poor elongation values 3%, compared to 7% where no such oriented grains were found