Enhanced strength and ductility in a highentropy alloy via ordered oxygen complexes

first_img Deformation mode. a) STEM image of the TiZrHfNb base HEA at 8% tensile strain (the yellow arrows indicate the coplanar dislocation arrays). b) STEM image of O-2 HEA at 8% tensile strain (the red arrows indicate the dipolar walls). c) STEM image of N-2 HEA at 8% tensile strain (the yellow arrows indicate the planar slip bands). Typical planar slip is observed in the base HEA and in the nitrogen-doped alloy variant N-2 HEA. However, wavy slip dominates deformation of the oxygen doped variant O-2 HEA, suggesting that oxygen addition leads to a plastic deformation mode dominated by wavy slip. The beam direction in a and c is [011] while that in b is [001]. d) Dislocation spacing of the TiZrHfNb base HEA and of the interstitially doped variants O-2 and N-2 HEAs probed during in situ TEM tensile experiments. The white arrows represent the dislocation spacing. The average dislocation spacing in the O-2 HEA is much smaller than that in the base HEA and in the N-2 HEA. The error bars are standard deviations of the mean. Credit: Nature, doi: 10.1038/s41586-018-0685-y Journal information: Nature To understand the underlying mechanism of this anomalous, interstitial solid-solution strengthening effect observed with oxygen doped materials, nanostructures of materials were investigated at the atomic scale. For this, the scientists first used synchrotron high-energy X-ray diffraction (XRD) patterns of the base HEA compared with the two alloy variants of O-2 and N-2 HEA. The results showed that addition of either nitrogen or oxygen to the base HEA did not change its single-phase body-centered cubic (b.c.c) structure. This observation was confirmed by electron back-scattering diffraction mapping. In the scanning transmission electron microscope high-angle annular dark field micrograph (STEM-HAADF) images of the O-2 HEA; light atoms were represented in dark contrast, while heavy atoms were imaged bright. Mechanical properties. a) Room-temperature tensile stress–strain curves for the as-cast TiZrHfNb (denoted as base alloy), (TiZrHfNb)98O2 (denoted as O-2) and (TiZrHfNb)98N2 (denoted as N-2) HEAs. σy is the yield strength (squares), σUTS is the ultimate strength (diamonds) and ε is the elongation (circles). The inset shows the corresponding strain hardening response (dσ/dε). A higher work-hardening rate is observed for the O-2 HEA variant (TiZrHfNb)98O2 compared to the base HEA TiZrHfNb and the N-2 HEA (TiZrHfNb)98N2. b) Changes in strength and ductility observed for the HEAs introduced here, relative to several types of established high-performance alloys. The reference systems are Ti6Al4V, β-Ti alloys, niobium, vanadium, interstitial free steel and 316 austenitic stainless steels. The alloys’ interstitial oxygen or nitrogen content is indicated for comparison. Credit: Nature, doi: https://doi.org/10.1038/s41586-018-0685-y Schematic diagram illustrating the mechanism of plastic deformation in the cubic structure of the oxygen-rich high-entropy alloy (HEA). a) In the oxygen-high entropy alloys, the ordered oxygen complexes (OOCs) acted on dislocations in mechanical strain studies. b) During the initial stages of plastic deformation, the planar slip still prevailed. c) Once the dislocations encountered severely distorted interstitial-enriched OOCs, cross-slip is promoted due to their strong pinning effects. d) This results in massive dislocation multiplications. e) More and more dislocations are pinned by OOCs, and dipolar walls emerged as the strain increased to promote work hardening of the material, eventually leading to higher ductility. Credit: Nature, doi: https://doi.org/10.1038/s41586-018-0685-y Researchers present new strategy for extending ductility in a single-phase alloy Engineering strong, tough (damage-tolerant) materials traditionally requires striking a compromise between hardness and ductility. In the new study, oxygen complexes were structurally ordered in nanoscale regions within the HEA characterized by oxygen, zirconium and titanium (O, Zr, Ti)-rich atomic complexes. Formation of these complexes was promoted by chemical short-range ordering among matrix elements within the HEAs. In face-centered cubic HEAs, carbon was reported to improve strength and ductility by lowering stacking fault energy and increasing lattice friction stress. By contrast, ordered interstitial complexes described by Lei et al. mediated a strain-harvesting mechanism with potential for specific use in Ti, Zr, and Hf (Hafnium)-containing alloys. Interstitial elements are usually highly undesirable in such metal alloys due to their embrittlement effects and since tuning the stacking fault energy and exploitation of thermal transfer had not previously led to property enhancement in other alloys. The novel study results therefore provided insight to the role of interstitial solid solutions and the associated mechanisms of strengthening metallic materials. The work is now published in Nature. Explore further Citation: Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes (2018, November 27) retrieved 18 August 2019 from https://phys.org/news/2018-11-strength-ductility-high-entropy-alloy-oxygen.html Oxygen is an abundant element that can form undesired impurities or ceramic phases in metallic materials, while doping the element on metal can render substrates brittle. During interactions with metallic alloys, oxygen takes a state between oxide particles and frequently occurring random interstitials to form ordered oxygen complexes. In a new study, materials scientists Zhinfeng Lei and co-workers observed that unlike in traditional interstitial strengthening, such ordered interstitial complexes could form high-entropy alloys (HEAs) with unprecedented enhancement in strength and ductility in compositionally complex solid solutions. When the scientists doped a model TiZrHfNb HEA with 2.0 atomic percent (2 at%) oxygen, they observed substantially enhanced tensile strength and ductility, breaking a longstanding conflict on strength and ductility trade-off. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.last_img read more