In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Budaraju Srinivasa Murty

Department of Metallurgical and Materials Engineering

Rahul John

Anirudha Karati

Mohan Muralikrishna Garlapati

Mayur Vaidya

Rahul Bhattacharya

Aluminum alloys

Annealing

Cobalt alloys

entropy

HIGH-ENTROPY ALLOYS

Iron alloys

Mechanical Alloying

Nanocrystals

Spark plasma sintering

ALLOY FORMATION

CONTAMINATION LEVELS

FCC phase

MILLING TIME

Nano-structured

Nanocrystallines

PHASE EVOLUTIONS

Nanocrystalline alloys

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Materials Science

High entropy alloys (HEAs) have emerged as promising class of materials having equiatomic or near equiatomic multicomponent configurations. Nanostructured HEAs have added a new facet to the development of these alloys, showing remarkable strength and functional properties. The present work examined the phase evolution and thermal stability of nanocrystalline CoCrFeNi and CoCrFeMnNi HEAs prepared through mechanical alloying (MA) followed by spark plasma sintering (SPS). After MA, both the alloys showed single phase FCC structure with minor fractions of tungsten carbide arising due to contamination from milling media. After SPS, the major phase remained as FCC in both the alloys along with Cr7C3 evolution. Phase stability of CoCrFeNi and CoCrFeMnNi HEAs, (MA powders and SPS pellets) were investigated in the temperature range 1073–1373 K up to 96 h. Formation of Cr7C3, concomitant with Cr-depletion in FCC matrix, was observed on heat treatment of MA powders. SPS alloys retained their mixture of FCC + Cr7C3 on thermal exposure with minimal change in the fraction of carbides. The presence of single phase field in their respective phase diagrams, similar atomic sizes of constituents and non-equilibrium nature of MA combined to lend a highly stable FCC phase in the nanocrystalline HEAs studied. © 2018 Elsevier B.V.

Journal of Alloys and Compounds

Phase evolution and stability of nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

Mechanically activated annealing was employed successfully for the synthesis of nanocrystalline half-Heusler (TiNiSn) alloy for the first time. TiNiSn could be synthesized by annealing, after 5 h of mechanical alloying of equiatomic elemental blend, at a temperature as low as 600 °C in 2 h. The nanocrystalline nature and resultant increased surface area coupled with large number of defects induced by mechanical alloying has resulted in a significant reduction in temperature and time of annealing in comparison to earlier reports (800 °C–400 h). This could lead to energy efficient low temperature synthesis of nanocrystalline half-Heusler alloys for thermoelectric and magnetic applications. © 2017 Elsevier B.V.

Materials Letters

Synthesis of nanocrystalline half-Heusler TiNiSn by mechanically activated annealing

The conventional way to form a multicomponent alloy requires mixing individual elements in a single step. Phase formation, therefore, is governed by inherent thermodynamic and kinetic factors of the system. We propose here a new approach for multicomponent alloy synthesis, which involves step by step addition of constituent elements. In the present work, this is illustrated through the formation of nanocrystalline equiatomic AlCoCrFeNi by mechanical alloying. For example, first, binary CoNi is formed by milling elemental Co and Ni powders. In the subsequent steps, Fe, Cr and Al are added to form ternary (CoNiFe), quaternary (CoNiFeCr) and quinary (CoNiFeCrAl) alloy, respectively. Three different classes of binaries have been selected as initial phases, namely, B2 (AlNi, AlCo and AlFe), BCC (FeCr) and FCC (CoNi and FeNi). Remaining constituent elements are added step-wise in varying sequences and equiatomic AlCoCrFeNi is obtained in the concluding step. The final AlCoCrFeNi alloy at the end of each sequence has varied fractions of BCC and FCC phases. For instance, AlNi + Co + Fe + Cr sequence is single phase BCC, whereas the sequence FeNi + Co + Cr + Al results in a mixture of BCC (75%) and FCC (25%) phases. The extent to which a particular element stabilizes a structure (BCC/B2/FCC) has also been elucidated. © 2017 Elsevier Ltd

Materials and Design

Influence of sequence of elemental addition on phase evolution in nanocrystalline AlCoCrFeNi: Novel approach to alloy synthesis using mechanical alloying

Nanocrystalline B2 aluminide (FeAl and NiAl) powders were synthesized using high energy ball milling and were consolidated using three different sintering methods: conventional pressureless sintering, microwave sintering and spark plasma sintering. The particle-particle contacts after sintering were found to have compositional gradient from interior to surface leading to formation of an Al-O rich phase. This Al-O rich phase formation is primarily attributed to oxygen pick-up during handling of powders and during sintering, leading to formation of amorphous alumina. The formation of amorphous phase is explained by examining the defect concentrations and their interactions. The composite developed with amorphous alumina as a second phase in an ordered B2 matrix can be of great interest for potential structural applications. © 2016 Elsevier Inc.

Materials Characterization

Formation of amorphous alumina during sintering of nanocrystalline B2 aluminides

Nanocrystalline CoCrFeNi high entropy alloy, synthesized by mechanical alloying followed by spark plasma sintering, demonstrated extremely sluggish grain growth even at very high homologous temperature of 0.68 Tm (900 °C) for annealing duration of 600 h. Mechanically alloyed powder had carbon and oxygen as impurities, which in turn led to the formation of two-phase mixture of FCC and Cr-rich carbide with fine distribution of Cr-rich oxide during spark plasma sintering. Sluggish grain growth is attributed to the Zener pinning effect from the fine dispersion of oxide, mutual retardation of grain boundaries in the presence of two phases, and sluggish diffusivity because of cooperative diffusion of multi-principle elements. © 2015 Elsevier B.V. All rights reserved.

Exceptional resistance to grain growth in nanocrystalline CoCrFeNi high entropy alloy at high homologous temperatures

Influence of mechanically activated annealing on phase evolution in Al0.3CoCrFeNi high-entropy alloy

Journal	Data powered by TypesetJournal of Materials Science
Publisher	Data powered by TypesetSpringer New York LLC
ISSN	00222461
Open Access	Yes