Effect of Cold Rolling on the Phase Transformation Kinetics of an Al0.5CoCrFeNi High-Entropy Alloy

Abstract

The solid state phase transformation kinetics of as-cast and cold rolling deformed Al 0.5 CoCrFeNi high-entropy alloys have been investigated by the thermal expansion method. The phase transformed volume fractions are determined from the thermal expansion curve using the lever rule method, and the deformed sample exhibits a much higher transformation rate. Two kinetic parameters, activation energy (E) and kinetic exponent (n) are determined using Kissinger-Akahira-Sunose (KAS) and Johnson-Mehl-Avrami (JMA) method, respectively. Results show that a pre-deformed sample shows a much lower activation energy and higher kinetic exponent compared with the as-cast sample, which are interpreted based on the deformation induced defects that can promote the nucleation and growth process during phase transformation.(FCC + BCC) Al 0.5 CoCrFeNi HEA with a balanced strength and plasticity are chosen to investigate the effect of cold rolling on the subsequent solid state phase transformation kinetics. Experimental ProceduresThe ingots with a nominal composition of Al 0.5 CoCrFeNi were prepared by melting pure elements (at least 99.95 wt.%) in a vacuum induction-melting furnace. The furnace is firstly heated to 500 • C and held for 2 h to remove the water vapor, and vacuumed to below 10 Pa. Then pure argon is backfilled to expel the rest of air until the vacuum goes back to standard atmospheric pressure. This process is repeated three times in order to gain an oxygen-free environment as much as possible. Afterwards, melting and casting is performed with the protection of high purity argon and the alloy is heated at about 1550 • C for 15 min. Approximately 15 kg ingot is produced by casting the melt into a steel crucible with a height of 180 mm, upper inner diameter of 140 mm and bottom inner diameter of 130 mm.Samples with the size of 8 × 20 × 70 mm 3 are taken from the center of the ingot. Then, the sample is cold rolled to a thickness reduction of 20%. Then, samples were machined to φ 6 × 25 mm for thermal expansion measurement both from as-cast and cold rolled plates. The thermal expansion curves were tested using a Netzsch ® DIL-402C dilatometer (Selb, Germany) under the protection of argon with constant heating rate of 4 K/min, 6 K/min, 8 K/min and 10 K/min.The microstructure and phases were characterized by scanning electron microscope (SEM, TESCAN MIRA3 XMU (Brno, Czech Republic), and the working distance and the energy of beam used are 15 mm and 20 kV, respectively) and X-ray diffractometer (XRD, DX 2700 (Dandong, China), and the voltage and current used during measurement are 40 kV and 30 mA, respectively), respectively. Uniaxial tensile tests are carried out with a MTS SANS CMT5105 (Shenzhen, China) mechanical tester at the strain rate of 10 −3 s −1 and the specimens are prepared along the rolling direction (RD) direction. Results and Discussion MicrostructureThe microstructure and XRD patterns of Al 0.5 CoCrFeNi high-entropy alloys at as-cast and 20% cold rolled (CR) condition are shown in Figures 1 and 2. Except for t...

Effect of Cold Rolling Reduction Rate on the Microstructure and Properties of Q&P Steel with a Ferrite-Pearlite Initial Structure

“…Figure 8 depicts the thermal expansion curve, which provides insights into the phase transition process occurring during Q&P. As the CRRR increases, the temperature of Ac1, representing the start of ferrite-to-austenite transformation, decreases continuously. This phenomenon can be attributed to the increased driving force for phase transformation due to the introduction of deformation energy storage through cold rolling [ 29 ]. The reduction in Ac1 temperature indicates a higher tendency for austenite formation with increasing CRRRs.…”

“…Figure 9 depicts the microstructure at 830 °C/100 s, revealing a decrease in the size of austenite grains with increasing CRRRs. This observation can be attributed to the higher number of nucleation points present in the cold-rolled samples during the austenite transformation process [ 29 , 32 ]. As a result, the size of the austenite grains at high temperatures is reduced.…”

Thermal–Mechanical Processing and Strengthen in AlxCoCrFeNi High-Entropy Alloys

“…Figure 4 exhibits the microstructures of ultrafine grains and/or nanostructured precipitates generated simply through cold deformation and annealing under different conditions (Guo et al, 2018;Gwalani et al, 2018;Shi et al, 2018;Hou et al, 2019;Wu et al, 2019;He et al, 2020). In the meantime, Wang et al (2018c) investigated the effect of cold rolling on the fcc to bcc transformation kinetics of the Al 0.5 CoCrFeNi HEA and concluded that pre-deformation induced defects could facilitate the nucleation and growth process during the phase transition compared with the as-cast sample.…”

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“…It caused grain refinement as well as hardness and electrical conductivity increases (up to 20% and 14%, respectively). Furthermore, the solid-state phase transformation kinetics of as-cast and cold rolling deformed Al 0.5 CoCrFeNi HEAs have been investigated by Wang et al [ 32 ] using the thermal expansion method. Lightweight AlCrMoTi and AlCrMoTiV HEAs were designed by Kang et al [ 33 ] and they were confirmed as solid solutions with minor ordered B2 phases.…”