The FCC to BCC phase transformation kinetics in an Al0.5CoCrFeNi high entropy alloy

Peng

et al. 2019

Metals

A novel graded material of a high-entropy alloy (HEA) FeCoCrNiMo was fabricated by spark plasma sintering (SPS) processing. After SPS, the HEA specimens consisted of a single face-centred cubic (FCC) phase in the center, but dual FCC and a tetragonal structure σ phase near the surface. Surprisingly, the sintering pressure was sufficient to influence the proportion of phases, and thus the properties of HEA samples. The hardness of the specimens sintered under the pressures of 30, 35, and 40 MPa increased gradually from 210 HV0.2, which is the single FCC phase in the center, to the maximum value near the surface as a result of the gradual increase in the fraction of the transformed σ phase. The σ phase, being a complex hard and brittle intermetallic particle to manipulate the properties of FCC-type HEA systems, which could be influenced by pressure, indicated a major possibility for designing gradient HEA materials.

Section: Phase Identificationcontrasting

confidence: 78%

Gradient Distribution of Microstructures and Mechanical Properties in a FeCoCrNiMo High-Entropy Alloy during Spark Plasma Sintering

Peng

et al. 2019

Metals

“…For CoCrFeMnNi, the oxide formed consists of Mn3O4 with Hausmannite phase which is in agreement with a previously obtained result for this alloy at 900 • C in air (Laplanche et al, 2016). For both CoCrFeNi and CoCrFeMnNi, the XRD patterns obtained after isothermal oxidation show one peak at ∼30 degrees, which appears to correspond to the ordered phase with B2 structure (Xia et al, 2015;Wang et al, 2017a), however, further experiments are needed in order to confirm this observation. product has not been observed in the experiments.…”

Section: Structures Of the Oxidessupporting

confidence: 90%

Nonlinear Oxidation Behavior in Pure Ni and Ni-Containing Entropic Alloys

et al. 2018

We performed a combined experimental and theoretical investigation of the oxidation behavior of pure Ni and of the following multi-component Ni-containing alloys with nearly equiatomic compositions: FeNi, CoFeNi, CoCrFeNi, and CoCrFeMnNi. The materials were exposed to air at ambient pressure and at a temperature of 800 • C for 150 min, their weight-gain due to oxidation was continuously monitored and the products of oxidation were subsequently characterized by XRD. The most common oxides formed have spinel or halite structure and the materials resistance to oxidation increases as: FeNi < CoFeNi < Ni < CoCrFeMnNi < CoCrFeNi. We found further that the oxidation-resistance of the materials does not correlate linearly with the number of elements present, instead the type of elements impacts significantly the materials susceptibility to oxidative damage. Cr is the element that imparted higher resistance to oxidation while Mn and Fe worsened the materials performance. In order to better understand the mechanisms of oxidation we employed thermodynamic equilibrium calculations and predicted the phase stability of oxides of the elements that are present in the materials, in different ranges of temperature, composition and oxygen activity. Additionally, we determined the phase compositions for the thermodynamically stable oxides at 800 • C. The results from the thermodynamic modeling are in good agreement with the experimental finds. The alloys with low resistance to oxidation such as CoFeNi and FeNi, form the Fe 3 O 4 spinel phase which tends to dominate the phase diagram for these materials. The presence of Cr increases the resistance to atomic rearrangement due to slow diffusion in the complex structure of Cr containing spinel phases. This causes the extremely high resistance to oxidation of the CoCrFeNi alloy. The presence of Mn in CoCrFeNi stabilizes the Mn 3 O 4 spinel, which reduces the oxidation-resistance of the alloys due to the high mobility of Mn.

“…The activation energy determined for FCC Al 0.25 CoCrFeNi high-entropy alloy is stable with the transformed volume fraction (198 ± 1 kJ/mol). There are just few results about the kinetic analysis of HEAs, e.g., Al 0.5 CoCrFeNi [24], and the activation energy for this alloy is quite different, as the value for the latter will strongly varies with the transformed volume (144-284 kJ/mol). The difference is due to the phase transition type is different, for the latter is FCC-BCC transition while in this study the transition is order-disorder transition.…”

Section: Discussionmentioning

confidence: 99%

“…In our previous research, the phase transformation kinetics of Al 0.5 CoCrFeNi HEA was inspected and reasonable heat treatment parameters were found [24]. Overall, kinetics analysis is absolutely important for high-entropy alloys to realize tuning microstructure and properties.…”

Section: Introductionmentioning

confidence: 96%

Phase Transformation Kinetics of a FCC Al0.25CoCrFeNi High-Entropy Alloy during Isochronal Heating

Wang

Chen

Yang

et al. 2018

Metals

Self Cite

The phase transformation kinetics of a face-centered-cubic (FCC) Al0.25CoCrFeNi high-entropy alloy during isochronal heating is investigated by thermal dilation experiment. The phase transformed volume fraction is determined from the thermal expansion curve, and results show that the phase transition is controlled by diffusion controlled nucleation-growth mechanism. The kinetic parameters, activation energy and kinetic exponent are determined based on Kissinger–Akahira–Sunose (KAS) and Johnson–Mehl–Avrami (JMA) method, respectively. The activation energy and kinetic exponent determined are almost constant, indicating a stable and slow speed of phase transition in the FCC Al0.25CoCrFeNi high-entropy alloy. During the main transformation process, the kinetic exponent shows that the phase transition is diffusion controlled process without nucleation during the transformation.