Novel multicomponent B2-ordered aluminides: Compositional design, synthesis, characterization, and thermal stability

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h. © 2020 by the author. Licensee MDPI, Basel, Switzerland

Suppression of σ-phase in nanocrystalline CoCrFeMnNiV high entropy alloy by unsolicited contamination during mechanical alloying and spark plasma sintering

CoCrFeMnNiV high entropy alloy (HEA) exhibits a high content of σ-phase (70 vol%) when produced by casting route. In the present work, a combination of mechanical alloying (MA) and spark plasma sintering (SPS) has been used to synthesize nanocrystalline CoCrFeMnNiV HEA where the formation of σ-phase has been avoided. Electron microscopy and atom probe tomography analysis indicated the formation of FCC structured HEA matrix along with (Cr,V) carbide (15 vol%) precipitation, without the presence of σ-phase in SPS processed alloy. Gibbs energy vs composition (G-x) diagrams of binary subsystems and possible carbides and oxides substantiate the absence of σ-phase during SPS of CoCrFeMnNiV alloy. Thus, the unsolicited contamination during MA-SPS route proves beneficial in suppressing the complex phase formation. © 2020 Elsevier B.V

Novel Multicomponent B2-Ordered Aluminides: Compositional Design, Synthesis, Characterization, and Thermal Stability

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h. © 2020 by the author. Licensee MDPI, Basel, Switzerland

Challenges in design and development of high entropy alloys: A thermodynamic and kinetic perspective

High entropy alloys (HEAs) have brought a renewed interest to thermodynamics and kinetics. Proper understanding and treatment of relevant quantities are important for the effective theoretical design framework. Since the ideal mixing treatment is useful, its validity is evaluated through checking the Lupis and Elliot effect on extensive binary data and temperature dependency effect on limited HEA data. The physical basis of different configurational entropy contributions and mismatch entropy are elucidated. New alloy possibilities such as immiscible HEAs are revealed. Influence of elemental atmospheres and crystalline nature of matrices on diffusion in HEAs are discussed. © 202

Studies on Kinetics of BCC to FCC Phase Transformation in AlCoCrFeNi Equiatomic High Entropy Alloy

Kinetics of face-centered cubic (FCC) phase evolution in equiatomic AlCoCrFeNi alloy has been studied in this work, measuring the phase fraction from X-ray diffraction (XRD). As-cast alloy showed a body-centered cubic (BCC)+B2 structure. Heat treatments performed at different temperatures showed the formation up-to 30 to 35 pct FCC phase between 1073 K and 1373 K. A systematic decrease in hardness from 540 to 390 HV10 with increase in temperature suggested an increase in the FCC volume fraction. Kinetics of FCC evolution were analyzed using the Johnson–Mehl–Avrami–Kolmogorov equation and Arrhenius law to calculate the activation energy for the phase transformation. Furthermore, a time-temperature-transformation diagram was constructed from the isothermal transformation studies. Detailed microstructural investigation suggests faster kinetics of FCC phase formation near dendritic boundaries compared to interdendritic regions. The Kurdjumov–Sachs orientation relationship between FCC and BCC phases suggested a coherent interface between these phases. Results of the present study pave the way to decide on heat treatment practices in AlCoCrFeNi alloy. © 2021, The Minerals, Metals & Materials Society and ASM International

Kinetics and phase formation during crystallization of Hf64Cu18Ni18 amorphous alloy

In this work, crystallization kinetics and phase formation of the Hf64Cu18Ni18 glassy ribbon were studied. It was observed that the ribbon crystallizes via two exothermic peaks. Further, the activation energy (E a) corresponding to each peak was calculated by Kissinger and Augis–Bennett methods. These methods showed nearly similar values of E a. The crystallization product of ribbon was identified by X-ray diffraction and discussed in view of prediction using the CALculation of PHAse Diagram (CALPHAD) approach. The identified phases indicate that the HfNi and Hf2Cu, Hf2Ni are formed after the first and second exothermic crystallization peaks, respectively. Further, the Avrami exponent (n) was calculated as 1.2 and 1.4 for the first and second crystallization peaks, respectively. Avrami exponent values show that crystallization takes place at non-random nucleation sites with either one or two-dimensional growth. © 2021 Informa UK Limited, trading as Taylor & Francis Group