Ravi Sankar Kottada

In the present study, influence of molybdenum and niobium additions on phase formation during mechanical alloying and spark plasma sintering of CoCrFeNi high entropy alloy was studied. Major FCC and minor BCC phase were observed after mechanical alloying of CoCrFeNi. However, major FCC and sigma phase were observed after spark plasma sintering. A maximum relative density of 95% was obtained with the hardness of 570 HV in CoCrFeNi HEA. The phase formation behavior was not significantly affected by the addition of molybdenum or niobium. However, addition of Mo to CoCrFeNi increased the hardness from 570 HV to 620 HV, and the hardness increased to 710 HV with combined addition of molybdenum and niobium. After sintering, major FCC phase with crystallite size of 60-70 nm was observed in all the compositions. Further, the microstructure and hardness retention was observed in CoCrFeNiMo0.2 with annealing temperature up to 800°C.

Evaluation of the diffusion coefficient of metal powders was attempted by using the power-law creep model in conjunction with the isothermal densification kinetics during spark plasma sintering (SPS). The diffusion coefficients obtained from the densification data of elemental Fe, Ni and Al powders are found to be higher than those reported in the literature. The higher values of diffusivity can be attributed to electric current effects. Our analysis demonstrates that it is possible to evaluate diffusion coefficients from experimental SPS densification data.

This study is focused on the evaluation of the thermal stability of a nanocrystalline Mg alloy, a relatively less explored topic. Guided by Darling et al. model which advises choosing of alloying elements based on their enthalpy of mixing and elastic enthalpy with respect to the parent element, Mo was chosen for stabilizing the nc Mg structure. High energy ball milling experiments were conducted on powders of Mg and Mo to achieve a composition of Mg–2at%Mo. Characterization of the ball milled powders using XRD and TEM indicated a lack of mixing of Mg and Mo and the microstructure was found to bear a mix of Mg and Mo phases. Kissinger's analysis using DSC yielded the activation energy of grain growth in the ball milled material as 73 kJ/mol which was similar to that of pure Mg, indicating that Mo did not alter the grain growth kinetics of Mg. The poor grain size stability of the nanocrystalline Mg–2Mo composition was also observed during spark plasma sintering studies conducted at 673, 723 and 773 K. The obtained results were evaluated in the light of the Murdoch and Schuh model and this revealed that the Mg–Mo alloy is not expected to be thermally stable due to the tendency of Mo to exist as a separate phase. This analysis appears to suggest that the Murdoch and Schuh model is more suitable for identifying thermally stable nanocrystalline compositions compared to the Darling et al. model.

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs.

The B2-Aluminide matrix (FeAl and NiAl) with in-situ Al2O3 reinforcement were synthesized using reactive milling. The oxides (Fe2O3 and NiO) were reduced by Al during high energy milling to form Al2O3. The 20 h ball milled powders were consolidated using spark plasma sintering (SPS). To understand the densification mechanisms during SPS, sintering was performed at various temperatures (750–850 °C) and applied pressures (25–100 MPa). The creep parameters are evaluated from the densification data obtained during SPS using the model proposed by Bernard and Granger. In addition, independent constant-stress compression creep studies were conducted on the dense SPS pellets. The creep studies were performed on the composites at 800 °C at different stresses (100–500 MPa). The densification studies and compression creep studies are correlated based on the creep parameters obtained from both these studies and corroborated by TEM studies of the crept samples. This correlation is found to be valid even for the in-situ composites. Thus, the analysis of densification data can be helpful in predicting the creep behavior and useful for designing the new creep resistant alloys or composites.

Joule heating as a primary heating source mechanism was probed during spark plasma sintering (SPS) of pure metal powders (Fe, Ni and Cu). Resistance to electric path was estimated from voltage-current measurements obtained online during these experiments. Resistance was observed to saturate at the same value irrespective of the type of metal powder, after attaining a sintering temperature of ∼0.3Tm. This saturation in resistance is attributed primarily to the Joule heating that occurs at the graphite-foil and punch in an SPS system.

Phase evolution and thermal stability of AlCoCrFe high entropy alloy with unsolicited carbon from the milling media were investigated. A single BCC phase was evolved during mechanical alloying, and BCC/B2 phases with Cr-rich M23C6 carbide was observed after spark plasma sintering. Unsolicited carbon during milling has led to the formation of M23C6 secondary phase after sintering. The alloy exhibits a very high hardness of 1050 ± 20 HV1 and excellent phase stability. The hardness reduction after annealing at 900 °C for 600 h is less than 10%, indicating excellent resistance to anneal softening.

Simultaneous spark plasma sintering (SPS) of metal powders (Fe and Ni) with their respective aluminide powders (FeAl and NiAl) was attempted to produce a cylindrical aluminide coating on a cylindrical metal core. The interdiffusion zone (IDZ) formed between the metal and the aluminide was analyzed to evaluate the diffusion coefficient. The composition variation and the actual temperature estimated at the interface were taken into consideration while evaluating the diffusion coefficient. The diffusion coefficients estimated based on the IDZ are found to be higher than those obtained by conventional methods but are in agreement with those obtained from previous SPS studies.

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.

The densification kinetics during spark plasma sintering (SPS) of FeAl and NiAl powders were analyzed using a model proposed by Bernard-Granger and Guizard [10]. Creep parameters obtained through densification data are in good agreement with those obtained from conventional creep experiments. Validity of the model was illustrated for aluminides in the form of deformation mechanism maps. This validation assures plausible confidence to predict creep behavior using densification data obtained during pressure assisted sintering of metallic alloys.