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.

     

Decomposition modes of austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels

The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M₃C or M₂₃C₆ precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M₂₃C₆ and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.