Phase stability, local chemical disorder and its effect on the mechanical properties of refractory high entropy alloys
Equiatomic TaNbHfZr and TaNbHfZrTi high entropy alloys (HEAs) have been investigated for their phase stability, short range clustering (SRC) type chemical ordering, other structural phenomena and and the effects on strengthening. Ab initio calculations have been done to determine the thermodynamic stability parameters. The as cast structures do not contain any local chemical order, but the high resolution transmission electron microscopy (HRTEM) studies reveal their locally distorted lattice. The local lattice distortion of the average structure in their as cast and annealed state is quantified by refining atomic displacement parameters (ADPs) from single crystal X ray diffraction (XRD) and powder neutron diffraction (ND) studies. The static component of the ADPs appears to dominate over the thermal ADP, suggesting a high degree of local lattice distortion in the HEAs. The lattice distortions cause solid solution like strengthening, which increases the hardness of the HEAs by a factor of 2 3 compared to the rule of mixture. The SRCs that form in the annealed HEAs have a planar profile perpendicular to the directions. The SRCs mainly are co clustering of Zr and Hf. The SRCs have a higher atomic volume causing local tetragonal lattice distortions, which in terms produce diffuse streak like intensities in the XRD datasets and in the fast Fourier transform (FFT) of the HRTEM lattice images. The presence of SRCs locally reduces the lattice potential energy of the structure and puts additional energy barrier to the movement of dislocations. This increases the hardness causing around 50% of additional strengthening in the annealed HEAs. Thermodynamic analysis shows that the relaxed structure containing the SRCs lower the enthalpy and the configurational entropy.