Phase decomposition and mechanical properties of pseudo-high entropy Zr65(Al,Fe,Co,Ni,M)35 (M=Cu, Ag or Pd) glassy alloys

Abstract

The extremely stable phenomenon to crystallization was recognized for multicomponent Zr65(Al,Fe,Co,Ni,M)35 (M = Ag or Pd) glassy alloys which are defined as a pseudo-high entropy (PHE) type with the features of positive or nearly zero heat of mixing and large atomic size mismatches among solute elements. The PHE glassy alloys exhibit two exothermic peaks (Tp1 and Tp2) on the DSC curve where their peak temperatures are 719 and 944 K for M = Ag and 727 and 929 K for M = Pd, respectively, showing a large temperature interval between Tp1 and Tp2. Even after the long-time annealing for 3.6–14.4 ks at 750 K above Tp1, no appreciable change was recognized in the X-ray diffraction patterns as well as the TEM and HRTEM images. The further increase in annealing temperature for 3.6 ks to 850 K near Tp2 causes the homogeneous precipitation of a metastable big cubic Zr2(Al,TM,M) (TM = Fe, Co, Ni) with a size of 3–5 nm. Upon heating to the temperature above Tp2, the glass + Zr2(Al,TM,Pd) phases change to Al16TM11Zr7 + ZrAl2 + PdZr. The hardness is about 485 in as-spun state, increases at the temperatures above Tp1, shows a maximum (632) in the glass + metastable Zr2(Al,TM,Pd) phases just below Tp2 and then decreases to 568 in Al16TM11Zr7 + ZrAl2 + PdZr phases. The high resistance to crystallization is due to the sluggish growth of crystalline nuclei resulting from the low atomic diffusivity and the necessity of long-range atomic rearrangements which are features for PHE glassy alloys. The Cu-containing glassy alloy does not belong to the PHE alloy and crystallizes easily to Zr6(TM,Cu)Al2 phase during the first exothermic reaction. The selection of PHE alloy composition is essential for the formation of the clustered glassy phase.