Abstract
The extremely stable phenomenon to crystallization was recognized for multicomponent Zr-65(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 (T-p1 and T-p2) 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 T-p1 and T-p2. Even after the long-time annealing for 3.6-14.4 ks at 750 K above T-p1, 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 T-p2 causes the homogeneous precipitation of a metastable big cubic Zr-2(ALTM,M) (TM = Fe, Co, Ni) with a size of 3-5 nm. Upon heating to the temperature above T-p2, the glass + Zr-2(ALTM,Pd) phases change to Al16TM11Zr7 + ZrAl2 + PdZr. The hardness is about 485 in as-spun state, increases at the temperatures above T-p1, shows a maximum (632) in the glass + metastable Zr-2(ALTM,Pd) phases just below T-p2 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 Zr-6(TM,Cu)Al-2 phase during the first exothermic reaction. The selection of PHE alloy composition is essential for the formation of the clustered glassy phase. (C) 2020 Elsevier B.V. All rights reserved.