The Crystallization Microstructures Of Zr₅₆Al₁₆Co₂₈ And Zr₇₀Al_12.5Fe_17.5 Alloy Melts And Supercooled Liquids

Zr-based bulk metallic glasses（BMGs）are promising high-performance structural and biomedical materials.Large GFA and high thermal stability are necessarily needed for their pratical applications.The Zr-based BMGs with exceptionally large GFA are multi-component alloys,which can be considered as the alloying modifications of the basics ternary systems of Zr-Al-TM（TM=Fe、Co、Ni、Cu,etc.）.Among the ternary alloy systems,Zr-Al-Fe and Zr-Al-Co are like systems showing similar characteristics in thermodynamics and phase diagram features,alloying chemistry and electronic structure factors as well,the GFA and the critical glass formation size associated with the two systems are significantly different,reported to be （?）1.5 mm for Zr₇₀Al_12.5Fe_17.5 and （?）18 mm for Zr₅₆Al₁₆Co₂₈.Their supercooled liquids are stable within a large temperature span when continuously heated at a certain rate.This paper studied the solidification microstructures formed during the liquid cooling down path,and the crystallization microstructures of metallic glasses during the reheating process for the BMG-forming alloys of Zr₇₀Al_12.5Fe_17.5 and Zr₅₆Al₁₆Co₂₈.X-ray diffraction（XRD）,Transmission Electron Microscopy（TEM）,and Electron Probe Microanalysis（EPMA）are used to examine the constituent phases and microstructure characteristic of the samples prepared by cooling the bulk liquids and reheating the BMGs of the two alloys.The experimental evidence would be useful for the understanding of the large GFA difference and thermal stability of the two systems.The results obtained in our experimens are presented in the following:（1）Solidfication microstructures of bulk alloy meltsThree distinct layers are seen to form in both the 5 g and 10 g Zr₅₆Al₁₆Co₂₈ alloy ingots.The portion of the ingot away from the copper hearth in the 5 g ingot is fully amorphous,while the contacted layer is crystallized,forming a spinodal-like microstructure.The unique microstructureis composed of hP²-Al₂CoZr₆（a=0.71 nm,c=0.71 nm）and cP-ZrCo（a=0.33 nm）phases which are coherently existed with the crystallographic orientation relationship of（300）_Al2CoZr6//（110）ZrCo,[（?）]_Al2CoZr6//[001]ZrCo,and with a length of 50-100nm in thickness.The portion of the ingot away from the copper hearth in the 10 g ingot is slightly crystalized,and a small amount of nanocrystals are formed on the amorphous matrix.The crystallization products are hP²-Al₂CoZr₆ and cP-ZrCo.The bottom part contacting the copper hearth of the sample is also fully crystalized.The 5 g ingot of the Zr₇₀Al_12.5Fe_17.5 alloy is fully crystallized,exhibiting grain boundaries and cotton-like constitutes in the interior of the grains.The crystalline phases are hP-Al₂FeZr₆（a=0.71 nm,c=0.35 nm）and tI-Zr₂Fe（a=0.64 nm,c=0.56 nm）.（2）Crystallization microstructures upon BMG reheatingAt various heating rates,the Zr₅₆Al₁₆Co₂₈ BMG exhibits two well separated exothermic peaks.It is found that crystallization does not occur in the first exothermic process,and the process is irreversible with an apparent activation energy of 292±10 kJ/mol.At a heating rate of 40 K/min,crystallization occurs at the onset temperature（873 K）of the second peak,synchrotron X-ray diffraction and TEM evidence indicate that the first crystallization product is hP²-Al₂CoZr₆.The nucleation rate is high,but the growth rate is low.At 973 K,crystallization has completed and the microstructure is characterized by forming ruderal-like hP²-Al₂CoZr₆ and block-like cP-ZrCo phase without coherent orientations.Isothermal annealing the BMG at 818 K for 40 s,4 min and 60 min finds one exothermic peak,and the sample maintains it amorphous structure.Isothermal annealing at 873 K,the BMG exhibits two exothermic peaks,the second peak corresponding to crystallization of hP²-Al₂CoZr₆ and cP-ZrCo which ceases to occur after 30 min.The isochronal and isothermal crystallization results results are consistent with each other.The rapidly quenched Zr₇₀Al_12.5Fe_17.5 glass shows a single peak mode of crystallization,which is associated with the formation of highly mixed hP-Al₂FeZr₆ and tI-Zr₂Fe nanocrystals of 50-100 nm size due to a high nucleation rate but a low growth rate.The apparent activation energy is 261±12 kJ/mol.Our experimental results tend to support the occurrence of phase separation in the highly supercooled Zr₅₆Al₁₆Co₂₈ liquid,accounting for the large GFA and high thermal stability.More convincing evidence is needed to confirm and to discuss the metastable spinodal mechanism thus supposed for the BMG alloy.