Crystallization Of Zr-Al-LTM(LTM= Fe,Co,Ni) Metallic Glasses

Zr-based bulk metallic glasses(BMGs)are mostly multi-component alloys,and extremely large glass-forming ability(GFA)appears at some specific compositions,at which centimeter scaled BMG samples are reached at low melt cooling rates.Zr-based BMGs exhibits high strength,high corrosion and wear resistance,which are potentially useful as biomedical and electronic devices materials.GFA has been a central issue concerning the composition design,preparation,property and application of BMGs.A large portion of multi-component Zr-based BMGs are the high-order alloying modifications of ternary Zr-Al-LTM(LTM=Fe,Co,Ni,Cu)alloys.These ternary systems show similar phase daigram characteristics,and share simialr alloying chemical factors and elctronic structure features as well.The BMG-forming compositions and the respective critical sizes(d_max)of glass formation are,however,greatly diffrerent.The known GFA criteria derived from thermodynamics and alloying chemistry are hard to explain the large separation of GFA in these ternary systems.Solid experimental evidence are highly demanding to clarify the underlying mechanism of the large separation in GFA.A metallic glass is formed while crystallizaion is completely depressed during liquid solidification,and inturn the kinetics of nucleation and growth of undercooled lquids governs the ease of glass formation,i.e.,the GFA.In the present work,crystallization of three typical BMGs of Zr70Al12.5Fe17.5(dmax=1.5 mm),Zr56Al16Co28(dmax=18 mm)and Zr60Al15Ni25(dmax=15 mm)are studied during liquid solidification as well as in the course of thermal annealing.The experimental techniques of Differential Scanning Calorimetry(DSC),X-ray Diffraction(XRD),High Resolution Syncrotron X-ray Diffraction(HR-SXRD),Small Angle X-ray Scattering(SAXS),Transmission Electron Microscopy(TEM),and Electron Probe Microanalysis(EPMA)are combined to examine the microstructure evolution and crystallization kinetics in the slowly cooled ingots,rapidly quenched ribbons and BMG rods.The chief experimental results are suammrized in the following:(1)Solidifcation microstructures of as-cast ingot alloysThe arc-melted ingot of Zr₇₀Al_12.5Fe_17.5 of 5 g in weight has been fully crystallized,showing a dual-phase microstructure composed of h P-Zr₆Al₂Fe(a=0.71 nm,c=0.35 nm)and t I-Zr₂Fe(a=0.64 nm,c=0.56 nm)phases;Zr₅₆Al₁₆Co₂₈ and Zr₆₀Al₁₅Ni₂₅ ingot samples of the same weight exhibit distinct microstructure separation by a sharp interface,the bottom part contacting the copper mold being crystallized and the upper part fully amorphous.The crystallization counterparts of Zr₆₀Al₁₅Ni₂₅ are h P-Zr₆Al₂Ni(a=0.79156 nm,c=0.3732 nm),t P-Zr₅Ni₄Al(a=0.717 nm,c=0.665 nm),and t I-Zr₂Ni(a=0.645 nm,c=0.52 nm),and those of Zr₅₆Al₁₆Co₂₈ are h P-Zr₆Al₂Co(a=0.71 nm,c=0.71 nm)and c P-Zr Co(a=0.33 nm).BMG formation in the slowly cooled Zr₅₆Al₁₆Co₂₈ and Zr₆₀Al₁₅Ni₂₅ ingots reflects the high stability of their undercoled liquids,whereas the Zr₇₀Al_12.5Fe_17.5 melt is vulnerable to crystal nucleation and growth.The microstructure separation phenomenon is also observed in the alloy ingots of Zr50Al10Cu40,Zr55Al10Ni5Cu30,Zr65Al7.5Ni10Cu17.5,Zr63.5Al9Fe4.5Cu23 and Zr_55.8Al_19.4Co_17.36Cu_7.44 associated with a large d_max(>1 cm).(2)Crystallization microstructure and kinetics of metallic glassesThe Zr₇₀Al_12.5Fe_17.5 metallic glass exhibits a single exothermic peak in the continuous heating DSC curve,and the apparent activation energy of crystallization is determined to be E=250±15 k J/mol.When isothermally annealed at 668 K,the HR-SXRD and TEM evidence indicate that nanocrystallization occurs and the first crystallization product is h P-Zr₆Al₂Fe.The nucleation rate is high,but the growth rate is low.The grain size does not change significantly with the increase of temperature and time.A size limit of growth is also observed for the nanocrystallites;The KJMA equation gives an Avrami index of 1.05?n?1.11 within the crystallization volume fraction of 0?6%,which is consistent with diffusion controlled growth of particles of appreciable initial volume;and within the volume fraction of10?45%,1.53?n?2.43,revealing diffusion controlled growth of small particles with decreasing nucleation rates.The activation energies of crystallization for the two stages are E=240±10 k J/mol,and E=325±10 k J/mol,respectively.The above evidence indicates that existence of quenched-in h P-Zr₆Al₂Fe nuclei in the Zr₇₀Al_12.5Fe_17.5 metallic glass,whereas they are absent in the Zr₅₆Al₁₆Co₂₈ and Zr₆₀Al₁₅Ni₂₅ metallic glasses.The DSC curve of Zr₆₀Al₁₅Ni₂₅ metallic glass exhibits two exothermic peaks at the heating rate of 5 K/min,which emerge into a single peak at higher heating rates.The corresponding apparent activation energy is E₁=332±9 k J/mol and E₂=296±12 k J/mol.After isothermally annealed to the first peak tip at 736 K,HR-SXRD and TEM examination shows that the first crystallization products are nanometer scaled h P-Zr₆Al₂Ni and metastable o C(a=1.356 nm,b=0.82 nm,c=0.57 nm)phases.The fully crystallized sample consists of h P-Zr₆Al₂Ni(a=0.79156 nm,c=0.3732 nm),t P-Zr₅Ni₄Al(a=0.717 nm,c=0.665 nm)and t I-Zr₂Ni(a=0.645 nm,c=0.52 nm)phasees.Upon heating at various rates,the Zr₅₆Al₁₆Co₂₈ metallic glass exhibits two well separated exothermic peaks associated with the activation energies of E₁=300 k J/mol and E₂=208k J/mol,respectively.HR-SXRD results show that crystallization does not occur in the sample annealed to the end temperature of the first exothermic peak,and the main diffraction peak of the amorphous structure has shifted to a lower diffraction angle along with the appearance of a weak shoulder peak at the high angle side.When annealed at 853 K,nano h P-Zr₆Al₂Co phase first precipitated,followed by nano c P-Zr Co.WAXS and SAXS results indicate that neither phase separation nor crystallization occurs in the first stage exothermic reaction.Comparison of the crystallization microstructures and kinetics data of the three metallic glasses finds that the initial stage of crystallization of their undercooled liquids preceeds by the precipitation of isostructural h P-Zr₆Al₂-LTM(Fe,Co,Ni)phases with high nucleation rates.The nucleation of h P-Zr₆Al₂Fe is readily occurred in the high temperature regime of the undercooled liquid,while this has been completely depressed in the cases of h P-Zr₆Al₂Ni and h P-Zr₆Al₂Co.The higher GFA of Zr₅₆Al₁₆Co₂₈ and Zr₆₀Al₁₅Ni₂₅ alloys is attributed to the high thermal stability of their undercooled liquids against nucleation of the hexagonal phases.