Glass Transition And Primary Crystallization Of Aluminum-Based Amorphous Alloy

Al-rich Al-TM-RE (TM: transition metals, RE: rare earth element) amorphous/nanocrystalcomposites have attracted a great deal of attention due to their enhanced tensile strength (1500MPa) with respect to the fully amorphous counterparts (1000 MPa). These excellent mechanicalproperties make this new family of Al-based amorphous alloys well studied for using as anadvanced engineering materials candidate for industrial application. It is found that some alloyshave glass transition but some no, which makes it difficult to understand the nature of glasstransition for Al-based amorphous. As for the theory of heterogeneous nucleation for Al-basedalloy, the composition and structure of heterogeneous nuclei are not clear. Glassy phaseseparation prior to primary crystallization is disputed. Because the heats of mixing of all atomicpairs in Al-TM-RE alloy are negatively large, it is impossible that a miscibility gap appears forthe liquid phase. Thus phase separation as a precursor stage for primary crystallization is noteasy to understand. Therefore, the further investigation for mechanism of formation of highdensity Al nanocsystal is necessary. So far, the glass forming ability is still poor, since theribbon and powder of fully amorphous can just be obtained. Composition optimization toimprove glass forming ability of Al-based alloy is one of hot topics.According to the hot topics of Al-based amorphous alloy, i.e. the nature of glass transition,the mechanism of primary crystallization and composition optimization, the glass transition andprimary crystallization of Al₈₈Gd₆Er₂Ni₄ and Al₈₈Gd₆Er₂Ni₄ amorphous alloy and the influenceof Be on the glass forming ability and thermal stability of Al₈₅Gd₈Ni₇ and Al₈₇Y₈Ni₅ alloy areinvestigated by using the combination of X-ray diffraction, transmission electron microscope,differential scanning calorimetry, energy filter transmission electron microscope and threedimensional atom probe-field ion microscope.The main results are presented as follow:The crystallization kinetics has a stronger dependence on the heating rate than the glasstransition dose and hence T_g is only detected at the heating rate above 0.67 K/s. The activation energies for primary crystallization are different depending on the state of starting phase, i.e.amorphous solid or supercooled liquid. However, the product of primary crystallization isα-Al.There is no glassy phase separation prior to primary crystallization in amorphous alloy. Theα-Al nanocrystals nucleate directly from a uniform amorphous matrix. There is no enrichmentof rare earth atoms at the interface betweenα-Al and amorphous matrix. This kind of interfaceserves as the heterogeneous nucleation site for subsequent nucleation from the rare earth richamorphous alloy at which the nucleation barrier is lower than in the amorphous matrix.With the decrease of rare earth content and the increase of Ni content, the onset temperatureof primary crystallization decreased due to existence of quenched-in nuclei, which promotedthe formation ofα-Al resulting in smaller size and higher density; and the temperature gapbetween first two crystallization was widened helpfully to avoid the formation of metalliccompound and to get the composite with high purity ofα-Al embedded into the amorphousmatrix within wide temperature range.The enrichment of rare earth atom at the interface betweenα-Al and amorphous matrix isdepended on the factors of starting phase of primary crystallization, annealing temperature andtime and the size of rare earth atom. There is enrichment of big rare earth atom when thestarting phase of primary crystallization is amorphous solid and at low annealing temperaturefor short annealing time.GFA and thermal stability of the amorphous phase showed best improvement in the regionof the Be/Al ratio lower than the binary eutectic composition in both Al₈₅Gd₈Ni₇ and Al₈₇Y₈Ni₅alloys. The best glass forming ability was obtained in the alloy (Al_99.75Be_0.25)₈₅Gd₈Ni₇. Furtherreplacement of Be caused a decrease in GFA due to the formation of aBe₁₃Gd phase. In the caseof Al₈₇Y₈Ni₅ alloy, the replacement of Al with a small content of Be suppressed the formationof Al₂Y andα-Al during melt spinning and obtained an enhancement of the thermal stability ofthe amorphous alloy. The critical wheel speed to obtain fully amorphous was decreased from 40m/s to 20 m/s.