Solid State Amorphization Reaction by Rod-Milling Al_xTa_<1-x> Powders and the Effect of Annealing

High thermal stable amorphous Al_xTa_ alloy powders with wide amorphization range (10≤x≤90) have been synthesized by rod-milling technique using a mechanical alloying (MA) method. During the first few kiloseconds (11-173 ks) of the MA time, the layered-composite particles of Al and Ta are intermixed and form an amorphous phase upon heating at about 680 K in a differential thermal analyzer by thermally assisted solid state amorphization (TASSA). The heat formation of an amorphous Al_xTa_ alloy via the TASSA process, ΔH_ has been measured as a function of the MA time. The crystallization characteristics indexed by the crystallization temperature, T_ and the enthalpy of crystallization, ΔH_ of the amorphous phase formed via the TASSA process are also investigated as a function of the MA time. Comparable with the TASSA process, a homogeneous amorphous Al_xTa_ alloy is formed after longer MA time (1080 ks). The amorphization process in this case is attributed to a mechanical solid state amorphization (MDSSA). At the end of the MA time (1080-1440 ks), the maximum heat of formation of an amorphous Al_xTa_ alloy via the MDSSA process, ΔH_, has been calculated. Moreover, the thermal stability characterized by the crystallization temperature, T_ and the enthalpy of crystallization, ΔH_, are also estimated. The role of amorphization via each process has been discussed

Synthesis of Amorphous Al-based Alloy Powders by Mechanical Alloying and Mechanical Disordering

Amorphous Al-based alloy powders have been synthesized by mechanical alloying (MA) of elemental Al and TM (TM; Ti, Zr, Nb and Ta), and mechanical disordering (MD) from crystalline intermetallic compound powders of Al_xTM_ respectively using both ball-milling (BM) and rod-milling (RM) techniques. The mechanically alloyed and mechanically disordered alloys were characterized by means of X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, differential thermal analysis, differential scanning calorimetry and chemical analysis. The results have shown that the crystal-to-amorphous transformation in the MD process occurs through one stage without compositional change, while the crystalline-to-amorphous formation in the MA process occurs through three stages with dramatic compositional change. At the early stage of the MA time, the starting elemental powders of Al and TM are agglomerated to form large composite particles , as large as 800 μm in diameter. These particles contain many thick layers of Al and TM metals. At the intermediate stage of the MA time, however, the powder particles are reduced in size and contain numerous narrow and well-arranged layers. During this stage of MA, a complete crystalline-to-amorphous phase transformation occurs by heating the alloy powders to about 700 K. This transformation occurs due to a thermally assisted solid-state amorphization (TASSA) between the elemental starting materials. Contrary to this, the formation of the amorphous phase at the final stage of the MA time occurs only due to the mechanically driven solid-state amorphization (MDSSA)