Study Of Microstructure And Thermal Stability Of Mechanically Alloyed Al/Ti Powders

ABSTRACT Al-Ti alloys are considered as attractive candidate materials for aerospace andengine domain, owing to their low density, elevated temperature strength andoxidation and corrosion resistance. However, their low ductility and poor formabilityprevent industrial application. Al-Ti intermetallics are usually fabricated byvacuum-melting method, which is required above the liquidus temperature of metal.The grain sizes of cast ingot are coarse and it is prone to produce cracks. In order toprepare this kind of intermetallics at room or low temperature, we adopt mechanicalalloying (MA) and reaction sintering technology, which contributes to produce finecrystal grains and expected Al-Ti alloys with high properties. This paper studied themicrostructure and thermal stability of mechanically alloyed Al/Ti powders. This paper firstly introduced the history of mechanical alloying and its techniquecharacteristics. Subsequently, the properties and prevenient research fruit of Al-Tialloys were reviewed in details. We prepared Al-x at.%Ti (x=6, 12, 25, 50, 80) powdermixtures synthesized by mechanical alloying. These powders were consolidated bycold pressing and then sintered. Measurements on series of Al1-xTix powder mixtures were performed by X-raydiffraction, scanning electron microscopy and differential thermal analyzer. Principalresults from them and relative analyses are as follows:1) For Al-6at.%Ti powders, the experimental results show that Al,Ti particle andgrain size decrease rapidly with the increase of milling time, and that the more millingtime, the more energy stored in the composite powders. Consequently, the activationenergy of the reaction for Al-Ti intermetallics decreases obviously. The 90h milledpowders at 660℃ for 6h heat treatment are mainly Al3Ti, with the existence of a littleAl2Ti,Al5Ti2.2) Strong combustion reaction occurs when Al-12&25at.%Ti powders are milled for50h. Because aluminum is more active element than titanium, and simultaneously, thegrain refinement of aluminum improves its ratio surface area, a large of Al2O3 phaseproduce. The range of this kind of reaction is about 10~30at.%Ti.3) For Al-50at.%Ti powders, X-ray diffraction (XRD) results show that the structureof a metastable Ti(Al) hexagonal close packed (h.c.p.) super-saturated solid solution is IIABSTRACTobtained after 140h ball milling. When milled for 300h, h.c.p phase transforms intoamorphous phase. The final equilibrium phases of Al-50at.%Ti powders which wereconsolidated by cold pressing and then sintered are AlTi and Ti3Al.4) For Al-80at.%Ti powders, X-ray diffraction (XRD) results show that the structureof a metastable Ti(Al) hexagonal close packed (h.c.p.) super-saturated solid solution isobtained after 250h ball milling.5) Gibbs free energy curves of various competitive phases are calculated andcompared according to the Miedema model, which explains how these phases proceed.The experimental results are well coincidental with thermodynamics theory.