Mechanical milling and annealing to produce nanoscale dispersion-strengthened cubic trialuminide composites

Intermetallic matrix composites composed of nanoscale dispersions of an H-phase carbide (Ti₂AlC) in a cubic titanium trialuminide, (Al,Cr) ₃Ti, matrix have been produced by mechanical ball milling followed by annealing. Initial mill charges consisted of prealloyed cubic trialuminide powder, excess Ti powder to control alloy losses to carbide formation, and an organic liquid process control agent (PCA). Milling produced powder particles comprised of a very fine mixture of cubic trialuminide, free Ti, and PCA. Although no carbide phases were present after milling, a very small amount of TiH₂ formed from reaction of the PCA with Ti. Carbon from the PCA enters into interstitial solid solution within the cubic trialuminide either during milling or annealing. Depending on processing conditions, AlCr ₂ can form prior to Ti₂AlC precipitation, whereas Al ₂Ti can form along with the carbide. These two intermetallic phases disappear during annealing at higher temperatures so that only the cubic titanium trialuminide and H-phase carbide remain.; A two phase equilibrium field was established between the (Al,Cr) ₃Ti and Ti₂AlC single phase fields, which means that a range of milling times exists where the two phase composite will form after annealing. With the proper initial powder charge, the overall composition will enter, and eventually pass through, this two phase field during milling because the carbon content arising from PCA incorporation increases with milling time.; The influence of a variety of milling parameters on the condition of the as-milled powder and microstructure of the annealed material has been studied in order to determine the most effective means for producing this nanoscale composite. In addition to dependence on milling time, the type and amount of PCA, the density of the balls used for milling, and the composition of the powder charge have all been found to have significant effects. Specifically, the degree of deformation imparted to the powder during milling increases with the density of balls, PCA level, and can change with type of PCA. Also, the amount of PCA incorporated into the powder during milling has been shown to increase with ball density and decreasing amount of PCA added to the charge.