Preparation of Ti-22Al-25Nb solid solution powders using mechanical alloying and solid solution mechanism analysis

Abstract

Solid solution powders of Ti-22Al-25Nb (at.%) were successfully prepared with Ti, Al, and Nb elemental powders via mechanical alloying (MA). The effects of milling time, stearic acid process control agent and Al2O3 reinforced particles on the morphology evolution, average particle size, crystallite size, solid solution and phase of mixed powders were studied. The solid solution mechanism and strengthening mechanism of Ti, Al and Nb mixed powders during MA were revealed. In addition, the strengthening effect of Al2O3 reinforced particles was clarified. The mixed powders at different milling times were characterized by X-ray diffraction and scanning electron microscopy. The microhardness of the solid solution powders was investigated, and the solid solubility of Al and Nb atoms in the Ti matrix was quantitatively calculated. The results revealed that the crystallite size of mixed powders without the addition of Al2O3 reinforced particles decreased with the increase of MA time, and was 18 nm after MA for 60 h. The solid solubility values of Al and Nb in the Ti matrix were 22.68 at.% and 9.49 at.%, respectively. The addition of Al2O3 reinforced particles could effectively refine the mixed powder particles, but had no obvious effect on the evolution of particle morphology; the values of load-bearing effects (ΔσLoad) and orowan strengthening (ΔσOrowan) were determined to be 27.5 MPa and 88.4 MPa, respectively. The microhardness values of the Ti-22Al-25Nb solid solution powders and Ti2AlNb-5 wt% Al2O3 nanocomposite powders were 435 HV and 465.8 HV, respectively. Compared to Al2O3 reinforced particles, stearic acid process control agent could significantly inhibit the welding between Ti, Al and Nb powders, making the powders more prone to disperse and break into small flakes with a smaller average particle size of 10 µm at 60 h. With the addition of stearic acid, the mechanical alloying process of Ti, Al and Nb element powders was inhibited, and the solid solubility of Al in the Ti matrix and the microhardness of the mixed powders after MA for 60 h were reduced to 16.31 at.% and 330 HV, respectively.