| A fundamental study of the phase transformations which occur in the Ti-Nb binary alloy system has been completed. Eight alloys in the range 20 to 70 at% Nb were investigated using transmission electron microscopy, light metallography and x-ray diffraction. Measurements of electrical resistivity and Vicker's microhardness also were performed. Emphasis was placed on the minimization of interstitial contamination in all steps of alloy fabrication and specimen preparation. In order to eliminate the effects of prior cold-working, the alloys studied were recrystallized at 1000C. Phase transformations were studied in alloys which had been quenched to room temperature after recrystallization, quenched to room temperature and then isothermally aged, and in those which had been isothermally aged without a prior room temperature quench. It was found that the microstructures of the quenched 20 and 25% Nb alloys were extremely sensitive to quench rate--with a fast quench producing martensite, a slow quench, the omega phase. The microstructures of the higher niobium content alloys were much less sensitive to quench rate. The microstructures of the isothermally aged 20 and 25% Nb alloys were found to be sensitive to prior thermal history. Alloys quenched to room temperature and then aged at 400C contained large omega precipitates, while those aged without an intermediate room temperature quench contained alpha precipitates. Isothermal omega phase precipitation was found in quenched 20 and 25% Nb specimens aged at 300, 350 and 400C. The 35% Nb alloys aged between 300 and 500C contained precipitates which could not be identified using electron diffraction, but were determined to be nuclei of the alpha phase. Aging at 450, 500 and 600C produced alpha precipitates in the 20 to 40% Nb alloys. The beta transus in this system could not be determined unambiguously. A phase diagram was calculated which fit the experimental data, but also predicted a small, but stable, beta phase miscibility gap. |
