Part I. The microstructural evolution in Ti-Al-Nb O + Bcc orthorhombic alloys

摘要

Phase transformations and the resultingmicrostructural evolution of near-Ti2AlNb and Ti-12A1-38Nb 0+ bcc orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcctransus, while, for Ti-12A1-38Nb, the processing temperaturewas supertransus. Phase evolution studies showed that thesealloys contain several constituent phases, namely, bcc, 0, and a2;when present, the latter was in small quantities compared to theother phases. The transmission electron microscopy (TEM),scanning electron microscopy (SEM), and X-ray investigationsof samples that were solutionized and water quenched were usedto estimate the phase fields, and a pseudobinary diagram basedon Ti = 50 at. pct was modified. The aging-transformationbehavior was studied in detail. For solutionizing temperaturesbetween 875℃ and the bcc transus, the phase composition andvolume fraction of the near-Ti2AlNb alloys adjusted throughrelative size changes of the equiaxed B2, 0, and a2 grains. Theaging behavior followed three distinct transformation modes,dependent on the solutionizing and aging temperatures.Widmanstatten formation was observed when a new phaseevolved from a parent phase. Thus, Widmanstatten 0 phaseprecipitated within the B2 phase for supertransus fully B2microstructures, as well as for subtransus ~2 + B2microstructures. Similarly, Widmanstatten B2 phase can formfrom a fully 0 microstructure, a transformation that has not beenobserved before. In the case of equiaxed 0 + B2 solutionized andwater-quenched microstructures, Widmanstatten 0-phaseformation occurred only below 875℃ . For the subtransus-solutionized and water-quenched microstructures, a secondaging transformation mode, cellular precipitation, was dominantbelow 750℃ . This involved formation of coarse and lenticular0 phase that grew into the prior B2 grains from the grainboundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus--solutionized and water-quenched microstructure transformed toa fully 0 microstructure at 650℃ . This microstructurereprecipitated B2 phase out of the 0 phase with continued agingtime. For Ti12A1-38Nb, Widmanstatten 0 precipitationremained the only transformation mode. It is shown thatsubtransus processing offers flexibility in controllingmicrostructures through postprocessing heat treatments.