Microstructural evolution and FCC twinning behavior during hot deformation of high temperature titanium alloy Ti65

Although the development of titanium alloys with working temperatures above 600?C faces enormous difficulties and challenges,the related research has not stopped.In the present work,detailed analyses on microstructure evolution and hot deformation behavior of a new temperature resistant 650?C titanium alloy Ti65 were investigated from micrometer scale to nanometer scale.The results revealed that lamellarαgrains gradually fragmentized and spheroidized during theα+βphase region compression and the orientation of the c-axis ofαgrains gradually aligned to radial directions,forming two high Schmid factors(SFs)value texture eventually with the increase of strain to 0.7.Moreover,there were some strengthening characters in theα+βphase region such as lenticularαsand nano silicide(TiZr)6 Si3.In theβphase region,fine equiaxed dynamic recrystallized(DRX)βgrains were formed.Besides,the variant selection ofαm′artensite followed Burgers orientation relationship during the compression process.The main deformation mechanisms of theα+βphase region were dislocation slip and orientation dependent spheroidization.Whereas,the deformation process in theβphase region was controlled byβgrain DRX.Interestingly,many nano scale FCC twins were generated at the interface ofαl′ath during deforming in theβphase region,which was firstly observed in Ti65 alloy.

Evaluation of Oxidation of Ti-Al and Ti-Al-Cr Coatings Arc-ion Plated on Ti-60 High-temperature Titanium Alloy

High-temperature titanium alloy for aeroengine compressor applications suffers from high-temperature oxidation and environmental corrosion, which prohibits long-term service of this kind alloy at temperatures above 600℃. In an attempt to tackle this problem, Ti-48Al （at. pct） and Ti-48Al-12Cr （at. pct） protective coatings were plated on the substrate of alloy Ti-60 by arc ion plating （ALP） method. Isothermal and cyclic oxidation tests were performed in static air at elevated temperatures. Phase composition, morphology of the coatings and distribution of elements were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray analysis （EDX）. The results showed that the Ti-48Al coating exhibited good isothermal oxidation resistance during exposure at 800℃, but poorer resistance against oxidation at 900℃. By contrast Ti-48Al-12Cr coating demonstrated excellent isothermal oxidation resistance at both temperatures. Cyclic oxidation tests performed at 800℃ indicated that resistance and no spallation of coatings was observed. But both coatings demonstrated good cyclic oxidation at 900℃ only Ti-48Al-12Cr coating demonstrated excellent cyclic oxidation resistance.

Influence of Nb Ion Im plantation on Oxidation Resistance of High Tem perature Titaniu m Alloy at 650 ℃

The oxidation rate of a high temperature titanium alloy in air at 650℃ could be decreased significantly by means of ion implantation of 3×10 16 and 3×10 17 ions/cm 2 Nb. The microstructure and alloy elements distribution in the oxidation scale of unimplanted and Nb implanted titanium alloy were investigated by using SEM, XRD and AES. The addition of Nb could reduce the number of point defects and decrease the solubility of oxygen in the alloy substrate. Therefore, the oxidation resistance of the alloy in air at 650℃ is remarkably improved.

Microstructure and tensile properties of Ti-62421S alloy plate with different annealing treatments

Ti-62421S （Ti-6A1-2Sn-4Zr-2Nb-lMo-0.2Si） is a novel short-time using high-temperature titanium alloy. The effects of annealing on microstructure and tensile properties of Ti-62421S alloy plate were studied through optical microscopy （OM）, electron probe microanalysis （EPMA）, transmission electron microscopy （TEM）, and tensile tests. The results show that, with annealing tem- perature increasing, the volume fraction of primary α（αp）- phase decreases while that of transformed β（βt）-structure and secondary α （αs）-phase increases. The room-temperature strength and plasticity are insensitive to annealing temperature. However, with annealing temperature increasing, the tensile strength decreases at 550℃, while increases at 600 and 650℃ instead. It is suggested that, at 550℃, the strengthening mechanism is mainly boundary strengthening and the biggest contributor is ap-phase by providing αp/β-boundary area. Above 600 ℃, the strengthening mechanism is grain strengthening, where αs-phase strengthens the β-phase.

Optimization of thermal processing parameters of Ti555211 alloy using processing maps based on Murty criterion

Isothermal compression testing of Ti555211 titanium alloys was carried out at deformation temperatures from 750 to 950 °C in 50 °C intervals with a strain rate of0.001-1.000 s^（-1）. The high-temperature deformation behavior of the Ti555211 alloy was characterized by analysis of stress-strain behavior, kinetics and processing maps. A constitutive equation was formulated to describe the flow stress as a function of deformation temperature and strain rate, and the calculated apparent activation energies are found to be 454.50 and 207.52 k J mol^（-1）in the a b-phase and b-phase regions, respectively. A processing map based on the Murty instability criterion was developed at a strain of 0.7. The maps exhibit two domains of peak efficiency from 750 to 950 °C. A ＊60 % peak efficiency occurs at 800-850 °C/0.001-0.010 s^（-1）. The other peak efficiency of ＊60 % occurs at C950 °C/0.001-0.010 s^（-1）, which can be considered to be the optimum condition for high-temperature working of this alloy.However, at strain rates of higher than 1.000 s^（-1）and deformation temperatures of 750 and 950 °C, clear process flow lines and bands of flow localization occur in the hightemperature deformation process, which should be avoided in Ti555211 alloy hot processing. The mechanism in stability domain and instability domain was also discussed.