Superplastic Behaviour of a Rapidly Solidified Al-17Si Alloy

A rapidly solidified Al-17Si alloy was pre-pared by the ultrasonic gas atomization andextrusion method.Its superplasticity was studiedunder constant cross-head speed tensile test condi-tion over the temperature range of 450℃ to 550℃and strain rate range of 1.67×10~(-4) to 6.17×10~(-2) s~(-1).The superplastic behaviour of theas-extruded and the thermomechanically treatedsamples was compared.It was found that thethermomechanical treatment was essential toachieving superplastic deformation.A maximumelongation of 445% was obtained at test tempera-ture of 550℃ and strain rate of 6.17×10~(-4) s~(-1).The microstructures before and after deformationwere studied using OM,SEM and TEM.Void for-mation on the primary Si phase interfaces wasfound to have detrimental effect on superplasticity.It was also noted that the primary Si phasecoarsened rapidly during superplastic deformation.The micromechanisms of superplasticity,phasecoarsening and void formation were discussed.     

Effect of Strain Rate on the Mechanical Properties of Ti-24Al-11Nb

The effect of strain rate on the bend ductility and notch fracture toughness of Ti-24Al-11 Nb wasstudied,it was found that the strain rate with a range of 1.17×10~(-5)～1.17 ×10~(-3) at 20℃ had nega-tive influence on both properties based on different microstructures.     

Cavitation Behavior during Superplastic Deformation of AZ31 Magnesium Alloy

Superplasticity of AZ31 Mg alloy at the temperature range of 250～450℃ and stain rate range of 0.7x10-3～ 1.4x 10-1 s-1 was examined through uniaxial tensile test. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of cavities and surface relief near fracture surface, respectively.It is shown that AZ31 Mg alloy starts to exhibit superplasticity from 300℃. The maximum elongation of 362.5%was obtained at 400℃ and strain rate of 0.7×10-3 s-1. There exist many O-shaped cavities and filaments at the boundaries near fracture surface. The fracture of filaments results in intergranular cavity and the model for the formation of intergranular cavities is proposed. The growth of cavities is plasticity-controlled and the serrated boundaries of intergranular cavities agree with the results of surface relieves.     

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract

High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10^?4 s^?1 of conventional cast and wrought Ti-1023 alloy to 3·3×10^?3 s^?1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10^?4 to 3·3×10^?2 s^?1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10^?3 s^?1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Microstructures and Tensile Properties of Isothermally-forgedTi-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr Alloys

Microstructures and room-temperature tensile properties of isothermally-forged γ-base (γ + α2)alloys in Ti-Al-Nb-Cr-V system with different heat treatments were investigated. The results show that the microstructures of Ti-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr (at. pct) alloys are mainly determined by heat treating temperature in the (cr + 7) tWo-phase field, and the joint additions of Nb, Cr and V in the Ti-47Al alloy afFect Ta significantIy. The microstructure of Ti-47Al alloy with additions of Nb, Cr and V (1~2 at. pct) can be dupIex or nearly-lamellar by a suitable heat treatment after isothermal forging at 1000℃ for over 50% plastic strains.Therefore its tensile properties can be improved at room temperature.     

Superplasticity and associated activation energy in Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The high temperature deformation characteristics of a commercial β -titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr have been studied in the temperature range 830–925∘C. The alloy exhibited superplasticity in a narrow temperature and strain rate range i.e. 850–865∘C and 5× 10^{− 5}–3× 10^{− 3} s^{− 1} respectively, with a maximum elongation of 634% at 855∘C. The superplastic behaviour in the alloy is considered to arise as a result of subgrain formation at the higher strain rates (region III) which enhances diffusional creep at lower strain rates (region II). The activation energy values for regions II and III were found to be close to the lower of the two activation energy values (129.2 KJ/mole) proposed to describe self diffusion in β -phase suggesting that the rate controlling mechanism during high temperature deformation of the alloy was that for lattice diffusion.     

Ti-6Al-4V合金热压缩过程中的动态再结晶

在变形温度为870~960℃、应变速率为5×10^-4 s^-1~5×10^-2 s^-1的条件下对Ti-6Al-4V合金进行单道次等温压缩实验,测出其应力-应变曲线并建立KM模型、Poliak-Jonas模型和Avrami模型,较为系统地描述了这种合金动态再结晶过程中的流变应力、临界应变量、组织演变动力学等的特征。将动态再结晶组织的转变体积分数引入Prasad功率耗散率模型,得到了Ti-6Al-4V合金动态再结晶过程中能量的变化规律并结合微观组织表征揭示了这种合金的动态再结晶机理。结果表明：随着变形温度的提高和应变速率的降低,Ti-6Al-4V合金的动态再结晶临界应变量减小,组织转变的体积分数增大。发生完全动态再结晶时的功率耗散率大于0.34,形核机制为位错诱导的弓出形核机制。     

TC18钛合金的超塑性行为与变形机制

通过高温拉伸实验研究TC18钛合金在温度为720~950℃,初始应变速率为6.7×10~(-5)~3.3×10~(-1)s~(-1)时的超塑性拉伸行为和变形机制。结果表明:TC18钛合金在最佳超塑性变形条件下(890℃,3.3×10~(-4)s~(-1)),最大伸长率为470%,峰值应力为17.93MPa,晶粒大小均匀。在相变点Tβ(872℃)以下拉伸,伸长率先升高后下降,在温度为830℃,初始应变速率为3.3×10~(-4)s~(-1)时取得极大值373%,峰值应力为31.45MPa。TC18钛合金在两相区的超塑性变形机制为晶粒转动与晶界滑移,变形协调机制为晶内位错滑移与攀移;在单相区的超塑性变形机制为晶内位错运动,变形协调机制为动态回复和动态再结晶。     

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al2O3 scale.     

一种新型近β型Ti-5.5Mo-6V-7Cr-4Al-2Sn-1Fe合金热变形行为

采用Gleeble-3800型热模拟试验机对一种新型近β型Ti-5.5Mo-6V-7Cr-4Al-2Sn-1Fe(质量分数/%)钛合金进行等温恒应变速率压缩实验。变形温度范围为:655~855℃,应变速率范围为:0.001~10s^-1 ,最大真应变为0.8。根据实验数据,建立了该合金的高温流变应力模型,计算出热变形激活能约为255kJ/mol,并绘制出热加工图。结合热加工图与材料的显微组织分析可知,在高应变速率(1~10s^-1 )条件下变形时,在热加工图上表现为材料的功率耗散值(η)低,为失稳区域,易产生绝热剪切带与局部塑性流动、开裂等现象。在应变速率小于0.01s^-1 和相变点( T β)温度以下(655~755℃)进行热变形时,组织变化主要以动态回复为主;在应变速率小于0.01s^-1 和 T β以上(755~855℃)进行热变形时,组织发生动态再结晶,且随着温度的升高,新产生的再结晶晶粒逐渐长大。在相变点附近(755~770℃),变形速率为0.001~0.003s^-1 区域内变形时,功率耗散值达到最大值,组织发生动态再结晶,该区域为合金热变形的“安全区”。     

Research on the Characteristics of Hot Deformation in BT20 Titanium alloy and Its Optimum Spinning Temperature Range

Hot compression tests were conducted on a Gleeble-1500 simulator to investigate the hot deformation behavior of BT20 Ti alloy (Ti-6Al-2Zr-1Mo-1V) in the temperature range from 550 to 1000℃ at constant strain rate in the range of 0.01～1 s-1, and then the optimum spinning temperature range was determined. Moreover, tube spinning experiments were executed to verify the reasonability of the optimum temperature range. The results show that the flow stress declines gradually with increasing deformation temperature and decreasing strain rate. In α β phase region the dynamic recrystallization is the main softening mechanism and in β phase region the hot deformation softening is controlled by dynamic recovery. In α βphase region with reducing strain rate dynamic recrystallization is fully developed. The optimum temperature of hot spinning is 850～900℃ and that of warm spinning is 600～650℃.Meanwhile, at the temperature above 600℃ tubular workpieces of BT20 Ti alloy have been spun without surface cracks and microstructure inhomogeneity, which proves that the optimum spinning temperature range obtained through hot compression experiments is reasonable.     

Diffusion Bonding of Ti_3Al Base Alloy

: The effects of diffusion bonding temperature and holding time on the joint strength of Ti3Al base alloy has been investigated in this paper. The shear strength of Ti-14Al-21Nb-3Mo-V alloy diffusion bonding joint under pressure of 12 MPa at 990℃ for 70 min was obtained to 797.6 MPa which approaches the base material strength. In addition, a short-time diffosion bonding process was studied in order to decrease the bonding cost. With the deformation of the specimens of 2.5% and the bonding temperature of 990℃ for 15 min, the bonding strength could reach 801 MPa.     

TiH2-based Ti-1Al-8V-5Fe PM alloys with different addition methods of alloying elements

The powder metallurgy high-strength Ti-1Al-8V-5Fe alloy (Ti-185 alloy) was investigated in this paper with different addition methods of alloying elements, Al, V, Fe pure elements powder (EP) or 1Al-8V-5Fe ternary master alloy powder (MAP), based on TiH₂ powders at the sintering temperature from 1150 to 1350°C. The results indicate that the Ti-185 alloy with the 1Al-8V-5Fe master alloy (Ti-185 MAP alloy) possesses the higher relative sintered density, less oxygen content, and less α-phase volume fraction versus the Ti-185 alloy with Al, V, and Fe pure elements (Ti-185 EP alloy). No matter where the sintering temperature is 1150, 1250, or 1350°C, Ti-185 MAP alloy invariably has the higher yield strength and hardness which have a strong relationship to its higher density and less volume fraction of softer α-phase in comparison with Ti-185 EP alloy.     

Thermomechanical Behavior of Ti-6Al-4V Alloy

Ti-6Al-4V, among the Ti alloys, is the most widely used. In the present work, the behavior of Ti-6Ak-4V alloy has beeninvestigated by the uniaxial hot isothermal compression tests and a series of dilatometric experiments were also carried out todetermine the transformation temperatures at different cooling rates. Specimens for hot compression tests were homogenizedat 1050℃ for 10 min and then quickly cooled to different straining temperatures from 1050 to 850℃. Cooling rates were chosenfast enough to prevent high temperature transformation during cooling. Compression tests were conducted at temperaturesfrom 1050 to 850℃ in steps of 50℃ at constant true strain rates of 10~(-3) or 10~(-2) s~(-1). The apparent activation energy forcompression in two-phase region was calculated 420 kJ·mol~(-1). Partial globularization of cr phase was observed in the specimendeformed at low strain rates and at temperatures near the transformation zone and annealed after deformation.     

固溶+时效处理对粉末冶金Ti-22Al-25Nb合金显微硬度的影响

采用放电等离子烧结技术(SPS)在950℃/80 MPa/10 min条件下制备粉末冶金Ti-22Al-25Nb(原子分数,%)合金作为初始材料,将其分别在940~1100℃、10~120 min和800℃/8 h条件下进行固溶处理和时效处理,研究了固溶+时效处理对粉末冶金Ti-22Al-25Nb (原子分数,%) 合金的微观组织和显微硬度的影响,并建立了显微硬度的演变模型。结果表明,随着固溶温度的提高和保温时间的延长B2相的晶粒尺寸增大、均匀度提高,在940~1010℃晶粒长大的速率最低,在1100℃晶粒尺寸的均匀度达到最大值0.84。板条O相的尺寸和数量对合金的性能有显著影响。在(B2+O)两相区时效后,其尺寸和数量显著影响合金性能的次生板条O相的体积分数提高、尺寸减小,尤其是相互交叉、缠结的O/O相数量的增多,使合金的显微硬度提高;在1060℃/60 min/Water cooling(WC)+ 800℃/8 h/Furnace cooling(FC)条件下处理的合金,其显微硬度达到最大值434.92 HV。     

Microtwin formation in the α phase of duplex titanium alloys affected by strain rate

The effect of tensile strain rate on deformation microstructure was investigated in Ti-6-4 (Ti-6Al-4V) and SP700 (Ti-4.5Al-3V-2Mo-2Fe) of the duplex titanium alloys. Below a strain rate of 10⁻² s⁻¹, Ti-6-4 alloy had a higher ultimate tensile strength than SP700 alloy. However, the yield strength of SP700 was consistently greater than Ti-6-4 at different strain rates. The ductility of SP700 alloy associated with twin formation (especially at the slow strain rate of 10⁻⁴ s⁻¹), always exceeded that of Ti-6-4 alloy at different strain rates. It is caused by a large quantity of deformation twins took place in the α phase of SP700 due to the lower stacking fault energy by the β stabilizer of molybdenum alloying. In addition, the local deformation more was imposed on the α grains from the surrounding β-rich grains by redistributing strain as the strain rate decreased in SP700 duplex alloy.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Effect of sputtered TiAlCr coatings on oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V

The effect of sputtered Ti-50Al-10Cr and Ti-50Al-20Cr coatings on both isothermal and cyclic oxidation resistance at 800 similar to 900 degreesC and hot corrosion resistance at 850 degreesC of Ti-24Al-14Nb-3V was investigated. Results indicated that Ti-24Al-14Nb-3V alloys exhibited poor oxidation resistance due to the formation of Al2O3+TiO2+AlNbO4 mixed scales in air at 800 similar to 900 degreesC and poor hot corrosion resistance due to the spallation of scales formed in Na2SO4+K2SO4 melts at 850 degreesC. Both Ti-50Al-10Cr and Ti-50Al-20Cr coatings remarkably improved the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy.     

Comparison of fatigue strengths of biocompatible Ti-15Zr-4Nb-4Ta alloy and other titanium materials

The fatigue strength of an annealed Ti-15Zr-4Nb-4Ta alloy at 1 × 10⁸ cycles was approximately 730 MPa. The fatigue strength of its alloy was much improved following an ageing treatment after a solution treatment. The tension-to-tension fatigue strengths of annealed Ti-6Al-4V, V-free Ti-6Al-7Nb, Ti-6Al-2Nb-1Ta, and Ti-15Mo-5Zr-3Al alloys at 1 × 10⁸ cycles were approximately 685, 600, 700, and 350 MPa, respectively. The ratios of fatigue strength at 1 × 10⁸ cycles to ultimate tensile strength for the α- and (α + β)-type Ti materials were higher than 65%.