High-temperature mechanical behavior of Ti-6Al-4V alloy and TiC p /Ti-6Al-4V composite

Mechanical behaviors at 538 °C, including tensile and creep properties, were investigated for both the Ti-6Al-4V alloy and the Ti-6Al-4V composite reinforced with 10 wt pct TiC particulates fabricated by cold and hot isostatic pressing (CHIP). It was shown that the yield strength (YS) and ultimate tensile strength (UTS) of the composite were greater than those of the matrix alloy at the strain rates ranging from approximately 10⁻⁵ to 10⁻³ s⁻¹. However, the elongation of the composite material was substantially lower than that of the matrix alloy. The creep resistance of the composite was superior to that of the matrix alloy. The data of minimum creep strain rate vs applied stress for the composite can be fit to a power-law equation, and the stress exponent values of 5 and 8 were obtained for applied stress ranges of 103 to 232 MPa and 232 to 379 MPa, respectively. The damage mechanisms were different for the matrix alloy and the composite, as demonstrated by the scanning electron microscopy (SEM) observation of fracture surfaces and the optical microscopy examination of the regions adjacent to the fracture surface. The tensile-tested matrix alloy showed dimpled fracture, while the creep-tested matrix alloy exhibited preferentially interlath and intercolony cracking. The failure of the tensile-tested and creep-tested composite material was controlled by the cleavage failure of the particulates, which was followed by the ductile fracture of the matrix.     

Microstructural Characterization of Ti-6Al-4V Metal Chips by Focused Ion Beam and Transmission Electron Microscopy

During machining, the cutting surface of a metal is subjected to high strain rates and temperatures. Due to the small mass of the formed chip, the metal is rapidly quenched, preserving the as-machined microstructure. These extreme conditions are reported to be favorable to form nanograin or ultrafine-grain microstructures. However, detailed investigation of this region is problematic due to the size of the chips and the difficulty in preserving the cutting surface microstructure during traditional transmission electron microscopy (TEM) preparation. This study investigates the use of focused ion beam (FIB) specimen preparation to preserve and TEM to image the microstructure of the secondary deformation zone (SDZ) at the cutting surface in chips of Ti-6Al-4V formed during machining. Use of the FIB allowed precise extraction of a side or transverse view specimen, which preserved the cutting surface to reveal an inhomogeneous microstructure resulting from the nonuniform distribution of strain, strain rate, and temperature. Initial imaging of a conventional TEM foil prepared from the plan view of the cutting surface revealed microstructures ranging from heavily textured to regions of fine grains. Using FIB preparation of a transverse foil, a layered microstructure was observed revealing a variation of fine grains near the cutting surface, which transitioned to coarse grains toward the free surface. At the cutting surface, a 10-nm-thick recrystallized layer was observed capping a 20-nm-thick amorphous layer.     

Hydrogen in β transformed Ti-6Al-4V

The internal reactions associated with hydrogen absorbed at ambient temperature by Ti-6A1-4V having a transformedβ microstructure were determined by using X-ray diffraction analysis. Below 650 ppm, the absorbed hydrogen was concentrated primarily in theβ phase causing an increase in the d(200) spacing and considerable X-ray line broadening. The a phase, however, was not significantly affected by the absorbed hydrogen showing no change in the d(1120) and d(1012) spacings and only a finite amount of line broadening. At approximately 650 ppm H, hydride precipitation began at thea-β interface. With increasing hydrogen content, theβ phase d(200) spacing continued to increase, the (200) X-ray line broadening reached a limiting value, and massive hydrides were formed. The data indicates that low concentrations of hydrogen absorbed by a-β titanium alloys can be detected by examining the X-ray line profile of theβ phase. W. D. HANNA, formerly with the McDonnell Douglas Astronautics Co.     

ECAP铜基原位复合材料的组织与性能研究

室温下采用等通道变形(ECAP)制备了 Cu-5.7%Cr原位复合材料,研究了其力学性能,电导率以及原位纤维的演变.结果表明:经过ECAP后,第二相Cr发生了旋转和伸长,逐渐演变为细小的纤维,且纤维逐渐倾向于与x轴平行.Cu-5.7%Cr合金的硬度,抗拉强度在2道次前显著增加,延伸率则迅速下降.2道次以后,硬度和抗拉强度的增加缓慢,延伸率则保持在20%左右.随着变形道次的增加,电导率逐渐降低,4道次后趋于稳定,8道次后达到80%LACS.     

Fretting wear behaviour of plasma nitrided Ti-6Al-4V fretted against unnitrided Ti-6Al-4V and alumina counterbodies

Ti-6Al-4V samples were plasma nitrided at 520°C in two environments (nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 h and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using two counterbody materials (unnitrided Ti-6Al-4V and alumina). Gross slip prevailed at a normal load of 4.9 N while mixed stick-slip prevailed at 9.8 N. Tangential force coefficient values of plasma nitrided samples were lower than those of unnitrided samples. The tangential force coefficient nearly stabilised after thousand cycles in case of samples tested against Ti-6Al-4V counterbody. On the other hand, it showed a continuously increasing trend in case of specimens tested against alumina counterbody. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen-hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in nitrogen. The samples plasma nitrided in nitrogen-hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume of the plasma nitrided samples was lower than that of the unnitrided samples. Owing to tribochemical reactions, the wear volume of unnitrided and nitrided samples fretted against alumina ball was higher than that of the samples fretted against Ti-6Al-4V. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume at different loads.     

置氢Ti-6Al-4V钛合金超塑性研究

通过采用Gleeble-1500D热模拟试验机进行超塑性变形试验,研究变形温度和应变速率对置氢TC4合金超塑变形性能的影响,利用XRD,SEM和TEM分析热氢处理改善钛合金超塑性能的机制.结果表明:置氢可降低超塑成形流变应力、变形温度,提高应变速率和m值;但只有适量的氢才有利于改善钛合金超塑性,即存在一个最佳置氢量;置氢0.35%H(质量分数)的TC4合金在800℃和3×10-3 s-1条件下仍有一定超塑性.分析表明,置氢钛合金超塑变形过程除晶粒转动和滑动机制外,位错滑移和孪生也作为辅助超塑变形机制.     

Microstructure Evolution and Mechanical Properties of Al-1080 Processed by a Combination of Equal Channel Angular Pressing and High Pressure Torsion

Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are the most promising severe plastic deformation (SPD) methods. Both methods impose very high strains, leading to extreme work hardening and microstructural refinement. In this paper, billets of Al-1080 were successfully processed by ECAP conducted for up to 10 passes, HPT at an applied pressure of 8 GPa for 5 revolutions, and a combination of ECAP and HPT (ECAP + HPT) at room temperature. The effects of the different SPD processes (ECAP, HPT, and ECAP + HPT) on the evolution of the microstructure and mechanical properties of Al-1080 were investigated. The HPT and ECAP + HPT processes were observed to produce finer grain sizes with greater fractions of high angle grain boundaries (HAGBs) than the ECAP alone. Although the grain sizes after HPT and ECAP + HPT were similar, the ECAP + HPT sample had more dislocations than the HPT sample. HPT after ECAP enhanced the mechanical properties (hardness, tensile strength, and ductility) of the ECAP-processed Al-1080, showing larger dimple size in the tensile fracture surfaces.     

Fabrication of Ti-6Al-4V Scaffolds by Direct Metal Deposition

Direct metal deposition (DMD) is a rapid laser-aided deposition method that can be used to manufacture near-net-shape components from their computer aided design (CAD) files. The method can be used to produce fully dense or porous metallic parts. The Ti-6Al-4V alloy is widely used as an implantable material mainly in the application of orthopedic prostheses because of its high strength, low elastic modulus, excellent corrosion resistance, and good biocompatibility. In the present study, Ti-6Al-4V scaffold has been fabricated by DMD technology for patient specific bone tissue engineering. Good geometry control and surface finish have been achieved. The structure and properties of the scaffolds were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tension test. The microstructures of laser-deposited Ti-6Al-4V scaffolds are fine Widmanstätten in nature. The tensile and yield strengths of the as-deposited Ti-6Al-4V were 1163 ± 22 and 1105 ± 19 MPa, respectively, which are quite higher than the ASTM limits (896 and 827 MPa) for Ti-6Al-4V implants. However, the ductility of the as-deposited sample was very low (～4 pct), which is well below the ASTM limit (10 pct). After an additional heat treatment (sample annealed at 950 °C followed by furnace cooling), both strength (UTS ～ 1045 ± 16, and YS ～ 959 ± 12 MPa) and ductility (～10.5 ± 1 pct) become higher than ASTM limits for medical implants.     

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.     

Deformation mechanisms in Ti-6Al-4V/TiC composites

The compressive behavior at room temperature of Ti-6Al-4V/TiC composites was examined at strain rates from 0.1 to 1000 s⁻¹. As little as 1 vol pct TiC particulates provided greater than a 20 pct increase in strength over that of the monolithic Ti-6Al-4V, while further additions of TiC did not provide proportional benefits. Microstructural examination before and after compression testing was instrumental in understanding the relative importance of the primary strengthening mechanism in the composites as compared to the monolithic material. A comparison of the various possible mechanisms clearly showed that the dominant mechanism was due to carbon in solid solution. At low strain rates, the failure process consisted of a progression of damage in the matrix and at particle-matrix boundaries, while at high strain rates, failure occurred along adiabatic shear bands. The composites had a greater susceptibility to adiabatic shear-band formation than did the monolithic material.     

等通道热挤压变形制备奥氏体不锈钢纳米级组织

 通过采用700 ℃等通道挤压法(ECAP法)对00Cr19Ni10奥氏体不锈钢实施变形,制备出晶粒尺寸在200~300 nm的超细晶组织,由此可使其抗拉强度与屈服强度显著增加。同时探讨了ECAP细化机理,对试验钢在等通道挤压变形中的微观组织演变过程进行了分析,发现其组织演变与滑移、孪晶以及动态再结晶有关。     

Environmental fatigue crack propagation in Ti-6Al-4V sheet

A study was made of environmental fatigue crack propagation in 2.5 mm thick Ti-6A1-4V sheet conforming to AMS 4911, and 2.2 mm thick IMI 318 conforming to BS TA 10. The environments were dry argon, normal air, distilled water and 3.5 pct aqueous NaCl. There were three alloy/orientation combinations: Ti-6A1-4V L-T, IMI 318 L-T and IMI 318 T-L. Test frequencies were 30 and 50 Hz, at which there was a general trend of higher crack growth rates in the order: argon, air, distilled water, 3.5 pct aqueous NaCl. For both dry argon and 3.5 pct aqueous NaCl there were large differences in crack growth rates at low δK values between the three types of specimen. There was a correlation between the texture and cleavage fracture and crack growth rates in 3.5 pct aqueous NaCl. This result is of considerable practical importance. For dry argon the ranking of specimen types could be explained by the relative importance of mechanical and environmental crack growth, using the structure-sensitive to structure-insensitive transition concept of Irving and Beevers.     

Environmental fatigue crack propagation in Ti-6Al-4V sheet

A study was made of environmental fatigue crack propagation in 2.5 mm thick Ti-6A1-4V sheet conforming to AMS 4911, and 2.2 mm thick IMI 318 conforming to BS TA 10. The environments were dry argon, normal air, distilled water and 3.5 pct aqueous NaCl. There were three alloy/orientation combinations: Ti-6A1-4V L-T, IMI 318 L-T and IMI 318 T-L. Test frequencies were 30 and 50 Hz, at which there was a general trend of higher crack growth rates in the order: argon, air, distilled water, 3.5 pct aqueous NaCl. For both dry argon and 3.5 pct aqueous NaCl there were large differences in crack growth rates at low δK values between the three types of specimen. There was a correlation between the texture and cleavage fracture and crack growth rates in 3.5 pct aqueous NaCl. This result is of considerable practical importance. For dry argon the ranking of specimen types could be explained by the relative importance of mechanical and environmental crack growth, using the structure-sensitive to structure-insensitive transition concept of Irving and Beevers.     

Fatigue and microstructural properties of quenched Ti- 6Al- 4V

The mechanical properties and microstructures of Ti-6A1-4V were determined for specimens heat treated at temperatures from 843 °C to 1065 °C for 10 minutes and water quenched; these properties were compared with those of α-β annealed specimens. Specimens heat treated at 900 °C and water quenched had higher fatigue lives by a factor of four to ten relative to the other treatments; in addition, this treatment resulted in high ductility, yield strength, tensile strength, and elastic modulus. Micro-structure studies utilizing optical and transmission microscopy showed that the improved fatigue lives were a result of a strain induced transformation of retained β to martensite. The amount of retainedβ and its relative stability were shown to depend upon the heat treatment temperature. The lower the heat treatment temperature below theβ transus the smaller the amount ofβ phase present before the quench and the richer theβ phase inβ stabilizer. The greater the concentration ofβ stabilizer in theβ phase the greater the probability that theβ phase was retained. High heat treatment temperatures resulted in a greater amount ofβ that was less stable and more probable to transform to martensite during a water quench.     

Transformation Superplasticity of Cast Titanium and Ti-6Al-4V

Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity. Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average strain-rate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation. The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.

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Ti-6Al-4V在步进轧制中的显微组织变化

对步进轧制在940℃α β区轧制Ti 6Al 4V合金棒材的锥形体取变形量不同的部位,利用光学显微镜、透射电镜观察其组织形貌及位错组态,分析了其变形机制,探讨了材料显微组织随变形量变化的规律及其形变硬化、回复、再结晶的演变过程.实验表明利用步进轧机加工,所得Ti 6Al 4V合金棒材芯部和边部的组织较均匀.     

Microstructural gradients in the superplastic forming of Ti-6Al-4V

The effect of microstructural gradients, introduced by local induction heating, on the behavior of a Ti-6Al-4V sheet in superplastic forming has been investigated. Heat treatment led to a change in the morphology of the α phase present at the start of superplastic deformation, which caused an increase in initial flow stress. This has a significant effect on the strain distribution. Trials using two axially symmetric shapes showed that the effect of microstructural gradients can be adequately predicted—via numerical modeling—and controlled. The technique has the potential to control the thickness distribution in formed parts.