Diffusion Bonding of Ti-6Al-4V Sheet with Ti-6Al-4V Foam for Biomedical Implant Applications

Advanced metallic bone implants are designed to have a porous surface to improve osseointegration and reduce risks of loosening. An alternative approach to existing surface treatments to create a porous surface is to bond separately produced metallic foams onto the implant. To assess the feasibility of this approach, a Ti-6Al-4V foam was diffusion bonded onto bulk Ti-6Al-4V in an argon atmosphere at temperatures between 1173 K and 1223 K (900 °C and 950 °C) for times between 45 and 75 minutes. These specimens were tested in tension to determine bond quality: failures occurred in the foam, indicating a strong diffusion-bonded interface. The quality of the bond was confirmed by metallographic studies, indicating that this approach, which can also be applied to creating of sandwich with porous cores, is successful.     

Cavitation-Induced erosion of Ti-6Al-4V

Cavitation-induced erosion has been examined in Ti-6A1-4V in the mill annealed, solution-treat and aged, and beta annealed conditions. Weight loss data show only small differences between heat treatments with the solution-treat and aged microstructure exhibiting the lowest weight loss rates. Sequential micrographs of the same specimen area as a function of erosion time show that initial fracture occurs along the α-β interfaces and along crystallographic slip bands in the α-phase. The early stages of erosion are also dependent on the orientation of the Widmanstatten colonies in the beta annealed condition. These observations strongly suggest that fatigue fracture is important, at least in the early stages of the cavitation erosion process. Depression of the softer α- phase also occurs at short exposure times, and this facilitates fracture and removal of the exposed material;i.e., β-phase or tempered martensite. Examination of the eroded surfaces in the later stages where considerable material has been removed shows little evidence of the underlying microstructure, despite the distinct differences in the micro-structures of the samples tested. Formaly Undergraduate Students at Michigan Technological University     

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.     

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.     

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.     

Low temperature creep of Ti-6 Al-4 V

Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded condition. Some aged specimens were tested after pre-straining. Creep stresses ranged from 40 to 90 pct of the aged material yield strength. Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded than in aged material. In general, pre-strains reduced creep, although a strain larger than 10^-3 was needed to do this at the highest creep stress. Activation areas A* were between 10 and 20 b², and thus were similar to tensile results on titanium and its alloys. The microstructural rationale applied to Ti-5 Al-2.5 Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain in this work. Formerly with Sandia Laboratories, Livermore, Calif.     

Producing basal textured Ti-6Al-4V sheet

This study determined the conditions under which Ti-6Al-4V sheet with a strong basal plane texture can be produced on a commercial basis. Texture development during all stages of processing starting with the forged sheet bar was followed using pole figures. The influences of sheet bar forging temperature, rolling temperature, rolling procedures and prior texture were examined. The deformation mechanisms by which a basal plane texture can form are discussed and it is shown that (1120) slip principally on {1010} prism planes can account for it. To demonstrate that texture can be controlled on a production basis, two 1.2 m×2.4 m×1.5 mm sheets from each of six heats of Ti-6Al-4V were rolled with a uniform basal plane texture.     

Superplastic deformation of Ti-6Al-4V alloy

The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (εkTd* ²/D_gbGb³) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.     

Cavitation during hot-torsion testing of Ti-6Al-4V

Hot-torsion testing was used to establish the cavitation behavior of a typical alpha/beta titanium alloy, Ti-6Al-4V, with a colony microstructure, during simple-shear deformation. For this purpose, sections of deformed specimens were examined by optical metallography, and by scanning and orientation-imaging microscopy (OIM). It was found that cavity nucleation occurred along prior beta boundaries as well as at triple points; in particular, most cavities nucleated along boundaries perpendicular to the axial direction of the specimen. Extensive growth was observed for cavities surrounded by both hard and soft orientations, with the soft colonies accommodating more of the imposed strain. At high degrees of deformation, dynamic globularization of the colony microstructure adjacent to the cavities was also observed. In addition, the metallographic observations revealed that the cavities did not grow in an equiaxed mode, but in an elliptical manner. A tensor describing the cavity-growth rate along the axial, radial, and hoop specimen directions was determined using measurements of individual cavity sizes. The cavity-growth behavior in torsion was compared to previous observations from hot-tension tests. This comparison indicated that the rate of cavity growth in shear was approximately one-tenth that in uniaxial tension. This finding is in broad agreement with models predicting the variation of the cavity-growth rate as a function of the ratio of the mean stress to the hydrostatic stress.     

Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy

Synthesis of nanocrystalline Ti-6Al-4V was explored using mechanochemical processing. The reaction mixture was comprised of CaH₂, Mg powder, anhydrous AlCl₃, anhydrous VCl₃, and TiCl₄. The milled powder (reaction product) primarily consisted of nanocrystalline alloy hydride having a composition (Ti-6Al-4V)H_1.942, along with MgCl₂ and CaCl₂ as by-products. Aqueous solutions of nitric acid, sulfuric acid, and 1 pct sodium sulfite were found to be very effective in leaching of the chlorides from the milled powder. The (Ti-6Al-4V)H_1.942 on dehydrogenation at 375°C resulted in nanocrystalline Ti-6Al-4V alloy powder.     

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α₂(Ti₃Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti₃Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength. since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH. KANAY GAZIOGLU, formerly with DFVLR, is deceased.     

Ti-6A14V合金表面改性技术

Ti-6Al4V合金作为一种重要的钛合金,其使用量占到了钛合金总使用量的75％～85％,但其耐磨性差、阻燃性差、疏水疏冰性能差、生物相容性不理想等性能缺陷在一定程度上限制了其在某些领域中的应用。首先对Ti-6Al4V合金在各个领域应用时,其性能缺陷的表现形式及危害进行了概述,然后介绍了目前改善Ti-6Al4V合金性能缺陷所普遍采用的以及具有创新性的表面改性技术,评述了部分表面改性技术的优缺点,最后提出了需对Ti-6Al4V合金表面改性技术进一步研究的方向。     

Excimer laser surface processing of Ti-6Al-4V

We have examined the effect of surface processing in air, using excimer laser light at 248 nm wavelength, on the oxygen content, microstructure, and surface hardness of Ti-6Al-4V. Processing with a single pulse results in the transformation of theα +β material toα′ martensite. Multiple pulse processing results in rapid incorporation of oxygen in the material. Oxygen initially dissolves in the material in the liquid phase. As the concentration exceeds the solid solubility limit during solidification, TiO particles precipitate. In contrast to equilibrium oxidation processes in Ti, only TiO is observed as an oxidation product; further processing results in increased oxygen incorporation and an increased volume fraction of TiO but no other oxides of Ti. The TiO particle size is a function of the oxygen concentration and the number of pulses, with some grain growth occurring after many pulses. The effects of solution hardening by dissolved oxygen and precipitation hardening by the TiO are identifiable as functions of oxygen concentration and mean free path between particles, respectively. A maximum surface hardness almost twice that of electropolished Ti-6Al-4V is observed.     

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.     

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.     

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.     

The kinetics of static globularization of Ti-6Al-4V

The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.     

Creep expansion of porous Ti-6Al-4V sandwich structures

Low-density titanium alloy sandwich structures consisting of a porous core and fully dense face sheets can be produced by consolidating argon gas charged powder compacts followed by not rolling and annealing to expand the gas-filled pores. Little is known about the rate of pore expansion, its dependence upon temperature, and the morphological evolution of the pore shape during expansion. In situ eddy current and laser ultrasonic sensors have been combined with metallographic and texture measurements to measure the relative density, the elastic moduli, and the microstructural evolution of Ti-6Al-4V sandwich panels during the annealing stage of low-density core (LDC) processing. The eddy current data indicated that expansion began during, the heating phase, reached a maximum expansion rate (Δ) of 2 × 10⁻⁵ s⁻¹ at approximately 685 °C, and had almost ceased (Δ < 1 × 10⁻⁶ s⁻¹) after annealing for 4 hours at 920 °C. The elastic moduli were found to decrease with increasing temperature and volume fraction of porosity. The initial (as-rolled) microstructure consisted of a lamellar α + β microstructure with an α-phase lath thickness of 2.0 μm and contained a distribution of oblate-shaped pores with aspect ratios of up to 10. During the expansion process, it recrystallized into an equiaxed α + β structure with an α-phase grain diameter of 7.5 μm with spheroidal pores with aspect ratios of up to 3. The combination of the two sensors was found to enable the in situ determination of both the porous cores relative density and its elastic properties. These are the two material indices that govern the elastic response of a sandwich structure.     

Durability of Fe-Co thin films on Ti-6Al-4V

The durability of Fe-Co thin films bonded on Ti-6Al-4V was studied as a function of layer thickness at ambient temperature. Interface toughness of the thin films was characterized by indentation and analyzed using an interface fracture model. The critical stresses for interface debonding and the fatigue life response of Ti-6Al-4V with and without Fe-Co thin films were evaluated by three-point bend fatigue at a stress ratio R of 0.1. The results indicated that the critical stress for interface debonding increased with decreasing layer thickness according to a critical energy release rate criterion. The Fe-Co thin films did not alter the fatigue life of the Ti-6Al-4V substrate. The presence of microcracks and interface debonds in the thin films did not affect the functionality of the sensor to detect strain via the inverse magneto-elastic effect. The overall durability of the films was in the range needed for practical application of the film as an embedded sensor.     

Creep fracture characteristics of weld-repaired cast Ti-6Al-4V

Constant load creep tests were conducted with cast Ti-6Al-4 V bars in air and vacuum at 315† and 650†C. Some of the bars were notched in their gauge section and heliarc-welded to simulate a weld repair in a casting. Results indicate that the welding has little influence on the time to rupture of the bars. Fractographic analysis of the bars indicates minor differences in the welded and unwelded fracture surfaces. However, these differences do not affect the microstructural influence on the creep and fracture processes. The prior beta grain, size and the alpha platelet size are shown to control the creep fracture of this alloy. These data show that weld repair of titanium castings for creep applications should be acceptable.