Influence of foreign particles on fatigue behavior of Ti-6Al-4V prealloyed powder compacts

The effect of three types of contaminants on the fatigue life of prealloyed Plasma Rotating Electrode Process (PREP) Ti-6A1-4V hot isostatically pressed (HIP’d) powder compacts was studied. Ultraclean powder was seeded separately with SiO₂, A1₂O₃, and 316 stainless steel (SS) contaminants of 50 μm, 150 μ, and 350 μm nominal size, a total of nine conditions. Seeded compacts were fatigue tested at room temperature and the results were compared to those of an unseeded baseline compact. It was found that a substantial loss in fatigue life occurred even at the smallest seeded contaminants used in this work. Angular nonmetallic SiO₂ and A1₂O₃ contaminants were found to be more detrimental to fatigue strength than spherical metallic 316 SS contaminants of the same size range also indicating a shape effect. The loss of fatigue life suggests that at the high stress levels there is almost no crack initiation period and fatigue lives are controlled mainly by crack growth.     

Microporosity in hot isostatically pressed Ti-6Al-4V powder compacts

The existence of micropores of less than 100 nm size is reported in hot isostatically pressed Ti-6Al-4V compacts that were expected to be fully dense. The micropores are in a film-like arrangement which traverses individual alpha grains. Several possible origins of the micropores are discussed. Of these, the dissolution of oxide film on prior powder particles appears most probable. Formerly Graduate Student at Case Western Reserve University, Cleveland, OH 44106.     

Influence of thermomechanical processing on low cycle fatigue of prealloyed Ti-6AI-4V powder compacts

A wide range of microstructures was generated using various thermomechanical processing sequences in Ti-6A1-4V Rotating Electrode Process (REP) powder compacts of low contaminant content. Low cycle fatigue results were found to be superior to those in higher contaminant compacts tested in a previous program. All microstructural groups showed fatigue strengths equivalent to those found in wrought alloy, with the beta-annealed condition being lowest as expected. Alpha + beta work and solution treatment resulted in an excellent fatigue strength of 875 MN/m² (127 ksi) at 10⁵ cycles; 85 pct of the UTS. In the five conditions tested, the fatigue strength increased with increasing tensile strength, decreasing grain size, and increasing volume fraction of low aspect ratio primary alpha. Most crack initiation sites were observed at the specimen surface. Only alpha + beta worked and solution-treated material exhibited subsurface initiations, none of which was associated with any defect or with a lower fatigue life. Although compacts contained some tungsten particles, in no case were they associated with crack initiation sites, indicating that they were innocuous in the conditions evaluated.     

Hydrogen-enhanced fatigue crack growth in Ti-6Al-4V ELI weldments

The effects of a hydrogen environment on the fatigue crack growth rates in Ti-6A1-4V ELI (STA) and weld material were determined in the temperature range of ambient to -200°F. The hydrogen environment resulted in an acceleration of the crack growth rate and a change in the fracture mode for both materials in the temperature range of ambient to -100°F. At -200°F, there was no significant difference between the crack growth rates obtained in helium and hydrogen gas. The degree of hydrogén-enhanced crack growth was found to be dependent on the crack tip stress-intensity range, temperature, and microstructure of the material. The data is consistent with an embrittlement mechanism involving hydrogen diffusing ahead of the crack front.     

Fatigue crack growth mechanics for Ti-6Al-4V (RA) in vacuum and humid air

The effects of environment and cyclic stress intensity factor on crack opening displacement and crack tip strain have been measured, and discontinuous crack growth has been observed directly under high resolution conditions in the Scanning Electron Microscope. This information is used in a crack tip failure model together with cyclic stress-strain, low cycle fatigue, and microstructural characteristics of the material to derive the expected crack growth increment. Agreement with measured striation spacings is reasonable.     

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.     

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.     

Effect of isothermal forging on microstructure and fatigue behavior of blended elemental Ti-6Al-4V powder compacts

The effect of isothermal hot forging (IHF) on microstructure, pore closure, and tensile and fatigue properties of Ti-6A1-4V blended elemental cold pressed and sintered powder compacts was investigated. Two types of sponge fines were used: (a) high chloride produced by the Hunter sodium reduction process (HP) and (b) low chloride produced by the electrolytic process (EP). The as-sintered HP compacts were 99 pct dense while the EP compacts were only 92 pct dense. All sintered preforms were isothermally hot forged below the beta transus temperature and reached almost full density. The microstructure of the HP forged compacts consisted of fine equiaxed alpha, while the EP forged compacts exhibited a coarse lenticular alpha structure after 30 pct reduction and a partially recrystallized structure after 68 pct reduction. It was found that EP compacts forged to a 30 pct reduction exhibited a low fatigue limit of 172 MPa (25 ksi), since the lenticular alpha morphology and the residual porosity resulted in premature fatigue crack initiation. On the other hand, a higher fatigue strength of 485 MPa (70 ksi) was obtained for EP compacts forged to a 78 pct reduction due to the mixed equiaxed/lenticular alpha morphology as well as removal of stress concentration features such as interparticle pore interfaces.     

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     

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.     

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.     

Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions

Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions.     

The effect of air and vacuum environments on fatigue crack growth rates in Ti-6Al-4V

Fatigue crack propagation studies in vacua of 1.33 mN m^-2, on Ti-6 A1-4V, at growth rates of 10^-7 to 10^-4 mm/cycle have shown that a threshold for growth exists at ‡K values of 6.3 to 7.6 MN m^-3/2. The value of the threshold level is microstructure dependent, but growth above this value was structure insensitive according to both growth rates and fracture surface observations. Some slow (≈ 10^-8 mm/cycle) crack extension was observed below the threshold values but prolonged cycling reduced the growth rate to a vanishingly small level. Fracture surface observations indicated that growth in this region was microstructure sensitive. Comparison with previously performed air work on the same material showed that while structure insensitive growth rates in vacuum were slower than those in air by a factor of 3 to 4, the low ‡K value structure sensitive rates were slower than the air ones by at least three orders of magnitude. A hypothesis is proposed to explain this in terms of a propagation mechanism for the structure sensitive mode of fatigue crack growth. Research Fellow, Department of Physical Metallurgy, University of Birmingham, England     

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstätten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane.