Room temperature creep of Ti-6AI-4V

Recent investigations have shown both that Ti-6A1-4V does and does not exhibit room temperature creep at low stress. In the present investigation three different microstructures (α β anneal, recrystallization anneal and β anneal) of Ti-6A1-4V were examined under dead weight torsional loading. The loading sequence was forward, reverse and second forward loading. It was concluded that even at a stress level well below the yield stress the alloy exhibits creep in forward loading and increased creep strain in reverse loading and second forward loading. Furthermore the rate of creep at constant stress was different in different microstructures of the alloy; the maximum creep rate occurred with the recrystallization anneal and the minimum creep rate occurred with the β anneal. At the maximum stress in forward loading there appeared to be a change in creep mechanism.     

Microstructural influences on superplasticity in Ti-6AI-4V

The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.     

Fatigue behavior of SiC reinforced Ti(6AI-4V) at 650 °C

Axial, low cycle fatigue properties of 25 and 44 fiber vol pct SiC/Ti(6Al-4V) composites, measured at 650 °C, were compared with the fatigue properties of unreinforced Ti(6Al-4V) at the same temperature. A prior study of the fatigue behavior of this composite system at room temperature indicated that the SiC fiber reinforcement did not provide the anticipated improvement of fatigue resistance of this alloy. At 650 °C, the composite fatigue properties degraded somewhat from those at room temperature. However, these properties degraded more for the unreinforced matrix at 650 °C with the result that the composite fatigue strength was two to three times the fatigue strength of the matrix alloy. The reasons for this reversal are discussed in terms of crack initiation at broken fibers and residual matrix stresses.     

Elevated temperature plastic anisotropy of Ti-6AI-4V plate

The influence of temperature on the planar and normal anisotropy parameters (ΔR andR, respectively) for mill annealed, duplex annealed, and cross rolled Ti-6A1-4V plate was investigated from 25 to 704°C (77 to 1300°F). Both parameters were assessed in terms of the plastic strain ratio (R), ratio of width to thickness strain at maximum load (~0.065 longitudinal strain) in tensile specimens oriented at 0, 45, and 90 deg to the rolling direction, and correlated with texture and microstructure. With increasing temperature, plates characterized by alpha deformation type basal plane textures exhibited significantly larger anisotropy variations than plate with a beta transformation type texture. This behavior was related to the degree of textural randomness and to a thermally induced transition in primary deformation mode from twinning to slip. Depending on texture, the results strongly suggest that working temperature may be utilized advantageously to alter the plastic anisotropy of Ti-6A1-4V plate for improved formability in a given fabrication operation.     

多孔TC4钛合金吸氢规律

利用管式氢处理炉采取固态气相渗氢法进行置氢实验,以研究多孔TC4钛合金置氢过程中吸氢量随置氢温度、置氢时间和相对密度的变化规律,并建立了相应的数学模型.结果表明:当多孔钛合金的相对密度较低时,吸氢量随置氢温度的升高而增加;当相对密度较高时,吸氢量与置氢温度的关系遵循Sievert's定律,与致密钛合金的吸氢特性一致;多孔钛合金随置氢时间的延长,吸氢量增加;随着多孔钛合金相对密度的增加,吸氢量降低.     

Enhanced superplasticity and strength in modified Ti-6AI-4V alloys

Although Ti-6A1-4V displays extensive superplasticity at 1200 K, lower superplastic forming temperatures are desirable. A study was conducted with the goal of modifying the composition of the Ti-6A1-4V alloy to lower the optimum superplastic forming temperature. Computer modeling results and previous experimental data suggested that additions to Ti-6A1-4V of beta-stabilizing elements which have high diffusivity in the beta-phase would permit lower superplastic forming temperatures. A series of modified alloys with 2 wt pct additions of Fe, Co, and Ni was prepared for experimental evaluation. The modified alloys achieved desirable microstructures for superplasticity at 1088 K,i.e., the grain size was approximately 5 μm and roughly equal volume fractions of the alpha- and beta-phases were present at the deformation temperature. The superplastic properties of the modified alloys were measured at 1088 K and 1144 K. The modified alloys produced values of flow stress, strain rate sensitivity, and total elongation at 1088 K approaching those of the base Ti-6A1-4V alloy at its standard superplastic forming temperature of 1200 K. In addition to lowering the superplastic forming temperature, the β-stabilizing additions also increased room temperature strength levels above those normally found for Ti-6A1-4V. Based on the room temperature and elevated temperature tensile properties, addition of selected beta-stabilizing elements to Ti-6A1-4V simultaneously raises resistance to deformation at room temperature and lowers resistance to deformation at elevated temperatures. This reversal in behavior is explained by considering the effect of beta-stabilizer additions on the deformation mechanisms at room temperature and at elevated temperatures.     

Surface hardening of Ti-6AI-4V alloy by electrochemical hydrogenation

Surface hardening of Ti-6A1-4V alloy can be performed by electrolytic charging in acid and basic solutions, with or without subsequent solution treatment, followed by dehydrogenation to obtain equiaxed α grains in a transformed β matrix. Surface hardnesses of the processed specimens are better than that of the mill-annealed specimen. The depth of the hardened layer depends on the processing parameters.     

High cycle fatigue of weld repaired cast Ti-6AI-4V

In order to determine the effects of weld repair on fatigue life of titanium-6Al-4V castings, a series of specimens was exposed to variations in heat treatment, weld procedure, HIP cycle, cooling rate, and surface finish. The results indicate that weld repair is not detrimental to HCF properties as fatigue cracks were located primarily in the base metal. Fine surface finish and large colony size are the primary variables improving the fatigue life. The fusion zone resisted fatigue crack initiation due to a basketweave morphology and thin grain boundary alpha. Multipass welds were shown not to affect fatigue life when compared with single pass welds. A secondary HIP treatment was not detrimental to fatigue properties, but was found to be unnecessary.     

Characteristics of sustained-load cracking and hydrogen effects in Ti-6AI-4V

Sustained-load cracking (SLC) characteristics of Ti-6A1-4V are significantly influenced by 1) exposure temperature, 2) hydrogen content, and 3) basal plane crystallographic texture. The stress intensity applied to precracked specimens did not play a major role in affecting crack initiation or crack growth rate except that there would appear to be a required minimum level. Increasing hydrogen content raises the temperature at which SLC occurs and increases crack growth rate. A model for SLC has been proposed based on hydride precipitation at the crack tip and subsequent crack propagation by creep, cleavage, andJor interfacial separation at the hydrideJmatrix interface. Formerly with the Boeing Commercial Airplane Co., is presently an Engineering Consultant.     

Void formation,void growth and tensile fracture in Ti-6AI-4V

Metallurgical and Materials Transactions A - The influence of microstructure on void formation, void growth and tensile fracture was investigated for the Ti-6A1-4V alloy, aged to yield strengths of...     

The micromechanics of fatigue crack growth at 25 °C in Ti-6Al-4V reinforced with SCS-6 fibers

Micromechanics parameters for fatigue cracks growing perpendicular to fibers were measured for the center-notched specimen geometry. Fiber displacements, measured through small port holes in the matrix made by electropolishing, were used to determine fiber stresses, which ranged from 1.1 to 4 GPa. Crack opening displacements at maximum load and residual crack opening displacements at minimum load were measured. Matrix was removed along the crack flanks after completion of the tests to reveal the extent and nature of the fiber damage. Analyses were made of these parameters, and it was found possible to link the extent of fiber debonding to residual COD and the shear stress for fiber sliding to COD. Measured experimental parameters were used to compute crack growth rates using a well-known fracture mechanics model for fiber bridging tailored to these experiments.     

The influence of stress trlaxiality on the damage mechanisms in an equiaxedα/β Ti-6AI-4V alloy

The influence of the stress triaxiality on void formation, void growth, and fracture was investigated for an equiaxed Ti-6A1-4V alloy. Void nucleation in theα phase was found to occur for a critical value of macroscopic plastic strain, whereas void nucleation at theα/β interface also depends on triaxiality. Under low triaxiality and important plastic strain, voids appear and grow in the area where the microshear bands develop, with an angle close to 45 deg to the stress axis in theα particles. In contrast, with high triaxiality, voids nucleate preferably at theα/β interfaces and grow perpendicular to the stress axis by a cleavage mechanism. In a middle range of triaxiality and plastic strain, voids nucleate inα because of the sufficient plastic strain and also at theαβ interfaces because of the sufficient triaxiality(X). Void growth occurs with an angle of 60 deg to the stress axis, sinceX is not high enough to create cleavage andε _p is high enough to provide a ductile growth. Two types of fracture were identified and reported on a fracture map: under low triaxiality, failure appears by plastic instability, whereas for high triaxiality, the instability is induced by a void-growth process discussed with the help of Rice and Tracey’s approach.     

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.     

Effect of hydrogen on creep behavior of Ti-6AI-4V alloy at room temperature

Metallurgical and Materials Transactions A - Room temperature creep was investigated for a commercial Ti-6Al-4V alloy containing 80 ppm to 720 ppm hydrogen. It was shown that dissolved hydrogen...     

Effect of hydrogen on creep behavior of Ti-6AI-4V alloy at room temperature

Room temperature creep was investigated for a commercial Ti-6Al-4V alloy containing 80 ppm to 720 ppm hydrogen. It was shown that dissolved hydrogen promoted the creep of this alloy at room temperature, markedly increasing both the creep strain and rate in the primary stage. The results indicated that dissolved hydrogen does not change the primary creep mechanism, but rather increases mobility of glide dislocations. It is suggested that dissolved hydrogen atoms soften the alloy at the beginning of deformation due to this increase in dislocation mobility. The initial deformation results in hydrogen build-up near the subboundaries due to the “sweeping” effect of the glide dislocations, thereby causing hydrogen embrittlement in a later stage of deformation. Formerly Visiting Scholar at the University of Delaware     

Surface residual stresses,surface topography and the fatigue behavior of Ti-6AI-4V

Fatigue tests have been conducted on Ti-6Al-4V from 293 to 589 K to determine the influence of surface residual stresses and surface topography on low and high cycle fatigue properties. Four types of machined surfaces as well as shot peened surfaces were included in the investigation. It was found that surface residual stresses play a key role in controlling the development of microcracks and, therefore, overall fatigue lives at both room and elevated temperature. X-ray measurement of the stability of surface residual stresses under thermal activation and/or cyclic loading demonstrated that, for the conditions studied, cyclic loading was primarily responsible for residual stress decay. In addition, the magnitude of the decay was dependent on the relationship between the sign of the residual stress and the sign of the imposed mean strain. Finally, it was demonstrated that the sharpness of machining grooves is more important than their depth in controlling fatigue resistance. The work was performed when all authors were affiliated with Pratt and Whitney Aircraft, Middletown, Connecticut.     

The importance of surface layer on fatigue behavior of a Ti- 6AI- 4V alloy

The effects of precycling and surface removal on the fatigue life and fatigue limit of a Ti-6A1-4V alloy were investigated. It was shown that both the fatigue life and fatigue limit were strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. Cycling with stress amplitude below the fatigue limit after precycling showed a dependence of the logarithmic number of cycles to failure on the fraction of prefatigue damage. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation, principally restricted to a surface layer of less than 100 μm. Depending on the severity of precycling and on the magnitude of the applied cyclic stress, the fatigue damage could be either partially or totally eliminated by surface removal. The α/β interphase region of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation.     

高温氧化对钛合金超塑性能的影响

研究了Ti-6Al-4V合金在800℃、850℃和900℃高温条件下进行拉伸试验时空气氧化对超塑性能的影响。通过光学显微镜、扫描电镜和X射线衍射分析了该钛合金氧化层的微观形貌和成分组成，并研究其在高温拉伸下的氧化机理。结果表明，高温氧化导致该合金在高温拉伸过程中表面产生氧化层，而在拉伸应力作用下氧化层断裂并向基体扩展，从而严重降低了Ti-6Al-4V合金的超塑性，但不会影响其抗拉强度及屈服强度。