Influence of stress state on cavitation during hot working of Ti-6Al-4V

Notched-tension tests were conducted on Ti-6Al-4V samples with a colony-alpha microstructure to determine the effect of the stress state on cavitation during hot working of alpha/beta titanium alloys. The experiments were complemented by finite-element-method analyses to establish the local stress state, strain, and damage factor for different areas within each sample. A critical damage factor, defined by a modified Cockcroft-and-Latham criterion, was found to be applicable for the prediction of cavity initiation for the different notch geometries. Measured cavity growth rates were also correlated to the stress state (i.e., ratio of mean-to-effective stress, σ _M /σ _e ) and compared to predictions of prior models. Model predictions showed reasonable agreement with measurements at low levels of stress triaxiality, but exhibited some deviations at higher values of σ _M /σ _e . The differences were attributed to differences in the properties of the present material and those assumed in deriving the models as well as the neglect of cavity interaction in one of the models. The results were summarized in terms of a processing map, which delineates the initiation of cavities and their size as a function of the stress state and effective strain.     

Effect of strain reversal on the dynamic spheroidization of Ti-6Al-4V during hot deformation

The effect of strain-path reversal on the kinetics of dynamic spheroidization during subtransus hot working was determined for Ti-6Al-4V with a colony α structure. Isothermal torsion tests were conducted at a temperature of 815 °C and a strain rate of 0.001 s⁻¹; strain-path reversals were achieved by applying forward and reverse torsion sequentially. The kinetics of spheroidization were measured as a function of the local (macroscopic) strain for monotonic-deformation, reversed-torsion, and double-reversed-torsion tests. Strain-path reversal led to a reduction in the spheroidization kinetics compared with monotonic deformation for a given total strain. The slower rate of dynamic spheroidization associated with strain-path reversals was ascribed to a reduced rate of sub-boundary formation/lower sub-boundary energies, which drive the boundary splitting process, and less sharp α/β interface curvatures, which control the coarsening process that also contributes to spheroidization.     

Cavitation and failure during hot forging of Ti-6Al-4V

The occurrence of cavity initiation and gross, free-surface fracture during subtransus hot pancake forging of Ti-6Al-4V with a transformed beta (colony) microstructure was established. Cavity initiation mechanisms were one of two distinct types. At temperatures approximately 75 °C or more below the beta transus temperature (T _β ), cavity initiation occurred at relatively low strains in the beta phase lying between the grain-boundary alpha phase and the lamellar colonies. By contrast, at temperatures near the transus (i.e., T≈T _β −25 °C), cavity initiation occurred at much larger strains as a result of microfracture of partially-to-fully globularized alpha phase. Finite element method (FEM) modeling of the pancake forging process revealed that secondary tensile stresses had been developed in the regions that had exhibited cavitation/fracture. The FEM analyses were used to correlate both the cavity initiation and the gross free-surface fracture results to previous observations from uniaxial hot tension tests in which identical damage mechanisms had been observed. The tensile work criterion of Cockcroft and Latham (C+L) gave moderately good (quantitative) correlation between the forging and uniaxial tension behaviors. An alternate comparison based on the Rice and Tracey cavity growth model gave reasonable predictions of free-surface fracture but tended to overestimate the incidence of subsurface cavity initiation.     

An experimental and theoretical investigation of the effect of local colony orientations and misorientation on cavitation during hot working of Ti-6Al-4V

Orientation in aging microscopy was used to determine the effect of local crystallographic texture on the size of cavities formed during hot tension testing at 815 °C and a strain rate of 0.1 s⁻¹ in Ti-6Al-4V with a colony-α microstructure. Cavities nucleated preferentially in the α-β interface along prior-β grain boundaries that were perpendicular to the tension axis, adjacent to colonies with a (hard) c-axis colony orientation parallel to the tension axis. Cavity growth was most rapid at locations where 20 to 40 pct of the area surrounding the cavity also had colonies with soft orientations (with the c-axis inclined to the tension axis). The constraint of the hard orientations and the strongly incompatible anisotropic deformation by prism and basal slip in the softer orientations appeared to facilitate cavity nucleation and growth in these local regions. To interpret these observations, a simple model was developed to quantify the effect of the misorientation between neighboring colonies on the partitioning of strain between them and the development of a local stress triaxiality. Estimates of the local strains and stress states were then incorporated into a plasticity-controlled cavity-growth model to estimate the cavity-growth rate, and thus cavity sizes. Predicted cavity sizes following initiation were very sensitive to the local strain and the hydrostatic stress through its effect on the cavity-growth parameter. The model was successful in differentiating growth rates according to local values of the Taylor factor.     

An analysis of cavity growth during open-die hot forging of Ti-6Al-4V

The effect of local deformation conditions on the cavitation behavior of Ti-6Al-4V during open-die forging was quantified using optical metallography and continuum finite-element-method (FEM) analysis of pancake forgings. The observations were interpreted using meso- and microscopic-scale models, which were used to predict the average cavity size as well the size of the largest cavities. The mesoscale model gave excellent predictions of the average cavity size at different locations of the workpiece, but grossly underestimated the size of the largest cavities by an order of magnitude. On the other hand, the micromechanical model, which accounted for the effect of local colony orientation on cavity growth, was capable of predicting the size of the largest cavities very well. Because the large cavities are the most deleterious with respect to subsequent processing and service performance, it was concluded that micromechanical models should be used to design primary hot working processes in order to minimize the size and number of such cavities.     

The effect of processing on plastic anisotropy of Ti-6Al-4V

The plastic anisotropy of Ti-6Al-4 V was examined after various thermomechanical treatments, including heat treating, rolling, and forging. Processing temperatures were varied from room temperature to 1340 K. The anisotropy, in terms of the strain ratioR, was measured by post-yield strain gages in the three principal directions, and results were correlated with the (0002) pole figures for each thermomechanical treatment. The plastic strain anisotropy, which was consistent with the basal pole orientation, was found to depend upon both the method and the temperature of mechanical working. The greatestR values occurred for the cold-rolled material where the basal poles rotate to within 15 deg from the sheet normal. In addition,R is not constant under uniaxial tension, but generally increases with the amount of plastic strain. The variation ofR with uniaxial strain depends upon the forming temperature, and the largest changes occur in samples that were rolled at room temperatures.     

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.     

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.     

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.     

Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V

Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T∼815 °C to 955 °C, ∼0.1 s⁻¹). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.     

Microstructure evolution during alpha-beta heat treatment of Ti-6Al-4V

A framework for the prediction and control of microstructure evolution during heat treatment of wrought alpha/beta titanium alloys in the two-phase field was established via carefully controlled induction heating trials on Ti-6Al-4V and accompanying mathematical modeling based on diffusion-controlled growth. Induction heat treatment consisted of heating to and soaking at a peak temperature T _p=955 °C, controlled cooling at a fixed rate of 11 °C/min, 42 °C/min, or 194 °C/min to a variety of temperatures, and final water quenching. Post-heat-treatment metallography and quantitative image analysis were used to determine the volume fraction of primary (globular) alpha and the nucleation sites/growth behavior of the secondary (platelet) alpha formed during cooling. The growth of the primary alpha during cooling was modeled using an exact solution of the diffusion equation which incorporated diffusion coefficients with a thermodynamic correction for the specific composition of the program material and which took into account the large supersaturations that developed during the heat-treatment process. Agreement between measurements and model predictions was excellent. The model was also used to establish a criterion for describing the initiation and growth of secondary alpha as a function of supersaturation, diffusivity, and cooling rate. The efficacy of the modeling approach was validated by additional heat treatment trials using a peak temperature of 982 °C.     

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.     

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.     

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.