A Self-Consistent Approach for Modeling the Flow Behavior of the Alpha and Beta Phases in Ti-6Al-4V

The flow behavior of the α and β phases in Ti-6Al-4V was interpreted in the context of a self-consistent modeling formalism. For this purpose, high-temperature compression tests were conducted at various temperatures for a single-phase α alloy (Ti-7Al-1.5V), a variety of near-β alloys, and the two-phase alloy Ti-6Al-4V, each with an equiaxed microstructure. The flow behavior of the α phase in Ti-6Al-4V was deduced from the experimental results of the single-phase α alloy. The flow behavior of the β phase, which was predicted by using the self-consistent approach and the measured flow behaviors of Ti-6Al-4V and Ti-7Al-1.5V, showed good agreement with direct measurements of the various near-β alloys. From these results, it was shown that the strength of the α phase is approximately three times higher than that of the β phase at temperatures between 1088 K and 1223 K (815 °C and 950 °C). It was also concluded that the relative strain rates in the two phases varies significantly with temperature. The usefulness of the approach was confirmed by comparing the predicted and measured flow stresses for other Ti-6Al-4V and near-α alloys.     

The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys

The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests, and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy was not observed. Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology.     

Effect of hydrogen on fracture and inert-environment sustained load cracking resistance of α- β titanium alloys

The fracture toughness and resistance to inert-environment sustained load crack propagation of α-β titanium alloys are usually reduced by increased hydrogen contents. The range of hydrogen contents over which either fracture toughness or threshold stress intensity for sustained load cracking was observed to decrease with hydrogen content is small (0 to 50 ppm) for Ti-6 Al-4 V, but further increases in hydrogen content can cause an increase in cracking rates. Sustained load crack propagation is characterized by a mixture of microvoid coalescence with cleavage, usually on a plane 12 to 15 deg from {0001} of the hep α phase with some {000l} cleavage. Cleavage apparently initiates ahead of the main crack front within a grains, usually near apparent α-β interfaces. Atmospheric moisture is inert with respect to sustained load cracking, that is, it does not cause stress corrosion cracking. Sustained load cracking was demonstrated in Ti-8 Al-1 Mo-1 V, Ti-6 Al-6 V-2 Sn, and several grades of Ti-6 Al-4 V.     

Comparison of the fatigue and fracture of α+β and β titanium alloys

The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical processing, maximum K _Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue crack initiation and propagation resistance (especially against small fatigue cracks).     

Surface cracking in α- β titanium alloys under unidirectional loading

Observations have been made of cracking which develops after small plastic strains in the Ti-6Al-2Sn-4Zr-6Mo and Ti-6A1-4V alloys at α-@#@ β interfaces, and a long slip band in α. Hydrogen and surface stresses and heat treat condition,i.e., solution treated or solution treated and aged have been ruled out. Cracking is attributed to intense slip commencing at the α-@#@ β interfaces, and progressing into the α, or at martensite-matrix interfaces.     

Stress induced hydrogen redistribution in commercial titanium alloys

The change in hydrogen concentration as a function of applied strain has been studied in commercial titanium alloys that included threeβ-phase, twoα-phase, and an (α + β)-phase Ti-6Al-4V alloy with differingα/β morphologies.Insitu measurements were made using a nondestructive nuclear technique on samples for which uniaxial compressive and tensile stresses were applied by four-point bending. Theβ-phase alloys exhibited hydrogen redistribution under an elastic stress gradient, but no further change was discernible accompanying plastic deformation. The extent of hydrogen concentration change for theβ-phase alloys was of the order of 4 to 6 pct for a 620 MPa stress gradient. This is less than would be predicted based on available data for the partial molal volume of hydrogen. Diffusion coefficients in a stress gradient were also determined and are consistent with those measured inβ-phase titanium at elevated and room temperatures. Within the experimental sensitivities there was no evidence of hydrogen redistribution with applied stress for theα-phase and Ti-6Al-4V alloys.     

The Effect of Friction Stir Processing on the Mechanical Properties of Investment Cast and Hot Isostatically Pressed Ti-6Al-4V

Friction-stir (FS) processing was used to modify the coarse, fully lamellar microstructure of investment cast and hot isostatically pressed (HIP’ed) Ti-6Al-4V. The effect of FS processing on mechanical properties was investigated using microtensile and four-point bend fatigue testing. The tensile results showed a typical microstructure dependence where yield strength and ultimate tensile strength both increased with decreasing slip length. Depending on the processing parameters, fatigue strength at 10⁷ cycles was increased by 20 pct or 60 pct over that of the investment cast and HIP’ed base material. These improvements have been verified with a statistically significant number of tests. The results have been discussed in terms of the resistance of each microstructure fatigue crack initiation and small crack propagation. For comparison, a limited number of fatigue tests was performed on α + β forged Ti-6Al-4V with varying primary α volume fraction and also on investment cast material heat treated to produce a bi-lamellar condition.     

Effect of Hypoeutectic Boron Addition on the <Emphasis Type="Italic">β</Emphasis> Transus of Ti-6Al-4V Alloy

In the present study, the β transus of boron-modified Ti-6Al-4V alloy was found to be almost equivalent to that of the normal alloy, although there is a difference in interstitial element content large enough to produce significant change. Compositional analysis confirms the scavenging ability of the boride particles that are present in the microstructure toward the interstitial elements. This factor can successfully retard the α → β phase transformation locally and increase the overall β transus of boron-added material.     

Differential scanning calorimetry study and computer modeling of <Emphasis Type="Italic">β</Emphasis> ⇒ <Emphasis Type="Italic">α</Emphasis> phase transformation in a Ti-6Al-4V alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.     

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.     

Corrosion Behavior of Friction Stir-Processed and Gas Tungsten Arc-Welded Ti-6Al-4V

The corrosion behavior of the investment-cast Ti-6Al-4V alloy in 5-pct HCl solution was investigated after gas tungsten arc welding and friction stir (FS) processing. The FS-processed samples exhibited superior corrosion behavior compared with the base metal and the arc-welded samples. The inferior corrosion resistance of the arc weldment was attributed to the acicular α and β microstructure and the alloying element partitioning between the phases. This was confirmed by scanning electron microscopy evaluations of the surface of specimens that had been immersed 50 hours in 20-pct HCl at 308 K (35 °C). In addition, the results indicated that vanadium as an alloying element has a detrimental effect on the corrosion performance of Ti-6Al-4V alloy in an HCl solution.     

Composition dependence of young’s modulus in Ti-V, Ti-Nb, and Ti-V-Sn alloys

The Young’s modulus of Ti-V and Ti-V-Sn alloys quenched from the β-phase region after solution treatment and cold rolling was investigated in relation to alloy compositions, microstructures, and constituent phases. The composition dependence of the Young’s modulus for quenched Ti-V binary alloys shows two minima of 69 GPa at Ti-10 mass pct V and 72 GPa at Ti-26 mass pct V. Between the two compositions, athermalω or stress-induced ω is introduced in retainedβ phase and increases Young’s modulus. That is, a low Young’s modulus is attained unless alloys undergoω transformation. In Ti-5 and -8 mass pct V, which under goα′ (hcp) martensitic transformation on quenching, the Young’s modulus further decreases by cold rolling, which can be reasonably explained by the formation ofα′ rolling texture. Comparing Young’s modulus in Ti-V binary alloy with that in Ti-Nb binary alloy, it is found that Young’s modulus is remarkably increased by athermal- or stress inducedω phase, and it shows a minimum when both martensitic andω transformation are suppressed during quenching in metastableβ alloys. The Sn addition to Ti-V binary alloy retards or suppresses athermal and stress-inducedω transformation, thereby decreasing Young’s modulus. Young’s modulus exhibits minimum values of 51 GPa in quenched (Ti-12 pct V)-2 pct Sn and of 57 GPa in cold-rolled (Ti-12 pct V)-6 pct Sn.     

The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V

The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and 2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities of titanium, cobalt, or nickel in the β phase are taken into account.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

Building a continuous cooling transformation diagram of β-CEZ alloy by metallography and electrical resistivity measurements

The phase transformations of Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe (β-CEZ) have been studied during continuous cooling after β-solution treatment. For this purpose, electrical resistivity measurements and metallographical examinations have been carried out, and the continuous cooling transformation (CCT) diagram of β-CEZ alloy has been plotted. The different kinds of β-phase decomposition schemes in β-CEZ alloy during continuous cooling have been investigated in detail. Two main morphological features of the α/β structure are involved, depending on the cooling rate: the basket-weave and the colony structures are observed for high and low cooling rates, respectively. For the intermediate cooling rates, the two morphologies coexist. Finally, a generalized scheme of the β → β + α transformation sequences during continuous cooling is presented.     

Probabilistic micromechanical modeling of fatigue-life variability in an <Emphasis Type="Italic">α+β</Emphasis> Ti alloy

The fatigue-life variability in an α+β Ti alloy (Ti-6Al-4V) has been examined through a probabilistic micromechanical model that treats the crack-initiation and growth processes at the grain-size level. First, a physics-based crack-initiation model is described. This is followed by a summary of a physics-based fatigue-crack-growth model. The combined model is applied to predict the variability of crack initiation and growth lives due to microstructural variations in Ti-6Al-4V. Finally, possible fatigue mechanisms or scenarios that can lead to the worst-case fatigue life are elucidated via probabilistic modeling of the fatigue-crack-initiation process, the driving force of the grain-sized cracks, as well as the intrinsic (closure-free) threshold and the closure-affected threshold of the small cracks. In the absence of preexisting cracks, the worst-case total fatigue life is obtained when two conditions are met: (1) the crack size at initiation is on the order of 1 to 2 times the grain size, and (2) the driving force (applied ΔK) exceeds the intrinsic threshold of the small cracks. The probabilistic results are also used to elucidate the conditions for the occurrence of dual fatigue limits in high-cycle fatigue (HCF) or giga-cycle fatigue.     

Textural changes during β→α and α→β→α transformations in a near-α titanium alloy

The near-α titanium alloy Ti-6Al-5Zr-0.5Mo-0.25Si has been rolled in the β- and (α+β)-phase fields. Texture studies have been performed on each of these materials in the as-rolled condition after air cooling from the finish rolling pass, with a view to examining the transformation texture β/(α+β)→α. One of the materials from each of the β and (α+β) rolled conditions has been heat treated in the β-phase field and air cooled (AC) to α phase in order to study the nature of the α → β → α transformation texture. Results indicate that transformation textures of the α phase are significantly different for both the β as well as the (α+β) rolling conditions. Heat treatment of secondary (transformed) α in the β-phase field and its further cooling to α phase leads to relatively weak texture for the β rolled materials. The results have been discussed in relation to the microstructural features and consequent variant selections and have been correlated with those observed in titanium and its alloys.     

In-situ measurement of continuous cooling β → α transformation behavior of CP-Ti

The β → α transformation kinetics of CP-Ti during continuous cooling was measured using a fully computer-controlled resistivity-temperature real-time measurement apparatus. Unlike the pure Ti case, the massive transformation occurs at medium cooling rates, about 90 °C/s to 600 °C/s. Its start temperature is estimated to be about 890 °C, which is close to the T ₀ temperature. The reason for the appearance of massive transformation in CP-Ti is because CP-Ti contains a significant amount of Fe as an impurity, which leads to the T ₀(β → α) vs composition curve being parallel to the composition axis due to its retrograde solubility. The martensitic transformation starts to occur at a cooling rate of about 500 °C/s, which is much slower than that (about 3000 °C/s) reported in a pure Ti case. This retardation effect of martensitic transformation is also believed to arise from the presence of Fe in CP-Ti, which is a strong β stabilizer.