The influence of microstructure and strain rate on the compressive deformation behavior of Ti-6Al-4V

This article reports on a study of deformation of Ti-6Al-4V in compression. In particular, two different microstructures, the equiaxed microstructure and the Widmanstätten microstructure, were generated from the same parent material and their properties were measured. The results show that at small strains, the mechanical response of samples with these microstructures is similar. The yield strength and the flow stress at a 0.05 true strain have similar values; these increase with increasing strain rate over the range of 0.1 to 1000 s^?1. However, samples with the Widmanstätten microstructure failed at a smaller strain than their counterparts with the equiaxed microstructure, and this difference increased with increasing strain rate. Examination of cross sections of samples deformed to different levels of strain showed that the deformation was inhomogeneous. As the sample barreled, the deformation built up on the surfaces of two cones of material whose apices met in the center of the sample. Cracks formed in the corners of the samples and propagated in toward the center. In samples with the equiaxed microstructure, short cracks and voids formed, but they were usually blunted at the grain boundaries. Long cracks were only observed immediately before failure. In samples with the Widmanstätten microstructure, cracks could grow within the laths more easily, and, as a result, longer cracks formed at lower strains. We propose that this difference leads to the differences in the failure strains for these two microstructures. Finally, examination of data in the literature, along with our own results, indicates that the interstitial content plays an important role in determining the yield stress of the material.     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms

Plastic flow behavior and microstructure evolution during hot working and heat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engineered Net Shaping (LENS?), process were established. To this end, isothermal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hipped) condition. The starting microstructures consisted of columnar grains with fine or coarse Widmanstätten (basketweave) alpha platelets. At subtransus temperatures, the flow curves of both microstructural conditions exhibited a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the kinetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti-6Al-4V with comparable alpha-platelet thicknesses. During supertransus heat treatment, moderately fine beta-grain microstructures were developed in samples that had been predeformed below the beta transus. Such a heat treatment for samples previously deformed above the transus gave rise to a nonuniform distribution of coarse beta grains, an effect attributed to critical grain growth.     

The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part I. Widmanstätten structures

An alpha + beta Ti-6Al-2Nb-lTa-0.8Mo alloy with an initial Widmanstätten structure was thermally treated to produce a wide range of microstructures. The effects of individual microstructural parameters on deformation behavior and mechanical properties were investigated. The results show that the Widmanstätten colony boundaries are major barriers to slip. However, the slip distance can be decreased to a distance equal to the thickness of acicular alpha by transforming the beta phase in the Widmanstätten structure to martensite by quenching from 950°C. The decrease in slip distance is accompanied by a 25 pct increase in yield strength with no loss in ductility. A large decrease in ductility occurs after excursions above the beta-transus. The development of both equiaxed beta grains during heating in the beta phase field and continuous grain boundary alpha during cooling in the alpha + beta phase field leads to strain localization along prior beta grain boundaries.     

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s^-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.     

The effect of microstructure and deformation behavior on the hot ductility of Ti-6Al-2Nb-1Ta-0.8Mo

Hot ductility of the alloy Ti-6Al-2Nb-lTa-0.8Mo has been correlated with microstructure and fracture behavior. Low hot ductility was found to be associated with strain localization within the grain boundary alpha phase, producing void formation along the prior-beta grain boundaries and inter-granular fracture. Microstructural features that appear to be critical to the strain localization process are beta grain shape and alpha phase morphology. For the case of Widmanstätten + grain boundary alpha phase morphologies, equiaxed prior-beta grains formed by annealing above the beta transus are required to produce significant strain localization. For the beta processed structure with elongated beta grains due to working above the beta transus temperature, the orientation of the grain boundary alpha phase limits strain localization due to low resolved shear stress. The martensitic Widmanst?ten alpha prime structure formed by quenching from above the beta transus temperature rapidly forms grain boundary alpha upon reheating to temperatures high in the alpha + beta phase field. This results in strain localization in the grain boundary regions in an apparently similar manner to that observed in the Widmanstätten + grain boundary alpha phase morphologies with equiaxed prior-beta grains.     

The effect of matrix strength on void nucleation and growth in a widmanstätten alpha-beta titanium alloy,CORONA-5

This paper presents the results of a study of the effect of matrix yield strength, at a constant Widmanstätten α microstructure, on the nucleation and growth of voids in an α-β titanium alloy, CORONA-5, Ti-5Al-4.5Mo-l.5Cr. Four microstructures with a retained β matrix and involving coarse or fine Widmanstätten α particles in coarse or fine β grains were used in different heat treatment conditions resulting in yield strengths from 765 to 1018 MPa. Void nucleation occurred atα/α boundaries, grain boundary α/matrix interfaces, α twin/matrix interfaces, and α twin/untwin boundaries. The void nucleation strain varied from 0 to 0.26 and was a function of both microstructure and yield strength. Void growth rates increased with yield strength for all microstructures except for those containing coarse α which decreased with yield strength. Intense shear was observed in colonies of fine α structures and was considered to be the cause of the observed rapid void growth rates in the fine β+ fine α microstructures. Surface cracking occurred in several aged conditions during straining. This cracking was attributed to strain concentration in α.     

Microstructure and properties of Ti-17 powder compact prepared through powder sintering and isothermal forging

Abstract

Powder sintering and dual isothermal forging were utilised to prepare titanium alloy. Owing to the disturbance effect of residual pores during sintering, the microstructures of two sintered Ti-17 powder compacts prepared with about 80–150 and ?150 mesh powders were composed of large residual pores and low aspect ratio of α platelets and small residual pores and high aspect ratio of α platelets respectively. Residual pores can be closed during the first isothermal forging above the β transus due to the excellent plastic deformation capability of the β phase. The microstructure can be effectively broken during the second isothermal forging below the β transus. Isothermal forging with 10^?2 and 10^?4 s^?1 strain rates can obtain uniform microstructure, whereas isothermal forging with 1 s^?1 strain rate just refined the microstructure non-uniformly. Closure of the residual pore and change of the microstructure’s morphology during dual isothermal forging effectively improved the ductility of the compact even though the oxygen and nitrogen contents exceeded the standard requirements.     

Effect of Initial Microstructure on High-Temperature Dynamic Deformation of Ti-6Al-4V Alloy

One of the attractive properties of Ti-6Al-4V alloy is control of microstructure through heat treatment to vary the mechanical properties. In this study, three different microstructures, Lamellar, Widmanstätten, and Martensitic morphologies, were created through heat treatment at a post-β transus temperature followed by cooling at different rates. With faster cooling rates, the microstructures evolved finer lamellae, smaller colony sizes, and thinner grain boundary layers. High-temperature dynamic compression was conducted on these specimens at a strain rate of 1000 s⁻¹ and temperatures in the range of 23 °C to 1045 °C. Flow stresses decreased linearly with colony size and grain boundary layer thickness, but increased with inverse square root of lamellar thickness. This strong correlation of flow stress to several microstructural feature sizes indicated multiple modes of deformation. All three microstructures showed identical thermal softening. The softening rate was intensified at elevated temperatures due to hcp → bcc allotropic phase transformation. Gangireddy modification to Johnson–Cook model could account for this augmented softening and the modified J–C model predicted the three microstructures to follow a similar thermal softening coefficient m = 0.8. The kinetics of phase transformation appear to be very rapid irrespective of the microstructural differences in the Ti-6Al-4V alloy.

     

The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part II. Equiaxed structures

The Ti-6Al-2Nb-lTa-0.8Mo alloy was processed to develop both near-basal and transverse textures. Samples were annealed at different temperatures to vary the equiaxed alpha grain size and the thick-ness of the grain boundary beta, and subsequently quenched in order to transform the beta phase to either martensite, tempered martensite, or Widmanstätten alpha + beta. The effect of microstructure and texture on tensile properties and on fracture toughness was investigated. In addition, yield locus diagrams were constructed in order to study the texture strengthening effect. The yield strength was found to be strongly dependent on the thickness and Burgers relationship of the transformed beta phase surrounding the alpha grains. A texture hardening effect as large as 60 pct was found for the basal-texture material but only 15 pct for the transverse texture material. These variations are asso-ciated with differences in deformation behavior.     

The mechanism of void formation,void growth,and tensile fracture in an alloy consisting of two ductile phases

An investigation has shown that it is possible to relate void formation, void growth, and tensile ductility to microstructural features in an α-β titanium alloy, Ti-5.25A1-5.5V-0.9Fe-0.5Cu, heat treated to a constant yield strength. Equations relating tensile void growth rates to microstructure for both equiaxed,E, and Widmanstätten plus grain boundaryα, W + ITG. B.,in aged β morphologies have been derived. A mechanism for void formation at α-β interfaces is presented which accounts for the observed fact that voids do not form at Widmanstätten α platelets. Tensile fracture is shown to be intergranular in nature and occurs when a critical crack length-stress relationship is satisfied. The amount of ductility achievable in a specimen depends upon the rate of void growth. If the rate is large, the void reaches a critical size for fracture at a lower applied stress and strain and hence the ductility is less.     

Effects of microstructural factors on quasi-static and dynamic deformation behaviors of Ti-6Al-4V alloys with widmanstätten structures

The effects of microstructural factors on the quasi-static tensile and dynamic torsional deformation behaviors in Ti-6Al-4V alloys with Widmanstätten structures were investigated in this study. Dynamic torsional tests were conducted using a torsional Kolsky bar for five Widmanstätten structures, in which microstructural parameters such as colony size and α lamellar spacing were varied by heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Under dynamic torsional loading, maximum shear stress was largely dependent on colony size, whereas shear strain at the maximum shear stress point was on colony size as well as α lamellar spacing. Adiabatic shear bands were found in the deformed area of the fractured torsional specimens, and their width was smallest in the structure whose colony size and α lamellar spacing were both large. The possibility of the adiabatic shear band formation was quantitatively analyzed in relation to microstructural factors. It was the highest in the coarse Widmanstätten structure, which was confirmed by the theoretical critical shear strain (υ _c ) condition for the adiabatic shear band formation.     

Fracture characteristics of Ti-6Al-4V and Ti-5Al-2.5Fe with refined microstructure using hydrogen

The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed α and coarse Widmanstätten α, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5Al-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed α at 1023 K, just below hydrogen saturated β (denoted β″ (H)) transus temperature, produces a microstructure of a, orthohombic martensite (denoted α″ (H)) and β (H). Hydrogenation at 1123 K, above β (H) transus, results in a microstructure of α″ (H) and β (H). Microstructure refinement during TCP results mainly from decomposition of α″ (H) and ;β (H) into a fine mixture of α + β during dehydrogenation. An alternative TCP method is below β (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated β (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed α or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstätten α, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5Al-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.     

Microstructure development during conventional and isothermal hot forging of a near-gamma titanium aluminide

The breakdown of the lamellar preform microstructure in the ingot metallurgy near-gamma titanium aluminide, Ti-45.5Al-2Cr-2Nb (atomic percent), was investigated. Microstructures developed during canned, conventional hot forging were compared to those from isothermal hot forging. The higher rate of deformation in conventional forging led to considerably finer and almost completely broken-down structures in the as-forged condition. Several nontraditional approaches, including the isothermal forging of a metastable microstructure (so-called “alpha forging”) and the inclusion of a short static recrystallization anneal during forging, were found to produce a more fully broken-down structure in as-isothermally forged conditions. Despite the noticeable microstructure differences after forging, conventionally and isothermally forged material responded similarly during heat treatment. In both cases, almost totally recrystallized structures of either equiaxed gamma or transformed alpha grains surrounded by fine gamma grains were produced depending on the heat-treatment temperature. Metallography on forged and heat-treated pancake macroslices was useful in delineating small differences in composition not easily detected by analytical methods.     

Effects of microstructure on the short fatigue crack initiation and propagation characteristics of biomedical α/β titanium alloys

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical α/β titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary α grain boundaries in an equiaxed α structure, whereas, in a Widmanst?tten α structure, initiation occurs within the α colonies and prior β grains, where α plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed α structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 μm), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanst?tten α structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an α structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of α colonies or prior β grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanst?tten α structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical α/β titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed α structure is changed by the volume fraction of primary α phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary α phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.     

The occurrence of shear bands in nonisothermal,hot forging of Ti-6AI-2Sn-4Zr-2Mo-0.1Si

The occurrence of shear bands in nonisothermal, hot forging of Ti-6Al-2Sn-4Zr-2Mo-0. ISi (Ti-6242) was investigated in order to establish the material properties and process parameters which generally lead to shear bands and shear cracks in conventional hot forging of metals. Upset compression tests on cylindrical samples and lateral sidepressing tests on long, round bars were performed to determine the modes by which flow localizes in deformation states ranging from axisymmetric to plane strain. In axisymmetric deformation, it was found that nonisothermal, hot compression leads to chill zones and bands of intense deformation separating the chill zones from the deforming bulk. For plane strain deformation, shear bands were found to initiate along zero extension directions and subsequently localize flow in a manner analogous to the formation and propagation of shear bands in isothermal, hot forging. For both deformation states, it was found that material properties, such as the flow stress dependence on temperature, and process parameters, such as forging speed and die temperature which strongly influence the amount of heat transfer, play critical roles in the flow localization process. A simple model quantifying these effects was developed to predict the occurrence and severity of the shear bands observed in the Ti-6242 alloy hot forged at various temperatures and rates. In addition, the occurrence of shear cracking under certain forging conditions was rationalized in terms of the chilling brought about by nonisothermal, hot forging conditions and the inferior workability of Ti-6242 at temperatures far below the transus temperature.     

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstätten structures on furnace or air cooling, fine Widmanstätten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstätten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.     

An investigation of the effect of fatigue deformation on the residual mechanical properties of Ti-6Al-4V ELI

Tensile properties, hardness, and Charpy impact toughness of Ti-6Al-4V extralow interstitial (ELI) with equiaxed α and Widmanstätten α structures at various stages of fatigue were investigated. Fatigue crack initiation characteristics of the same alloy were also investigated in this study. In the equiaxed α structure, fatigue cracks initiated mainly at the interface between primary-α grains, while in the Widmanstätten α structure, they initiated across α plates at an angle of around 45 deg to the stress axis. Specimens with the Widmanstätten α structure fractured before adequate fatigue hardening was achieved because a multitude of microcracks readily formed. Specimens with the equiaxed α structure fractured after adequate fatigue hardening developed. Tensile strength, 0.2 pct proof stress, and hardness increased clearly with increasing stress cycles and fatigue steps, particulary in the low-cycle fatigue (LCF) region, while impact toughness and elongation showed a reverse trend. It is suggested, therefore, that the dislocation density multiplies more rapidly near the specimen surface during the early stages of fatigue, while during the later stages of fatigue, dislocation density increases near the center of the specimen. Also, the dislocation multiplication will continue until saturation of the entire specimen has occurred.     

Self-consistent modeling of the flow behavior of wrought alpha/beta titanium alloys under isothermal and nonisothermal hot-working conditions

A self-consistent model was applied to predict the plastic flow behavior during hot working of alpha/beta titanium alloys with wrought (equiaxed alpha) microstructures as a function of the flow behavior and volume fractions of the individual phases. For this purpose, constitutive relations that incorporated composition-dependent strength coefficients were determined for the alpha and beta phases. With these constitutive relations and measurements of the specific compositions and volume fractions of the two phases at hot-working temperatures, the flow stress dependence on temperature under nominally isothermal conditions and the (average) strain rates in the individual phases were predicted for Ti-6Al-4V. The effect of temperature transients during hot deformation on the flow stress under nonisothermal (conventional) forging conditions and under nominally isothermal, high strain-rate conditions was also established using the self-consistent modeling approach. In these instances, the effect of a rapid temperature drop or rise, respectively, on the retention of a metastable microstructure was quantified. The predicted flow behaviors showed good agreement with experimental measurements.     

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10^?3 s^?1. During deformation in the higher temperature region, fine Widmanstätten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.