Microstructure and fracture toughness of the aged ,β-Ti Alloy Ti-10V-2Fe-M

Fracture mechanics and tensile tests have been performed on the metastable β-Ti alloy Ti-IOV-2Fe-3AI. A variety of microstructures was established by several combinations of forging and heat treatment resulting in different types, morphologies, and volume fractions of the a-phase which precipitates from the matrix-β phase. Both fracture toughness and ductility are strongly reduced by increasing hardening by the secondary a-phase. An elongated primary a-phase (α _p ) shows higher toughness compared to a globular α _p -phase. A thick, continuous subgrain boundary a-film lowers the toughness significantly. For microstructures without primary a a grain boundary α-film does not affect the toughness, while the ductility is drastically reduced. Very attractive combinations of fracture toughness and ductility were found for a microstructure without primary a and without grain boundary α. The results are discussed based on the fractographic observations, and a model is proposed which includes the effect of microstructure and slip distribution on the crack nucleation, the crack growth path, and the crack deviation.     

Effect of mean stress (stress ratio) and aging on fatigue-crack growth in a metastable beta titanium alloy, Ti-10V-2Fe-3Al

The effect of mean stress, or the stress ratio (R), on the fatigue-crack growth (FCG) behavior of α-aged and ω-aged microstructures of the beta titanium alloy Ti-10V-2Fe-3Al was investigated. While the mean stress had a negligible effect on the FCG behavior of the α-aged microstructure, a strong effect was observed in the ω-aged microstructure. In particular, the values of the threshold stress-intensity range (ΔK _th ) exhibited a strong dependence on R in the ω-aged microstructure, while this dependence was weak in the α-aged microstructure. These effects seem to arise primarily from fracture-surface roughness-induced crack closure. The crack closure levels for the α-aged microstructure were found to be very low compared to those for the ω-aged microstructure. Transmission electron microscopy and scanning electron microscopy studies of microstructures and fracture surfaces were performed to gain insight into the deformation characteristics and crack propagation mechanisms, respectively, in these microstructures. The microstructure-induced differences in FCG behavior are rationalized in terms of the effect of aging on slip and crack closure.     

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.     

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

This work was undertaken to examine the effect of increasing matrix strength at constant equiaxed microstructure on void nucleation and growth in the titanium alloy, CORONA-5, Ti-4.5Al-5Mo-1.5Cr. A martensite and a beta matrix were used in the as-quenched and the heat treated conditions. For each matrix, fine and coarse alpha sizes were produced and a third size of alpha was used for the as-quenched condition of the martensite series. The processing procedures produced an aligned alpha structure which was most pronounced in the fine structure. Void nucleation occurred in an aligned fashion and took place predominantly atα /martensite orα/β interfaces. An explanation is offered for the aligned nucleation in terms of nonuniform deformation of the banded structure which appeared most prominently after heat treatment to produce the coarser microstructure. An incubation strain was found for both types of matrices. The incubation strain increased for the interface in the following order: martensite/martensite,α /martensite, andα/ β. The incubation strain for martensite/martensite interfaces was relatively independent of the matrix strength. Void growth as a function of true strain was generally found to occur in two stages, a slow stage I and a more rapid stage II. Stage II growth occurred as a result of coalescence of voids growing toward one another from nearby particles. Stage II growth was more rapid for the martensite matrix than for theβ matrix. For the martensite matrix void growth rates could not be accounted for either on the basis of strength or strain hardening rates. However, the longest void growth rate was found to increase as the function λ ^N /d _α ^L increased. λ^N is the interparticle spacing normal to the tensile axis and _α ^L is the alpha particle size parallel to the tensile axis. For the beta matrix void growth rates increased with increasing yield s trength and decreased with increasing strain hardening. It was not possible to relate fracture strength to an extrapolated longest void at fracture as was done in earlier studies. This is explained in terms of the nonuniformity of fracture paths observed in the alloy.     

Low cycle fatigue behavior of Ti-Mn alloys: Cyclic stress-strain response

The cyclic stress-strain response at constant total strain has been determined for a series of binary Ti-Mn alloys which have been heat treated at a constant temperature of 700 °C to produceα,α-β, and β phase microstructures where the chemical composition ofα andβ phase has been kept constant. The α phase hardens during cyclic straining and the amount of hardening increases with strain. Theβ phase initially softens but hardens with increasing number of cycles. Theα -β alloys show a mixture of these behaviors with softening at low strains and hardening at higher strains. Softening in theα -β alloys is not expected, particularly in the higherα alloys, because plastic strains are much higher inα than inβ at low strains. Transmission electron microscopy has indicated that this behavior is due to dislocation rearrangement inα phase. It has not been possible to ascertain the reason for softening ofβ phase, which may be due either to an increased mobile dislocation density or metallurgical instability of the alloy or both.     

Phase transformation and microstructural evolution in Ti-44Al-4Nb-4Zr alloy during heat treatment

Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology of the transformation products for the three phase transformations studied,α →α ₂, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled.     

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.     

Phase transformations in Ti-6.8Mo-4.5Fe-1.5Al

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 °C to 750 °C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy. Artificial aging following solution treatment and water quenching initially involved growth of the athermal ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the α phase.     

Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma titanium aluminide alloy

The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal, constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between 80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s⁻¹ on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains of γ + α ₂ + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling of the lamellae, and the formation of equiaxed particles of γ + β ₂ + α ₂ on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions.     

Numerical models of creep and boundary sliding mechanisms in single-phase, dual-phase, and fully lamellar titanium aluminide

Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α₂+γ, and fully lamellar (FL) α₂+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.     

A study of void nucleation,growth, and coalescence in spheroidized 1518 steel

The ductile fracture of a spheroidized 1518 steel has been investigated using three types of tensile specimens — smooth tensile, notched tensile, and plane-strain tensile. It was found that void nucleation has two different modes (Type I and Type II) depending on local conditions, the most important of which are the size, shape, and distribution of the particles. By identifying the low-strain-range nucleation behavior (Type I), it was possible to determine the value of plastic strain, ε_N, after which void nucleation at average-sized carbide particles (Type II) begins; ε_N is 0.45 for the smooth tensile case, 0.30 for the notched, and 0.25 for the plane strain. The critical stress for Type II void nucleation, σ_c, is of the order of 1200 MPa. Void growth depends on the macroscopic stress-strain state: longitudinal growth is given by a linear function of applied plastic strain, ε_p, whereas lateral growth shows a linear dependence on the triaxial stress, σ_T. When the local value ofV _f reaches a critical volume fraction of voids (V _f ^cri = 5 ± 0.5 pct), void coalescence occurs in a catastrophic manner, leading to final separation within a highly localized zone. The stress concentration caused by the notched tensile specimen geometry and the localized mode of plastic flow caused by the constraint of the plane-strain state in a Clausing-type specimen were found to affect the substeps of void nucleation, growth, and coalescence. Formerly Graduate Research Assistant, Division of Engineering, Brown University. Formerly Professor, Division of Engineering, Brown University, Providence, RI.     

Effects of interstitial oxygen on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb with fully lamellar and duplex microstructures

The effect of added oxygen in the range of 1000 to 4000 wt ppm on the microstructures of a Ti-48Al-2Cr-2Nb alloy has been investigated and compared to the microstructures for a high-purity alloy. For specimens cooled from theα phase, interstitial oxygen stabilizes the lamellar microstructure with respect toγ grains, with higher than equilibrium values for theα ₂ volume fraction. For specimens cooled from theα +γ phase field, the lamellar microstructure still tends to be favored by oxygen, but it is found that the phase volume fractions are not significantly different from equilibrium values. This suggests that interstitial O essentially reduces the kinetics of theα toα +γ transformation. Thus, interstitial oxygen will have a strong effect on microstructures obtained by continuous cooling fromα, but significantly less on those, such as the duplex microstructure, obtained by long treatment in theα +γ phase field. In general, increased O content is well correlated with reduced ductility. Finally, the role of interstitial oxygen on this phase transformation is discussed.     

Observations on void nucleation and growth in α/β Ti-Mn alloys

Void nucleation and growth was studied in three binary equiaxed α-β Ti-Mn alloys containing 1.8 wt pct Mn (alloy 2), 3.9 wt pct Mn (alloy 3), and 5.8 wt pct Mn (alloy 4) given heat treatments to vary the alpha size at constant volume fraction of alpha. Void nucleation rates expressed as number of voids per unit volume,N _v, increased exponentially with true strain, ε. WhenN _v was normalized with respect to the number of alpha particles or grains per unit volume, N_α ^T,N _v/N_α ^T was found to be largest for the largest alpha size in each alloy series. Void size distributions as a function of strain for one alloy containing 3.9 wt pct Mn (alloy 3 given heat treatment B,3B) were presented and, as expected, the largest number of voids occurred at the smallest void sizes. Void growth rates for alloys 3 and 4 were found to increase with increasing particle size and this was ascribed to decreasing constraints to slip with increasing particle size. For alloy 2C with the largestα grain size void growth rates were smallest and this behavior was attributed to the growth inhibiting effects of multiple twinning. Evidence was adduced to show that nucleating voids grow more rapidly than established voids. T. V. Vijayaraghavan, Formerly Graduate Student, Polytechnic University, Brooklyn, NY     

Strain hardening at high strain in aluminum alloys and its effect on strain localization

The modes of strain localization in the tensile testing of a sheet sample are diffuse necking, localized necking and, in some materials, localization in an unstable shear band. In a tensile test of a rate insensitive material, the normalized strain hardening parameter,H = (1/σ)(dσ/dε) has the values ofH = 1 for diffuse necking andH = 0.5 for localized necking. Curves ofH vs strain were obtained up to large values of plastic strain using the hydraulic bulge test. The materials selected were commercially important sheet alloys in the condition normally used for forming. It is shown that the materials have similarH vs strain curves in the range of uniform tensile straining, but the curves diverge widely at higher strains whereH falls below 1. This has important consequences on strain localization behavior. The limit strains of the alloys in simple tension and punch stretching show reasonable correlation with their values ofH and those alloys which are susceptible to catastrophic shear failure have low values ofH at high strains. Strain rate sensitivity adds to or subtracts from theH values obtained in this study and has an additional influence on strain localization. Formerly with Alcoa Laboratories, Alcoa Center, PA 15069, U.S.A     

The microstructure of rapidly solidified Ti-Fe melt-spun ribbons

Ti-Fe binary alloys were rapidly solidified by the melt-spinning technique, and four compositions were examined: Ti-5 wt pct Fe, which is the critical composition for theβ to ω athermal transformation; Ti-10 wt pct Fe, which represents a hypoeutectoid composition; the eutectoid composition Ti-15 wt pct Fe; and Ti-20 wt pct Fe, as an example of a hypereutectoid alloy. The Ti-5 wt pct Fe rapidly solidified ribbons are composed of two different structures. The first consists of α′-martensite plates inβ matrix and the second, athermal ω particles inβ matrix. The Ti-10, 15, and 20 wt pct Fe alloys are also composed of two structures. These areβ grains and isothermal-like ω particles inβ matrix. A solidification model is suggested which explains the existence of two different microstructures at the same composition and the for-mation of two kinds of ω particles.     

Superplastic deformation,cavitation, and fracture of microduplex Cu-Ni-Zn alloys

A study has been made of the superplastic behavior during tensile straining of two α/β Cu-Ni-Zn alloys (nickel silvers). Cavitation occurred during deformation and has been studied using metallographic and density techniques. Cavities nucleated at α/β boundaries and triple points involving two phases, and cavity growth and interlinkage led to brittle superplastic fracture. Density studies showed that the volume of cavities increased with increasing strain, but was relatively independent of strain rate and temperature. The results were consistent with a high rate of cavity nucleation in the early stages of deformation, followed by a grain boundary sliding mechanism of growth.     

The fracture of ordered Zr3AI polycrystals

Metallographic studies of Zr₃Al polycrystals deformed in tension have established that failure occurs by grain boundary fracture at low temperatures (<200 K), by microvoid coalescence at intermediate temperatures (≃295 to ≃875 K) and by grain boundary sliding following dynamic recrystallization at high temperatures C>875 K). The ductile fracture mode is composed of three separate processes —the nucleation, the growth and the coalescence of microvoids. Microvoid nucleation occurs by the cleavage of hard, secondphase Zr₂Al particles and by particle/matrix separation, while void growth occurs by the concentration of plastic strain at the crack tip. Both void nucleation and growth occur throughout plastic deformation and generate the damage which eventually causes ductile fracture.     

The migrational characteristics of the cusp-oriented α/κ interface in the Cu-Si system

The mobility of the coherentα/κ boundaries in the copper-silicon system was studied in a Cu plus 5.16 wt pct Si alloy with a lamellar microstructure. A new resistometric technique was employed to study the isothermal kinetics ofα/κ interphase boundary migration as a function of driving force at two reaction temperatures, 702° and 568°C. The driving force was shown to be linearly related to the temperature change, δT, and the experimental results show that the functional relationship between mobility and δT exhibits an initial linear dependence followed by a sharp rise in mobility at higher values of δT. Upquench and down-quench results were found to be equivalent, implying that the mechanism for boundary motion is independent of whether the boundary is moving into theα or into theκ phase. The results demonstrate that theα/κ interface does not move normal to itself by the long range bulk diffusion of silicon at low values of δT during the initial periods of the reaction times. A pole mechanism with its associated dislocation node configuration, consisting ofa/2〈112〉 andc[00.1] pole dislocations anda/6〈112〉 transformation partial, is proposed as the mechanism for the migration of the coherent fcc/hcp boundary in the range of small δT. The results obtained at the different reaction temperatures are consistent with the mechanism proposed, and quantitative agreement exists between the experimental and predicted mobilities. The sharp rise in mobility at larger values of δT could not be explained in terms of the pole mechanism. Attempts to rationalize this behavior in terms of a two dimensional nucleation model were not successful since unrealistically low values of surface energy would be required. Formerly with Carnegie-Mellon University Formerly Professor, Carnegie-Mellon University This paper is based on a portion of the Ph.D. Thesis submitted by J. K. TIEN to the Department of Metallurgy and Materials Science, Carnegie-Mellon University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgy and Materials Science.     

The relationship between microstructural and plastic instability in AI-4.0 Wt Pct Cu alloy

The investigation of the effect of plastic deformation on the stability of theθ′ precipitates in an aluminum-4.0 wt pct copper alloy was performed. The alloy was produced by directional solidification, with Ti added as a grain refiner. Hot compression tests were performed at 200 °C in the strain rate range of 10_-3 to 10_-5 s₁ and equivalent strain up to 0.7 on specimens that had been initially heat treated, also at 200 °C, in order to obtain a uniform distribution of theθ0′ precipitates within the matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the plastically deformed specimens revealed a very heterogeneous distribution of strain. Also, the regions with localized strain contained randomly distributedθ precipitates of nearly equiaxed shape without any preferred orientation relationships to the matrix. Thus, the plastic deformation initiated the transformationθ′ →θ. The flow stress was reduced in the regions in which this transformation had occurred, which further accentuated the localization tendency of the strain. The combined process,θ′ →θ transformation/strain localization, thus developed in an avalanching way.     

Quantitative analysis on boundary sliding and its accommodation mode during superplastic deformation of two-phase Ti-6Al-4V alloy

A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size and α/β volume fraction during superplastic deformation of a two-phase Ti-6Al-4V alloy. A load relaxation test has been performed at 600 °C and 800 °C to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600 °C and is well described by the plastic state equation. Whereas, at 800 °C, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 μm) well agrees with the isostress model, while that for large-grained structures (11 μm) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries is lowest in the α/β boundary, and increases on the order of α/β≪α/α≈β/β, which plays an important role in controlling the superplasticity of the alloys with various α/β phase ratios.