Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)

Of the β-isomorphous Ti-X alloy systems, Ti-Ta is one of the least studied. In the current work, the microstructure and mechanical properties of Ti-40 wt pct Ta (Ti-15 at. pct Ta) are investigated. Annealing at 810 °C produces a two-phase microstructure consisting of Ti-richa idiomorphs in a continuous Ta-rich β matrix; this suggests the β-transus temperature is higher than indicated by the most recently published phase diagram. Water quenching from 810 °C causes the β phase to partially transform to orthorhombic martensite (α), while furnace cooling yields secondarya The primary α formed isothermally remains unchanged in both cases. Subsequent aging causes transformation of the martensite to type 1a plus residual β, with a corresponding increase in strength and decrease in ductility. The maximum ductility (20 pct elongation) occurs in the water-quenched condition in which metastable β is retained. Analysis of the true stresstrue strain data suggests that transformation-induced plasticity may contribute to the enhanced ductility of the water-quenched material.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

The compctition between martensite and omega in quenched Ti-Nb alloys

A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison.     

Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy

The microstructural evolution and grain refinement mechanisms of a Ti-10V-2Fe-3Al alloy, β-solution quenched and cold forged (CF) to strains of 0.1, 0.35, and 1.2 have been investigated using optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the stress-induced martensitic transformation became a predominant deformation mode in the metastable Ti-10V-2Fe-3Al alloy during cold forging. These martensites α″ repeatedly divided the original β parent phase into a large number of micron-sized blocks when the forging strain was 0.1. Shear bands were observed to traverse α″/β lamellae and resulted in a significant grain refinement of the β phase, as the forging strain increased to 0.35. The degree of grain refinement inside shear bands was higher than the outside. Nanocrystalline and amorphous structures were produced in local areas of the original β phase, when the forging strain rose to 1.2. This dramatic grain refinement in the metastable Ti-10V-2Fe-3Al alloy could be attributed to the stress-induced martensitic transformation promoting the initiation and growth of shear bands across α″/β lamellae. More dislocations were produced and accumulated inside grains to accommodate plastic deformation. The crystal structure was collapsed and an amorphous structure was formed as soon as the dislocation density was accumulated to a critical value of 10¹⁴/cm². Moreover, some of the reverse martensitic phase transformation, α″→β, was observed to contribute to grain refinement of Ti-10V-2Fe-3Al alloy as well.     

The tantalum-palladium constitution diagram

The Ta-Pd system was investigated over the entire composition range by metallography, X-ray diffraction and electron microprobe analysis. The solubility limits of terminal and intermediate phases and solidus temperatures were determined. α-Ta dissolved ∼20 at. pct Pd at 2550°C and ∼10 at. pct Pd at 1000°C; α-Pd dissolves ∼22 at. pct Ta at 1730°C and ∼18 at. pct Ta at 1000°C. The presence of four intermediate phases a, (β-U type), α-TaPd (TiCu type), TaPd₂ (MoPt₂ type), and TaPd₃ (TiAl₃ type) was confirmed; they melt or decompose (α-TaPd) at about 2550, 1410; 1800, and 1770°C, respectively. In addition, an equiatomic high temperature phase, β-TaPd was found which melts at ∼1720°C and may be an extension of and isomorphous with the α-Pd solution. Seven three-phase reactions are described. Formerly with Massachusetts Institute of Technology     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

The compctition between the alpha and omega phases in aged Ti-Nb alloys

The compctition between the stable α phase and metastable ω phase to precipitate in a metastableβ phase matrix was investigated in the present study. Four binary Ti-Ni alloys with compositions between 20 and 35 at. pct Nb were air cooled to room temperature from 1000 °C and then aged at temperatures between 300 and 500 °C. For aging temperatures of 400 °C and lower it was found that the quench before aging enabled ω phase precipitates to grow to the exclusion of α precipitates. When specimens were directly aged at 400 °C only α precipitates were observed. Precipitates which could not be identified using SAD were observed in specimens of the 30 and 35 at. pct Nb alloys. All precipitation reactions became more sluggish as the niobium content of an alloy was increased. The results reported here form the second part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Part III. The tensile behavior of Ti-Al-Nb O+Bcc orthorhombic alloys

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pct and Nb concentrations between 22 and 38 at. pct has been investigated for temperatures between 25 °C and 650 °C. Several microstructural features were evaluated in an attempt to identify microstructure-property relationships. In particular, the effects of the phase volume fraction, composition, morphology, and grain size were examined. In addition, the constitutive properties were evaluated using single-phase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O)+body-centered-cubic (bcc) microstructures. The disordered fully-bcc (β) Ti-12Al-38Nb microstructure, produced through heat treatment above the β-transus, exhibited a room-temperature (RT) elongation of more than 27 pct and the lowest yield strength (YS-553 MPa) of all the alloys studied. The ordered fully-bcc (B2) microstructures, produced through supertransus heat treatment of near-Ti₂AlNb alloys, exhibited fracture strengths up to 672 MPa and low elongations-to-failure (ε _f≤0.6 pct). Thus, increasing the Al content, which favors ordering of the bcc structure, significantly reduces the ductility of the bcc phase. Similar to the ordered B2 microstructure, the ordered fully-O Ti₂AlNb microstructures exhibited intermediate RT strength (≤704 MPa) and ε _f (≤1 pct). The O+bcc microstructures tended to exhibit strengths greater than both the fully-O and fully-bcc microstructures, and this was attributed to the finer grain sizes in the two-phase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O+bcc microstructures containing greater bcc-phase volume fractions tended to exhibit greater elongations yet poorer elevated-temperature strengths. A higher Al content typically resulted in larger elevated-temperature strengths. For the Ti-12Al-38Nb bcc-dominated microstructures, fine O platelets, which precipitated during aging, provided significant strengthening and a reduction in ε _f for the Ti-12Al-38Nb alloy. However, large RT elongations (ε _f>12 pct) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 vol pct O phase. Morphology did not appear to play a dominant role, as fully-lath and fully-equiaxed two-phase microstructures containing the same phase volume fractions exhibited similar RT tensile properties. The slip and cracking observations provided evidence for the ductile and brittle characteristics of the single-phase microstructures, and the slip compatibility exhibited between the two phases is an important part of why O+bcc microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.     

Structure-property relations in a metastable β Ti alloy containing Si

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)_xSi_y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)_xSi_y precipitates, b) the equilibrium silicide, and c) the α-phase.     

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.     

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.     

Young’s modulus and mechanical properties of Ti-29Nb-13Ta-4.6Zr in relation to α″ martensite

An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (M _s) temperature. A critical grain size of 40 μm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low M _S temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.     

Effects of Cu content and preaging on precipitation characteristics in aluminum alloy 6022

Effects of Cu content and preaging treatments on precipitation sequence and artificial aging response in aluminum alloy 6022 were investigated using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness tests. It was found that Cu induces the formation of Q and its precursor metastable phases and has a beneficial effect on the kinetics of artificial aging. For the alloy with 0.07 wt pct Cu, the precipitation sequence is GP zones → needlelike β″ → rodlike β′ + lathlike Q′ → β + Si. On the other hand, the precipitation sequence in the alloy with 0.91 wt pct Cu is GP zones → needlelike β′ → lathlike Q′→Q+Si. For the artificial aging condition of 20 minutes at 175 °C, which is the typical automotive paint bake condition, suitable preaging treatments were found to significantly reduce the detrimental effect of the natural aging on artificial aging response.     

Mutual effects of hydrogenation and deformation in Ti-Nb alloys

Alloying of Ti-based alloys with hydrogen is used to modify the microstructure and improve mechanical properties. In this study, hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte at high fugacities. This research investigated in detail, by means of X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal desorption spectroscopy (TDS), and microhardness tests, the influence of hydrogen at high fugacities on the phase stability, desorption behavior, and microhardness in Ti-Nb (20 to 45 wt pct Nb) alloys before and after deformation. Hydrogenation of Ti-Nb was found to exhibit a significant effect on the phase stability as well as the microhardness of Ti-Nb alloys. Hydrogenation of Ti-20 wt pct Nb led to precipitation of (Ti,Nb)H _x in the metastable α″ matrix. In Ti-Nb alloys with 40 or 45 wt pct niobium, hydrogen stabilized the bcc β phase, but destabilized the hcp ω phase. With increasing hydrogen content, an expansion of the lattice constant of the β phase occurred, followed by the formation of (Ti,Nb)H _x . The influence of hydrogen on the microhardness of Ti-40 wt pct Nb and Ti-45 wt pct Nb shows only a minor effect, whereas Ti-20 wt pct Nb exhibits significant softening in response to hydrogen charging up to 3000 ppm. The TDS showed that deuterium desorption strongly depends on the niobium content and the deformation treatment prior to or after charging. The observed results should provide further insight on the mutual effects and the resulting micromechanism of hydrogenation and deformation in Ti-Nb alloys. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

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.     

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct). Formerly Research Assistant, Department of Materials Science, University of Southern California