Dynamic fracture behavior of Ti-6Al-4V alloy with various stabilities of βphase

The effect of stability of the body-centered cubic (bcc) β phase on the dynamic fracture behavior of Ti-6Al-4V alloy at room temperature and 77 K has been studied. The presence of a highly unstable β phase in the quenched alloy leads to a decrease in both the dynamic fracture toughness and the crack propagation energy, and this decrease bccomes more pronounced when test temperature is reduced to 77 K. Somewhat improved fracture characteristics were obtained by applying anneal procedure to receive a fully stable β phase. The highest fracture toughness as well as the greatest crack propagation resistance were observed in the air-cooled grade, where the lattice parameter of the bcc phase was intermediate between those pertaining to quenched and annealed Ti-6Al-4V alloys. The effect is attributed to the vanadium content in the β phase, which is sufficiently high to suppress deformation-induced transformation. On the other hand, the V content should be low enough to retard ductile-brittle transition, typical for the bcc metals at cryogenic temperatures. As a result, marked toughening can be achieved, so that the lowest application temperature of high-strength titanium alloys containing the bcc phase can be decreased significantly. Formerly Assistant Professor, Department of Production Systems Engineering, Toyohashi University of Technology     

Notched Tensile and Impact Fracture of Ti-15-3 Laser Welds

The notched tensile strength (NTS) and impact toughness of Ti-15V-3Cr-3Sn-3Al (β-type titanium alloy Ti-15-3) laser welds aged at temperatures ranging from 590 K to 866 K (317 °C to 593 °C) were determined, and the results were compared to those of unwelded Ti-15-3 plates aged at the same temperature. At a given aging temperature, α precipitates in welded specimens were finer and exhibited higher hardness than those in unwelded specimens. Among the tested specimens, the weld aged at 644 K (371 °C) was most susceptible to notch sensitivity. In those welds aged at or above 755 K (482 °C), the coarse columnar structure was prone to interdendritic fracture during notched tensile tests, which reduced the NTS of the weld relative to that of the unwelded plate aged at an equivalent temperature. Of the tested specimens, the weld that was not subjected to the postweld aging treatment possessed the highest impact toughness among the specimens.     

Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti-4Al-4Mo-2Sn-0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite (α′) is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (α″) on rapid cooling. Finally, below 1123 K, the β phase is stable—no martensitic transformation occurs on rapid cooling. This transition from α′ → α _primary + (α′ + β _retained) → α _primary + (α″ + β _retained) → α _primary + β _metastable + ω, with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature β phase.     

Phase transformations and tensile properties of Ti-10V-2Fe-3AI

The effect of heat treatment on the microstructure of the metastableα-Ti alloy Ti-10V-2Fe-3Al has been studied using light and electron metallography, analytical electron microscopy, and X-ray diffraction. A survey of the effects of microstructure on tensile properties of the alloy also has been conducted. It has been found that the alloy contains inclusions which are rich in Ti, S, Si, and P. The alloy has been shown to form ω-phase both athermally and isothermally. The isothermal ω can have either an ellipsoidal or a cuboidal morphology. The reasons for this are enumerated. The formation of α-phase has been studied, and three distinct modes of formation are described. A stress induced orthorhombic martensite also has been observed. The effect of this stress induced product on tensile behavior is discussed. The relative roles of inclusions and α-phase precipitates in the tensile fracture also have been examined. Formerly at Carnegie-Mellon University, is now at Brown, Boveri & Cie, AG, Baden, Switzerland,     

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.     

Microstructural characterisation of near-α titanium alloy Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si

Microstructural stability in the near-α titanium alloy (alloy 834) containing Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si (in weight percent), in the β and(α + β) solution-treated and quenched conditions, has been investigated. The β transus for this alloy is approximately 1333 K. Solution treatment in the β phase field at 1353 K followed by quenching in water at room temperature resulted in the formation of α′ martensite platelets with high dislocation density and stacking faults. Thin films of β are found to be sandwiched between interface phases, which, in turn, are sandwiched at the interplatelet boundaries of lath martensite. The interface phase is a subject of much controversy in the literature. Solution treatment at 1303 K in the(α + β) phase field followed by quenching in water at room temperature resulted in the near-equiaxed primary α and transformed β. Both the β and(α + β) solution-treated specimens were aged in the temperature range of 873 to 973 K. While aging the —treated specimen at 973 K,(α + β)-treated specimen, even at a lower temperature of 873 K for 24 hours, caused precipitation of suicides predominantly at the interplatelet boundaries of martensite laths. Electron diffraction analysis confirmed them to be hexagonal suicide S₂ witha = 0.70₂ nm andc = 0.36₈ nm. The above difference in the precipitation could be attributed to the partitioning of a higher amount of β- stabilizing elements as well as silicide-forming elements to the transformed β in the(α + β) solution-treated condition. However, ordering of theα′ phase was observed under all of the aging conditions studied. The ordered domains were due to the longer aging times, which cause local increases in the level of theα-stabilizing elements. Formerly Research Associate, Department of Metallurgical Engineering, Baranas Hindu University.     

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.     

Improvement in fracture toughness of Ti-4.5Al-3V-2Mo-2Fe through microstructural optimization

In order to improve the observed low fracture toughness of β-rich α+β-type Ti-4.5Al-3V-2Mo-2Fe annealed at the temperature of 1123 K, a two-step cooling (TSC) after solution treatment at that temperature was proposed instead of air cooling. Solution treatment plus aging (STA) and slow furnace-cooling (SFC) treatments were also carried out on the same alloy for comparison. It was found that the relatively higher fracture toughness (J _IC ) is obtained by the TSC treatment, that is, by slow cooling the alloy at a cooling rate of 0.075 Ks⁻¹ from a temperature of 1123 to 723 K and, subsequently, water quenching to room temperature. The J _IC of the two-step-cooled alloy has the same value as that of the alloy annealed at 993 K after having first been annealed at 1123 K (duplex annealing (DA)), which has been previously observed to have high fracture toughness. The J _IC value can also be improved by STA, but it is still lower than that of two-step-cooled alloy. The relatively lower J _IC is obtained in slow-furnace-cooled alloys. The J _IC of slow-furnace-cooled alloys decreases monotonously with decreasing cooling rate for cooling rates less than 0.075 Ks⁻¹. It is suggested that the factors responsible for increasing J _IC> in two-step-cooled and solution treated-and-aged alloys are the plate-like α and secondary α, respectively, which appears in the β phase during cooling and aging, respectively. The presence of such kinds of transformation products increases the crackdeflection effect and, thus, increases fracture toughness.     

The effect of hydrogen on mechanical properties in Ti-10V-2Fe-3Al

The effect of hydrogen on the mechanical properties of the metastable β alloy Ti-10V-2Fe-3Al was examined. The material was beta annealed and water quenched (B/WQ) to yield a nominally all-β microstructure, with a small volume fraction of athermal omega present. Tensile and notched bend bar tests were performed with differing levels of hydrogen concentration (~0 to >30 at. pct) obtained by Sieverts (gaseous) charging prior to beta annealing. The β phase was transformed to orthorhombic alpha double prime martensite (β") upon deformation. The volume fraction and morphology of the alpha double prime depended on the hydrogen concentration. The deformation-induced martensitic transformation changed from being stress-induced to being strain-induced with increased hydrogen concentration. High hydrogen concentrations also resulted in changes in fracture mode. At high hydrogen concentrations, where little or no martensite formed upon deformation, “intrinsic” (i.e., independent of microstructural modification) hydrogen effects were observed in the β phase. These intrinsic hydrogen effects, deleterious in nature, were taken to be evidence of hydrogen embrittlement in the body-centered cubic β phase. Formerly at Carnegie Mellon University.     

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).     

Fracture and toughening mechanisms in an α2 titanium aluminide alloy

The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α₂ + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJ _IC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α₂ matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α₂ + β interface due to strain incompatibility resulting from impinging planar slip originated in α₂. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering of the initiation toughness (theK _IC value) but a drastic increase in the crack growth toughness and the tearing modulus.     

An Investigation of the fracture behavior of gamma-based titanium aluminides: Effects of annealing in the α + γ and α2 + γ phase fields

The effects of annealing in the α + γ and α₂ + γ phase fields on the microstructures and fracture properties of Ti-48A1 and Ti-49Al-3.4Nb are discussed in this article. Annealing of the niobium-containing alloy in the α₂ + γ phase field results in the precipitation of ⇌₂ and Nb₅Si₃ predominantly at the grain boundaries. The precipitation decrease the grain boundary cohesion, thereby promoting intergranular separation. Precipitation also decreases the tensile strength and ductility of Ti-49Al-3.4Nb compared to that of the binary alloy. The possible role of interfaces in the transmission of slip is also discussed, and micromechanical models are applied to the prediction of tensile behavior and fracture toughness. Formerly Scientist, McDonnell Douglas Research Laboratories Formerly Director and MDC Fellow, McDonnell Douglas Research Laboratories     

The Effect of Yttrium on Ti-5111 Gas Tungsten Arc Welds

Much interest has developed in the near-α titanium alloy Ti-5Al-1Sn-1V-1Zr-0.8Mo (Ti-5111) for naval applications. When gas tungsten arc welded with filler metal that has the same chemical composition as the base metal, however, the weld FZ tends to be harder and less ductile than the base metal, which may make the weld susceptible to failure. This behavior may be attributed to the presence of oxygen impurities and the large prior-β grain size in the weld. In this investigation, the addition of a small amount of yttrium to the weld filler metal can decrease hardening and increase the ductility of Ti-5111 welds, which is beneficial for weld performance. Microstructural and chemical analyses of unmodified and yttrium-modified Ti-5111 welds are presented along with results from mechanical testing of the welds.     

Effect of grain size on the tensile behaviour and fracture toughness of Ti-10V-4.5Fe-3Al beta titanium alloy

The tensile and fracture toughness behaviour of Ti-10V-4.5Fe-3Al alloy in the beta solution treated condition was investigated as a function of β grain size and the behaviour was compared with that of Ti-10V-2Fe-3Al alloy. It was found that this alloy, unlike the Ti-10V-2Fe-3Al alloy, did not exhibit any stress induced martensitic transformation during tensile testing. This was attributed to the higher stability of the β phase in this alloy as compared to that in Ti-10V-2Fe-3Al alloy because of the higher iron content. The yield strength, ultimate tensile strength, total elongation and fracture toughness in this alloy were found to decrease with increase in grain size. The yield strength of this alloy was found to obey the Hall-Petch relationship with grain size, similar to the observation in Ti-10V-2Fe-3Al alloy. However, the σ_i value was higher and k value lower than that observed in Ti-10V-2Fe-3Al alloy. The fracture toughness in Ti-10V-4.5Fe-3Al alloy was also found to increase linearly with inverse square root of grain size; however, the slope was significantly less than that observed in Ti-10V-2Fe-3Al alloy.     

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.     

High temperature low cycle fatigue in beta processed Ti-5Al-5Sn-2Zr-2Mo-0.25Si

The effects ofβ heat treatment andβ-work on microstructure and high temperature low cycle fatigue properties of Ti-5Al-5Sn-2Zr-2Mo-0.25Si were investigated. Strain control tests at total strains ranging from 1.2 to 2.5 pct were conducted at 427, 482, and 538 °C with frequencies of 10 and 0.4 cpm. The results showed shorter fatigue life at the higher temperature and lower frequency for all microstructural conditions.β-worked material with shorterα-platelet structure showed the highest fatigue strength for all test conditions. At these high temperatures, fatigue cracks initiate along theα/β interfaces with longer initial cracks in theβ-annealed condition which had longer interfaces. It is suggested that oxygen diffusion along theα/β interfaces is responsible for surface connected interfacial cracking leading to the observed temperature and frequency dependence. The better HTLCF life of theβ-worked material is related to the shorter initial interface cracks. Formerly with the Metals and Ceramics Division, AFML, W-PAFB, OH 45433     

Age Hardening of α- βTi-6AI-2Sn-4Zr-6Mo

The aging behavior and corresponding strengthening mechanisms were studied in an α plus retained β structure of the Ti-6Al-2Sn-4Zr-6Mo alloy. When quenched from 1123 K the resulting structure is a mixture of equiaxed α plus retained β phases. Slip was found to be the only deformation mode and was extremely heterogeneous in nature for the asquenched structure. Plate-like α started to precipitate along α-β and β-β boundaries when the alloy was aged at 873 K. Dislocations were found at the leading edge of precipitating α as well as in the adjacent β matrix and inside the precipitated α. With aging, the retained β transformed into an interlaced structure of platelike α, and the slip characteristics change from heterogeneous to homogeneous both in the β matrix and the primary α. Ernest Levine, formerly with Union Carbide Corporation, is now Staff Engineer, IBM Corporation, East Fishkill     

Microstructural stability on aging of an α + β titanium alloy: ti- 6ai- 1.6zr-3.3mo- 0.30si

The development of the microstructure on aging of an (α + β) type titanium alloy containing 6A1-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The β-transus temperature of this alloy is approximately 1243 K. Solution treatment in the β-phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite struc-ture. The martensitic structure was confirmed to be orthorhombic (α″) using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation ofs ₂ silicide particles and thin films of β sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters α= 0.70(1) nm andc = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation ofs ₂ silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of β and acicular martensite. The relative sizes of silicide precipitates and width of thin films of β phase increase with increasing aging time. The sites for silicide precipitation are mainly at α′-α′ boundaries, α-β interfaces, and sometimes within regions of transformed β. The kinetics ofs ₂ silicide precipi-tation in this alloy is faster than in commercial near-α titanium alloys. This is attributed to the presence of Mo, a strong β stabilizer. Formerly Reader, Department of Metallurgical Engineering, Centre of Advanced Study, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India     

Subcritical crack-growth under sustained load in Ti-6AI-6V-2Sn

The subcritical sustained-load cracking (SLC) behavior of beta annealed, recrystallization annealed, and solution-treat-and-aged Ti-6Al-6V-2Sn was studied in dry argon and moist air. The effects of microstructure, internal hydrogen concentration, specimen orientation, and specimen thickness on threshold stress-intensity and crack-growth rate were determined under increasing stress-intensity conditions using wedge-opening load (WOL) specimens, and the fracture morphologies were studied by scanning electron microscopy. The SLC threshold stress intensities are lower and crack growth rates are higher in solution-treatand-aged condition than in beta-annealed and recrystallization conditions. The plane strain conditions increase the susceptibility to SLC. The texture effects on SLC are intensified when the cracking plane is close to the basal plane. For low interstitial-hydrogen concentrations ≈10 ppm, sustained-load crack growth is controlled by creep at the crack tip. Increased hydrogen concentration results in enhanced cleavage, lower threshold stress-intensity, and accelerated crack-growth. Of the possible mechanisms for hydrogen-assisted SLC in Ti-6Al-6V-2Sn, the most likely is that involving preferential segregation of hydrogen to beta phase, leading to fracture by α-β interface separation. Formerly with McDonnell Douglas Research Laboratories.