Β-phase Field Research Articles

A preliminary study on precipitation hardening of Ti–38.5Al–4Fe and Ti–38.5Al–4Fe–5Nb–0.2B intermetallic alloys via heat treatment

The present paper is devoted to the effect of heat treatment including quenching from the single β-phase field followed by ageing on the microstructure, hardness (microhardness) and strength properties of the intermetallic alloys Ti–38.5Al–4Fe and Ti–38.5Al–4Fe–5Nb–0.2B. The heat treatment led to formation of nanosized precipitates of the intermetallic phases α2-Ti3Al, γ-TiAl and τ2-Al2TiFe. Mechanical compression tests and hardness (microhardness) measurements showed considerable strengthening and hardening of the alloys.

The effect of processing on the 455 °C tensile and fatigue behavior of boron-modified Ti–6Al–4V

This work investigated the effect of nominal boron (B) additions of 0.1 wt.% and 1.0 wt.% on the elevated-temperature (455 °C) fatigue deformation behavior of Ti–6Al–4V (wt.%) for maximum applied stresses between 250 and 550 MPa ( R = 0.1, 5 Hz). The alloys were evaluated in the as-cast condition as well as the cast-and-extruded condition. Boron additions resulted in a dramatic refinement of the as-cast grain size, and larger boron additions resulted in larger titanium-boride (TiB) phase volume percents. For the as-cast alloys, the B-containing alloys exhibited longer average fatigue lives than those for Ti–6Al–4V, which was suggested to be related to their increased strength and stiffness due to the addition of the strong and stiff TiB phase. The longest average fatigue lives were exhibited by the Ti–6Al–4V–0.1B alloy, which also exhibited the greatest elongation-to-failure value. The extrusions, which were performed in the β-phase field, resulted in a significantly smaller grain size, a smaller α-colony size, and finer α-lath width compared to that for the as-cast B-modified alloys. The TiB whiskers were aligned in the extrusion direction and the α-phase was also strongly textured such that the basal plane was predominately oriented perpendicular to the extrusion axis. Together these microstructural features were responsible for the significantly higher 455 °C yield strength, ultimate tensile strength, and fatigue strength exhibited by the cast-and-extruded alloys compared with the as-cast alloys. In the extruded condition, B-addition did not improve the tensile or fatigue strength.

An evaluation of high-temperature phase stability in the Ti–Al–Nb system

Ti–Al–Nb alloys have shown promising properties for high-temperature applications. Although the phase diagram for this system has been well researched, there is still controversy over some aspects, including the extent of the β-phase field. We have studied an alloy with the composition 37Ti–44.5Al–18.5Nb, which based on existing phase diagrams is expected to solidify as the γ-TiAl phase. Experimental observations revealed that this alloy solidified as the β-phase, suggesting an extended β-phase field beyond the previously reported phase boundary.

Microstructural characterization of Ti–6Al–4V machining chips after remelting and severe deformation

Abstract The present work was initiated to recycling the machining chips of Ti–6Al–4V alloy as part replacement of titanium sponge and expensive alloying materials for manufacturing Ti-alloy ingots. The technology includes a two-step processing route involving an initial remelting of machining chips into ingot by vacuum-induction melting furnace followed by conversion of the ingot into slab by severe hot working processes (forging and large strain hot rolling). Optical microscope was used to investigate the microstructures. The results show that when deformation process is carried out in the α + β-phase field, a much finer α + β-structure can be obtained, because the primary α-phase limits the growth of the β-phase. Both the primary α-phase and β-phase deform simultaneously and the β grains are easier to recrystallize.

Development of grain structure in β-phase field during friction stir welding of Ti–6Al–4V alloy

The electron backscattered diffraction technique was employed to examine the microstructural evolution in the high-temperature β-phase field during friction stir welding of a Ti–6Al–4V alloy. Reconstruction of the β-grain structure demonstrated that development of the grain structure is presumably governed by grain elongation and transverse grain subdivision. Orientation measurements in the retained β-phase showed that the material flow may be described in terms of a simple-shear deformation arising from the {1 1 0}〈1 1 1〉 slip.

Processing, Microstructure, and Thermal Expansion Measurements for High Temperature Ru-Al-Cr B2 Alloys

AbstractThe B2 intermetallic compound RuAl has a melting temperature above 2000 °C and is a candidate for high temperature structural applications. A large extension of the B2 phase field is found in the Ru-Al-Cr system as was documented by the characterization of arc-melted and heat treated alloys. Two compositions consisting of Ru-35Al-19Cr and Ru-20Al-38Cr (at. %) were directionally solidified in an optical floating zone furnace. Depending upon the processing conditions, single phase, polycrystalline, B2 microstructures could be produced. The coefficient of thermal expansion (CTE) was measured from room temperature to 1250 °C for the Ru-20Al-38Cr alloy, and an average value of 11×10-6 K-1 was found. Additionally, the thermal conductivity was measured as 27 W/mK at room temperature for the Ru-20Al-38Cr B2 alloy and as 89 W/mK for binary RuAl.

Creep Deformation Mechanisms in Ru-Ni-Al Ternary B2 Alloys

The creep behavior of five Ru-Ni-Al ternary alloys with compositions across the ternary RuAl-NiAl B2 phase field has been investigated within a temperature range of 1223 to 1323 K. These alloys exhibited exceptional creep strength compared to other high melting temperature intermetallics. A continuous increase of the melting temperature and creep resistance with increasing Ru/Ni ratio was observed. Post-creep dislocation analyses revealed $$ {\left\langle {{\text{100}}} \right\rangle } $$ and $$ {\left\langle {{\text{110}}} \right\rangle } $$ edge dislocations in the Ni-rich alloys, while uniformly distributed $$ {\left\langle {{\text{100}}} \right\rangle } $$ jogged screw dislocations predominated in the Ru-rich ternary alloys. Transient creep experiments, dislocation substructures, and constitutive modeling indicate a transition of the creep mechanism from viscous glide-controlled to jogged screw-controlled deformation with increasing Ru/Ni ratio.

Effect of Thermo-Mechanical Processing on Texture and Elastic Modulus of Ti-Nb-Si Alloys for Biomedical Application

Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. Optical microstructure observation and X-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-cold rolled sample we have found well-developed α-fiber texture components which are frequently observed in bcc-structured metals and alloys. In recrystallized sample, rotated cube texture component was weakly detected. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.

Hot Deformation Behavior and Microstructure Evolution of Ti-6.5Al-1.5Zr-3.5Mo-0.3Si with an Equiaxed α+β Starting Structure

The hot deformation behavior of TC11 with an equiaxed α+β pre-form microstructure was investigated by hot compression tests in the temperature range 800°C–1090°C and strain rate range 1×10-310s-1. Several approaches have been used in this investigation, which include analysis of shapes of stress–strain curves, kinetic analysis and microstructure observation. The experimented results showed that, (1) In the β phase field, the alloy exhibits dynamic re-crystallization. The apparent activation energy in this domain is estimated to be 183kJ/ mol, which is close to that for self-diffusion in β. The re-crystallized grain size may be governed by competing of diffusion and dynamic re-crystallization rate at deforming conditions. (2) In the α+β phase field, the alloy exhibits super-plastic deformation behavior at 0.001-0.01s-1 and 980°C -850°C, and the apparent activation energy estimated in this domain is about 600 kJ/mol, which is much higher than that for self-diffusion in α-titanium due to β volume fraction is not constant over the test temperature range.

Effects of microstructure on fatigue crack growth behavior of Ti–6Al–2Zr–1Mo–1V ELI alloy

In this paper, fatigue crack growth behavior of Ti–6Al–2Zr–1Mo–1V ELI alloy with differences in the lamellar structure was investigated at room temperature. Heat treatments were performed in order to produce the microstructures that vary in α lamella width and colony size. Results show that, with adequate mechanical properties, the microstructure formed through air cooling from the β-phase field presents better fatigue crack propagation behavior than microstructures formed by other heat treatment paths. The fatigue crack growth proceeds along the planes either perpendicular or parallel to the long axis of α lamellae. The explanation for this behavior is related to prismatic slip along the a 1 [21¯1¯0] and a 2 [112¯0]/a 3 [1¯21¯0] directions.

Microstructure and texture evolution of Ti-Nb-Si based alloys for biomedical applications

Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. In as-quenched sample, relatively random distribution of pole figure was detected without showing a specific texture component. In as-cold rolled sample, however, it is found well-developed several texture components consisting of rotated cube, α-fiber and γ-fiber texture components which are frequently observed in bcc-structured metals and alloys were found. Therefore, texture components developed in the present alloys are closely related to the deformation of β-phase even though small amount of α″ phase co-exist in the microstructure. In recrystallized sample, α-fiber texture component is weakly detected while the other texture components, rotated cube and γ-fiber components, appears to be relatively unchanged. No additional texture components were detected besides those texture components observed in the cold rolled samples.

Omega phase transformation – morphologies and mechanisms

Abstract Various morphologies of the ω phase are observed in the α and the β phases of group-IV elements and their alloys where formation of this phase is induced by various means, namely, quenching from the β-phase field, isothermal annealing, under high static as well as dynamic pressure, and irradiation. The presence of various morphologies, such as ellipsoidal, cuboidal, granular or plate shapes, under different conditions is demonstrated. It is shown that various factors, including shuffling of atoms, shearing of atomic planes, long-range diffusion of solute atoms, change in volume during transformation, etc., influence the formation of different morphologies of the ω phase. The underlying mechanisms in each case are discussed. Ordered ω phases, which exist as equilibrium phases in various alloy systems, are shown as a case of superimposition of the concentration wave on the displacement wave.

General approach to phase stability and elastic properties of β-type Ti-alloys using electronic parameters

A general correlation between the phase stability and the elastic properties in β(body-centered cubic)-type Ti-alloys is investigated with the aid of two electronic parameters. The extent of the single β-phase field is clearly represented using these parameters. It is also shown that the addition of Al, O, and Sn is effective in modifying greatly the stability of the β-phase, because they work as the β-stabilizing elements in these β-type alloys, resulting in significant changes in the elastic properties of alloys.

Failure analysis of a titanium strut tank support in the space shuttle Columbia

A titanium midtank strut support from the Columbia space shuttle was submitted for failure analysis. Stereomicroscopy, light optical microscopy, and scanning electron microscopy with energy-dispersive spectroscopy were used to investigate the failed part. Evidence of melting was found on the fracture surface. Additionally, the high temperature associated with the melting was combined with cooling from the β-phase field, as indicated by the formation of a martensitic microstructure in the surrounding material. Craters were found on the surface of the part due to impact of foreign particles containing iron, nickel, and chromium. Failure of the titanium strut support was due to the severe high temperatures encountered as a result of the shuttle breakup and re-entry into the earth’s atmosphere.

COMPRESSION CREEP BEHAVIOR OF B2 AL-NI-RU TERNARY ALLOYS

AbstractThe compression creep behavior of five Al- Ni-Ru ternary alloys with compositions across the NiAl-RuAl B2 phase field has been investigated within the temperature range of 950 °C to 1050 °C. A continuous increase of the melting temperature and creep resistance with increasing Ru/Ni ratio in these alloys was observed. Post-creep dislocation analyses identified the presence of <100> and <110> edge dislocations in the Al-deficient alloys. Conversely, jogged <100> screw dislocations predominated in the Ru-rich ternary alloys. Dislocation substructures and transient experiments suggest a transition of the creep mechanism from viscous glide controlled to jogged screw motion in these alloys.

Enthalpies of Formation in the Al—Ni—Ru System by Direct Reaction Synthesis Calorimetry.

Abstract The enthalpies of formation of ternary compounds in the Al–Ni–Ru system have been determined by high temperature reaction calorimetry. The composition dependence of the enthalpy of formation and lattice parameter of the compounds with B2 structure were determined in the region of 0.40 ≤ Al ≤ 0.5 mole fraction. Unusual behavior is observed for the composition dependence of the enthalpy of formation suggesting that formation of a miscibility gap in the B2 phase field occurs. For some compositions, the experimental enthalpy data therefore represent the formation enthalpy of the metastable B2 phase.

Microstructure evolution of near-α titanium alloys during thermomechanical processing

In this work, near-α Ti-1100 alloys with TiC (1 vol.%) ceramic particles reinforcement were fabricated. After casting, ingots breakdown was performed at 1150 °C. The volume fraction of the primary α-phase in the microstructure as a function of heated temperature, and the β t temperature of this material, were examined by metallographic method. The ingots after breakdown were forged using various thermomechanical processing parameters. According to whether the deformation was performed in the β-phase field, the α + β-phase field, or the upper α-phase field, four types of hot deformation are recognized: deformation in the β-phase field; deformation started in the β-phase field and finished in α + β-phase field; deformation in α + β-phase field; deformation in the upper α-phase field. The characteristics of the microstructural evolution are different for the four types of deformation process. The final microstructures of this alloy after thermomechanical processing are depended on two factors. One is the thermomechanical processing process, and the other is the cooling process after hot deformation. The result will deepen the understanding of the microstructural characteristics of near-α titanium alloy during thermomechanical processing and optimize the microstructural control.

Enthalpies of formation in the Al–Ni–Ru system by direct reaction synthesis calorimetry

The enthalpies of formation of ternary compounds in the Al–Ni–Ru system have been determined by high temperature reaction calorimetry. The composition dependence of the enthalpy of formation and lattice parameter of the compounds with B2 structure were determined in the region of 0.40 ≤ Al ≤ 0.5 mole fraction. Unusual behavior is observed for the composition dependence of the enthalpy of formation suggesting that formation of a miscibility gap in the B2 phase field occurs. For some compositions, the experimental enthalpy data therefore represent the formation enthalpy of the metastable B2 phase.

Thermomechanical response of a powder metallurgy Ti−6Al−4V alloy modified with 2.9 Pct Boron

The thermomechanical response of Ti−6A1−4V modified with 2.9 pct B produced by a blended powder metallurgy route was studied with isothermal constant strain-rate hot compression tests in the temperature range 850°C to 1200°C and strain rate range 10−3 to 10s−1. Detailed analyses of the flow stress data were conducted to identify various microstructural deformation and damage mechanisms during hot working by applying available materials modeling techniques. In the α+β phase field, cavitation at the matrix/TiB interfaces and TiB particle fracture occurs at low strain rates (<10−1 s−1), while adiabatic shear banding also occurs at high strain rates. At low strain rates, the β phase deforms superplastically due to the stabilization of a fine grain size by the TiB particles. Grain boundary and matrix/TiB interface sliding with simultaneous diffusional accommodation are observed to contribute to the β superplasticity. The deformation behavior at high strain rates in the β-phase field is similar to that of the α+β phase field, with microstructural manifestations of extensive cavitation at the matrix/TiB interfaces and TiB particle fracture.

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

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