Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196 °C to 1200 °C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600 °C and 800 °C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600 °C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The workhardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200 °C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600 °C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900 °C) is thought to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating.     

Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196°C to 1200°C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600°C and 800°C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600°C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The work-hardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200°C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600°C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900°C) is though to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating. Z. JIN, formerly Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory     

Chemically induced reduction: A viable process for synthesizing γ-TiAl based intermetallic matrix composite powders containing nanocrystalline TiC

A chemically induced reduction process has been developed for synthesizing intermetallic matrix composites (IMCs) consisting of titanium aluminide and titanium carbide. The process involves the reduction of metal chlorides (TiCl₄ and AlCl₃) with metallic lithium in polar organic solvents such as acetonitrile (MeCN) and tetrahydrofuran (THF) to form a colloidal precursor. The as-prepared precursors have been either directly heat treated in ultra-high-purity argon (UHP-Ar) or pretreated in hydrogen (H₂) followed by further heat treatment in UHP-Ar. The powders have been characterized primarily using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results of the structural analyses conducted on the heat-treated precursors derived using MeCN as a solvent indicate the formation of either single-phase titanium carbide (TiC) or a composite mixture of γ-TiAl and nanocrystalline TiC, depending on the heat-treatment conditions. The formation of TiC is related to the strong interaction between TiCl₄ and the polar organic solvents, resulting in the formation of adducts which contain primary Ti-C linkages. Pretreatment of the precursors derived using MeCN as a solvent in H₂ promotes the removal of carbon and results in the formation of the composite mixture of γ-TiAl and TiC after subsequent Ar treatment at 1200 °C. At this stage, washing the pretreated powders in water helps to minimize and even eliminate any impurity phases to a large extent, leaving behind phase-pure composites containing γ-TiAl and TiC after the final Ar treatment. However, extended pretreatment in H₂ appears to be ineffective toward removal of additional carbon and leads to formation of hydride-phase impurities. On the other hand, the reductive reaction conducted using THF as a solvent results in minimizing the amount of carbon while inducing the formation of γ-TiAl during direct Ar treatment of the precursors. This is because of the weaker interaction between TiCl₄ and THF. Transmission electron microscopy was used to characterize the size distribution of the constituent phases. The analysis shows that the composite synthesized using these chemical approaches consist of discrete nanocrystalline TiC particles (<20 nm) that are uniformly distributed intermixed with submicron sized γ-TiAl (0.1 to 0.2 μm). Thus, the new chemical process proposed in this study demonstrates the potential for synthesizing in situ composites containing fine distribution of γ-TiAl and nanocrystalline TiC. Such composites could potentially exhibit unique mechanical properties and deformation behavior useful for high-temperature structural applications.     

Melting of γ-TiAl in the alumina crucible

Titanium aluminide alloys with unique properties as well as high strength to weight ratio, excellent oxidation resistance and acceptable high temperature mechanical properties can be used as a high temperature structural material and a competitor for super-alloys. In this research, production process of Ti-48Al-2Cr (numbers indicate atomic percent) intermetallic in an induction furnace with argon atmosphere, alumina crucibles with different purities and several ingot casting time, were investigated. Microstructure of produced ingots was studied by optical microscope and scanning electron microscope (SEM) with EDX analyzer and phase analysis were studied by XRD method. Results show that microstructures are completely lamellar and by increasing cooling rate, interlamellar space will decrease. In the structure, three morphologies of alumina particles could be seen that are: spherical, cluster and lathed. Volume fraction of Al₂O₃ particles will increase by increasing the holding time of melt and crucibles with higher SiO₂ content will react more severe with melt.     

Effect of Rare Earth on Microstructure of γ-TiAl Intermetallics

The rare earth (RE) elements (Ce, La) were added to binary Ti-47% Al alloys (atomic fraction) by Induction Skull Melting. The element Ce of 1.0 atomic percent was added individually, and La of 0.2 atomic percent was added individually. This article studied the influences of rare earth metal (Ce, La) on microstructure of as-cast TiA1 based alloy by XRD, SEM, EMPA and TEM measurement methodology. The results show that most of rare earth-rich phases (AlCe, Al-La) are uniformly distributed in grain boundary in the shape of discontinuous network, and some particles of rare earthrich phases within the grains are mainly ellipsoids. In addition, rare earth element can obviously refine the grain size and the lamellar thickness of as-cast TiAl based alloy samples. The grain size of Ti-47Al-1.0Ce-0.2La alloy reaches about 30～80μm, and the lameUar thickness of its γ phase and α2 phase are less than 200 and 20nm, respectively.     

Structure and Properties of Detonation Coatings Based on γ-TiAl

Investigations of a coating ― substrate composite before and after oxidation in air at 900°C revealed that the main structural features were: formation of an Al₂O₃ scale on the surface of the TiAlCrSc(γ) coating (as a result of oxidation) and of inner layer at its interface with a 90% titanium substrate (as a result of diffusion in the composite). The observed phenomenon was caused by a Kirkendall effect resulting in the formation of titanium-enriched phases, apparently Ti₃Al and α-Ti, which have a broad homogeneity range. The formation of a diffusion zone in the system with displacement of the Kirkendall plane in the direction of the substrate has a positive effect on the adherence of the coating. Furthermore, the filling of vacancies and pores in the coating with additional material supplied by the diffusion of titanium should have a favorable effect on the strength and durability of the coating, particularly its fatigue resistance.     

Ductile reinforcement toughening of γ-TiAl: Effects of debonding and ductility

The effects of reinforcement debonding and work hardening on ductile reinforcement toughening of γ-TiAl have been examined. Debonding has been varied by either the development of a brittle reaction product layer or by depositing a thin oxide coating between the reinforement and matrix. The role of work hardening has been explored by comparing a Nb reinforcement that exhibits high work hardening with a solution hardened TiNb alloy that exhibits negligible work hardening. It is demonstrated that a high work rupture is encouraged by extensive debonding when the reinforcement exhibits high work hardening. Conversely, debonding is not beneficial when the reinforcement exhibits low intrinsic ductility due to an absence of work hardening.     

Tem analysis of square-shaped dislocation configurations in the γ′ phase of a Ni-based superalloy

Straight sessile dislocations are observed in the γ' phase of a Ni-base superalloy deformed at 975°C. They are shown to be prismatic pure edge dislocations, lying along 〈110〉 directions, with a Burgers vector $\text{b}\text{= 2a〈001〉}$. We propose a formation mechanism and a non-planar core structure which accounts for the observations.     

Comparison of experimental and theoretical electronic charge distribution in γ-TiAl

Using critical voltage electron diffraction, Fox has recently determined the lowest seven X-ray structure factors of γ-TiAl (L1₀ structure). We present here a comparison of these accurately measured (0.15%) structure factors with first-principles local density calculations, finding an agreement within 0.7% and an r.m.s. error of 0.013 e/atom. While such measurements are limited to the first few structure factors ϱ(G) (where G is the crystal momentum), theory is able to obtain ϱ(G) for arbitrarily high G. If we construct charge density deformation maps by Fourier summations up to the lowest measured G, the calculated and experimental density deformation maps agree very closely. However, if we include in the theoretical density deformation map high G values (outside the range accessible to experiment), qualitatively different bonding patterns appear, in particular between Ti atoms. Systematic study of the total, valence, and deformation charge densities as well as comparison with results for NiAl in the hypothetical L1₀ structure elucidate the bonding patterns in these transition metal aluminides.     

The effect of impact damage on the room-temperature fatigue behavior of γ-TiAl

The relationship between impact damage and the fatigue behavior of γ-TiAl has been examined. Axial fatigue specimens fabricated from cast Ti-47.9Al-2.0Cr-1.9Nb (to be referred to as 48-2-2) and Ti-47.3Al-2.2Nb-0.5Mn-0.4W-0.4Mo-0.23Si (to be referred to as WMS) alloys were damaged by impact under controlled conditions with a 60 deg wedge-shaped indenter to simulate assembly-related damage in low-pressure turbine blades. The level of damage produced was quantified and found to correlate well with the peak load of the impact event. The WMS alloy exhibited a greater resistance to impact damage due to its higher yield strength and lamellar microstructure. A measure of the ambient-temperature fatigue failure stress in the alloys was obtained by standard fatigue testing employing a step-loading approach. The failure stress of the WMS alloy was greater than that of the 48-2-2 alloy in the undamaged state. The relationship between impact damage and failure stress was examined using a threshold-based approach. These studies indicate that, for damage levels below a transitional flaw size, the failure stress is near that for undamaged specimens. At damage levels greater than the transitional flaw size, the failure stress can be adequately approximated using the threshold stress-intensity range (ΔK _TH ) from long-crack growth testing. Fractographic studies were performed to investigate impact damage and crack-advance mechanisms, which match those observed in other alloys tested at room temperature.     

Ductile phase toughening mechanisms in a TiAlTiNb laminate composite

Toughening mechanisms in a laminate composite composed of alternating layers of brittle γ-TiAl and ductile TiNb reinforcements were studied. The TiNb phase comprising about 20% of the composite volume, contributed to toughening by both crack renucleation and bridging mechanisms, yielding a steep resistance curve and effective toughness more than ten times higher than the matrix value. In part, the extraordinary toughening is derived from large scale bridging effects, which occur when the size of the bridging zone is not small compared to the crack and specimen dimensions. Large scale bridging model predictions based on independent evaluations of the fundamental composite properties, including the reinforcement stress-displacement function, were in good agreement with the experimental observations. We demonstrate how the models can be used to optimize the composite properties for specific applications.     

The structure of complex monoborides in γ-TiAl alloys with Ta and B additions

Fibrous primary borides grown from the melt in a Ti-48Al-9Ta-4.3B at.% alloy were characterized by conventional and high resolution transmission electron microscopy. The as-cast borides show predominantly the B27 crystal structure of TiB, but with twice as much Ta as Ti in the metal sublattice. The borides contained numerous planar faults parallel to the (100)_TiB planes, which multiplied and evolved into long platelike precitates with the structure of TaB (B_f) upon prolonged heat treatment. The initial faults could be described as rotational twins on the (100)_TiB plane, giving rise to monolayers of the B_f structure; migration of planar diffusion regions along the needle axis subsequently coarsens the TaB regions. Experimental and computer-generated diffraction patterns were consistent with the orientation relationship anticipated from the twinning model, (100)_TiB|(100)_TaB and [010]_TiB|[001]_TaB. High resolution images showed that the B_f precipitates are fully coherent with the parent B27 matrix and only a few atomic layers thick, but extend over many micrometers along the axis of the boride needles. Moreover, microchemical analysis revealed that the structural transformation occurs without apparent solute partitioning. The evolution of the boride structures is discussed in terms of phase stability, growth kinetics and lattice energy considerations.     

Anisotropy of the critical resolved shear stress of a superalloy containing 6 vol.% L12-ordered γ′-precipitates

Single crystals of the γ′-strengthened nickel-base superalloy NIMONIC PE16 have been compression tested in the temperature range 683–1143 K. Four different orientations of the specimens have been studied: [0 0 1].[1¯23].[011] and [1¯11]. They were either in the homogenized, single-phase state or in the peak-aged state. The critical resolved shear stress (CRSS) of the homogenized specimens was isotropic at 683 K. The CRSS of the peak-aged specimens, containing 6 vol.% of L1₂-long-range ordered γ′-precipitates of 25 nm radius, was anisotropic at 683 K and at 989 K: the [001]-orientated specimens were the softest ones, the CRSS increased as the orientation moved towards [011] or [11¯1]. This is the same orientation dependence found for the CRSS of single-phase L1₂-ordered materials. The interpretation of the anisotropy of the CRSS of NIMONIC PE16 follows that given for single-phase L1₂-long-range ordered materials.