Polycrystalline Ti-50 and Ti-52A1 (in at.%) alloys, with large grain sizes (50Al-300μm, 52Al-500μm) and containing low (∼250 wt.ppm) levels of oxygen were deformed under compression over a wide temperature range (77-1173°K). The 0.2% proof stress-temperature profiles comprise of three distinct regimes: Regime I (between 77-∼600°K), regime II (between ∼600-1073°K), and regime III (above 1073°K). Deformation temperature influences the types of dislocations present, the nature of superdislocation dissociations, and the morphological characteristics of both ordinary and superdislocations. Collectively, experiment and theory suggest that the flow properties at low temperatures (regime I) are controlled by lattice friction, whereas at higher temperatures (regime II) the properties are dislocation obstacle controlled.

Quantitative TEM observations have been made on a series of gamma Ti47Al51Mn2 polycrystals that were deformed at different temperatures. Special attention has been given to determining the statistical variation of defect densities that occur at the different temperatures. The results, which are in good agreement with mechanical testing [J], indicate that three different mechanisms control deformation in this alloy: lattice friction and the formation of faulted dipoles at low temperatures, the pinning of ordinary dislocations at intermediate temperatures, and the bowing and looping of dislocations at high temperatures. The anomalous flow strength of this alloy has been found to be related to the intrinsic pinning of ordinary dislocations. Details of this pinning and subsequent unzipping process, which are the basis for the new local-pinning-unzipping (LPU) models[2-4], are outlined within.

The fine structure and the character of a dislocations on prism planes have been determined for room-temperature deformed polycrystalline Ti3Al using a combination of experimental and computational techniques of transmission electron microscopy (TEM). Within the resolution limits of the weak-beam technique the fine structure of prism plane a dislocations in Ti3Al is found to be planar and to comprise only a single ribbon of antiphase boundary and no experimentally observable stacking faults.

In situ deformation experiments have been performed on a two-phase Ti-23.7Al-9.4Nb (%at) alloy at room temperature. Dislocation glide processes in each phase are studied and discussed. An exemple of dislocation transmission through an interface is analysed in detail.

In Ni3Al alloys the yield stress increases with increasing temperature up to a maximum stress, reached at the temperature TP. Below TP glide occurs on primary octahedral planes whereas above TP glide on primary cube planes is dominating. Therefore this change of glide planes was considered to determine TP.

TEM investigations were carried out to study the mechanisms that control TP in Ni3Al specimens with [001] compression axis. In this case no shear stresses are acting on any of the cube glide systems. The TEM results show a pronounced change of the dislocation structure on the {111} glide planes below and above the peak. The breakdown of the barriers (locked screw dipoles) starts when the diffusion is high enough to facilitate their annihilation. The dynamic recovery of the locked screws occurs by multiple cross-slip via octahedral planes. It is proposed that, in this and other ordered alloys, the value of TP is diffusion controlled.

The strain rate sensitivity ß of the flow stress τ is associated with workhardening and β=(δτ/δln ε) is proportional to the workhardening increment τh = τ - τy, where τy is the strain rate independent yield stress. The temperature dependence of β/τh reflects changes in the rate controlling mechanism. At intermediate and high temperatures, the hardening correlates with the density of [101] dislocations on (010). The nature of the local obstacles at room temperature is not established.

A transmission electron microscope (TEM) study of dislocation reactions that take place during the first few percents of permanent strain at room temperature is presented. The nature of the dipolar segments, a noticeable feature within the deformation microstructure, is elucidated. It is determined that antiphase boundary (APB) tube formation is unlikely to stem from the annihilation between a mobile superdislocation and an immobilized Kear-Wilsdorf (KW) configuration, at variance from what has been expected so far. A clear relationship between APB tubes and superlattice stacking fault (SSF) dipoles is pointed out.

The stress and temperature dependence of the dislocation velocity v(σ, T) has been studied experimentally in Ni3AI single crystals. The mobility of individual dislocations was measured by the stress pulse-etching technique at temperatures between 77 and 873 K at the resolved shear stresses between 25 and 150 MPa over a range of dislocation velocities from 3×l0−7 to 3×10−5 m/s. Fresh dislocations were introduced and revealed by a selective etchant on {001} surfaces of electropolished specimens annealed at 1473 K for 120 hours in argon. The specimens were deformed in three point bending around <110> bending axis at a constant temperature. The dislocation arrays moved in two acting <110> {111} primary glide systems. The motion occurred over a range of applied stresses close to the macroscopical yield stresses y The dislocation velocity decreased as temperature increased, in perfect analogy to the experimental data on y obtained before 1 on the same Ni3Al.. No detectable violation of the Schmid law was observed. The stress dependence of the dislocation velocity was rather steep resulting in a relatively large activation area of A=100-200b2. Such an activation area is greater than that one could expect in a kink-mode crystal with predominant Peierls mechanism for dislocation motion, although it is considerably less than A measured in other materials controlled by local obstacles.

Ni3(Al,Ta) single crystals have been deformed in compression in the temperature range of the flow stress anomaly (293–780K). The strain-rate sensitivity (SRS) of the flow stress has been characterised by using a technique of repeated stress relaxations that allows for the measurement of the true (or effective) activation volume (Veff). When measured at the conventional critical resolved shear stress (CRSS), Veff exhibits as a function of temperature a sharp discontinuity close to 470K. When the temperature is held constant (420K), the discontinuity of Veff occurs along the stress-strain curve at approximately 3% strain; the stress for both discontinuities is approximately the same. These results suggest a change in the rate controlling mechanism that is dependent on stress as much or more than temperature.

The yield stress of a specimen prestrained at 294 K increases only slowly or is constant up to a temperature above which the yield stress of a virgin specimen is greater. Above that temperature, the yield stress tends to increase as for a virgin crystal. For sufficiently high prestresses, the yield stress anomaly disappears completely. The results are interpreted in terms of a model in which the operation of Frank-Read sources is rate controlling.

The deformation substructure and mechanical response of the LI2 intermetallic, Ni3Al, subjected to shock-wave loading was studied. Following shock prestrains to shock pressures of approximately 14.0 GPa, 23.5 GPa, and 47.2 GPa, the reload yield strength of Ni3Al was measured to be 750 MPa, 1250 MPa and 1500 MPa, respectively. The effective hardening in the shock-loaded Ni3Al exceeds that quasistatically obtained when deformed to roughly the same equivalent strains. Coarse planar slip on {111} type planes, a high density of stacking faults, and deformation twins with a {111} type twinning plane were observed in the shocked material. An increasing propensity for twinning and stacking fault formation was observed with higher shock peak pressures.

The dissociated structure of the a〈101〉 superdislocations in Ni3Al is predicted as a function of temperature. The temperature dependence of the relevant fault Helmholtz free energies and elastic constants are calculated within the quasiharmonic approximation using the embedded atom method. The results of these calculations are then incorporated into anisotropic elasticity theory-based calculations of the dissociation distances. The cross-slip activation enthalpy is estimated and found to decrease by 24% at 600 K when compared with its 0 K value. The calculations point to the need to perform experiments to address this temperature dependence.

Embedded atom method molecular dynamics simulations of low Σ grain boundaries in Ni3Al are presented. The results show that the grain boundary plane has a larger effect on grain boundary energy than the Σ value, rigid body translations and stoichiometry. Assessment of the energies of Σ3n (n ≥ 1) grain boundaries in Ni3Al for various grain boundary planes indicates that only the Σ3 grain boundary is energetically preferred. The implications of this result for the development of the twin limited structure based on energetic considerations are discussed.

One of the fundamental questions concerning Ni3Al is why doping with boron improves the room temperature ductility of the polycrystalline material. Boron is thought to prevent environmental embrittlement and increase the cohesive strength of grain boundaries since it changes the fracture mode from intergranular to transgranular. This change in cohesive energy must be reflected in the bonding changes at the grain boundary which can be probed using spatially resolved electron energy loss spectroscopy (EELS). We have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24 using EELS, EDX and ADF imaging in a UHV STEM. Ni-enrichment is seen in a 0.5–1 nm wide region at large angle grain boundaries, both in the absence and presence of B. EELS shows that B segregation can vary along the interface. The Ni L2, 3 core edge fine structure which is sensitive to the filling of the Ni d-band, shows only the boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. These results demonstrate that boron segregation increases the cohesive energy and hence improves the fracture resistance of the grain boundary, by making the bonding at boundaries similar to that in the bulk. The measured changes in d band filling may also affect the local solubility of hydrogen.

Auger electron microscopy (AES) analysis of boron doped Ni3Al have shown boron segregation at grain boundary (GB). Boron segregation was enhanced with increasing bulk boron content independent of stoichiometry, suggesting that hypo-stoichiometric alloy is intrinsically ductile even without boron. A slight Ni cosegregation is confirmed using atom probe field ion microscope (APFIM). In Ni3(Si,Ti), no distinct composition fluctuation was identified between matrix and GB vicinity. These results suggest that atomic bonding atmosphere modification from covalent to metallic in the vicinity of GB is one of factors to modify ductility for Ll2-type intermetallics.

The application of TiAl-based alloys as an exhaust valve material would allow automotive engines to operate at higher temperatures with increased efficiencies. Development of these materials at Ford initially concentrated on the Ti-48A1-1V (at%) system. This included; 1) room and elevated temperature fatigue, 2) creep and 3) tensile ductility optimization. Results from this test program in conjunction with other available data, previous ceramic experience and limited engine testing led to the conclusion that the major roadblock to implementation was not optimizing mechanical properties, but rather low cost and reliable valve manufacturing technology. When a cost effective manufacturing technology is developed, then the emphasis will shift to ensuring the product durability. Hence, the focus of the current program is the development of valve manufacturing technologies, in particular valve blank fabrication. Currently, casting appears to be the lowest cost alternative for valve blank fabrication. This paper reviews the technology development process as it pertains to TiAl-based valves.

Lightweight gamma titanium aluminide automotive valves may significantly improve engine performance and fuel economy. However, low cost mass production processes must be developed to realize these gains. Permanent mold casting processes and low cost heat treatments are being developed to enable commercial introduction of TiAl automotive valves. Microstructural development, tensile, creep, and high cycle fatigue properties of permanent mold Ti-47Al-2Nb-1.75Cr (at%) castings are compared with investment castings of similar size. A recommendation is made to use as-HIP castings for components not designed to damage tolerant criteria. This work was performed as part of a vertically integrated team effort to transfer aerospace TiAl technology to the automotive market.

The objective of this study is to evaluate the high temperature behavior of γ TiAl-based alloy sheets containing AI2O3 particles, produced by a combination of vacuum induction melting, use of a CaO crucible, and direct sheet casting, over a wide temperature range. Alumina particles, having a tendency to coagulate during solidification of a TiAl ingot, are finely dispersed due to the disturbance of high frequency induction, and frozen without having enough time to grow in size by direct sheet casting. The TiAl sheet thus produced shows remarkable high temperature tensile strength which exceeds that of conventional ingots having the same composition and various different structures. This is determined to be attributable to the dispersion strengthening of finely dispersed AI2O3 particles whose diameter is from 100 to 500nm. Moreover, because of the small size of these alumina particles, the TiAl sheet does not show any significant retardation in high temperature ductility, which is often the case in conventional ceramic-reinforced intermetallic compound composites.

Room temperature tension-tension fatigue crack growth experiments were perfomed on single edge notch specimens of Ti-48Al-2Cr sheet materials produced on industrial scale with different microstructures ranging from near gamma to fully lamellar. Crack extension was monitored using a travelling optical microscope and a DC potential method. Crack paths and fracture surfaces were investigated by SEM. Fracture toughness data were determined by monotonic loading of previously fatigued samples.

Morphology, fine structure, and chemistry of phases formed in cast Ti-47Al-2Cr-2Nb-0.8B due to boron addition were examined in detail. Boron acts an inoculant, refining and stabilizing the cast grain size. A curving ribbon-shaped phase was present throughout the material, related to the ordered phase Ti2AlCr. The ribbon phase had the B2 structure, with a0∼0.318nm. Many ribbons had a low degree of order, i. e. the material was a BCC mixture of Ti, Al, Nb, and Cr. Ribbon phase composition was variable, measured to be: Ti-(20–30)Al-(2–4)Nb-(5–16)Cr. TiB2 was present in small amounts, always in intimate contact with the ribbon phase. A (100)ribbon ‖ {1100}TiB2, (011)ribbon ‖ {1122}TiB2 orientation relationship was observed. It is proposed that the ribbon phase is a remnant of the high-temperature titanium aluminide β-phase, stabilized by dissolved boron.