H.L. Fraser

Processing, microstructure, and properties of co-continuous alumina-aluminum composites

Abstract A novel co-continuous composite of Al 2 O 3 and Al has been developed, consisting of approximately 65% (by volume) of the ceramic phase. It is formed by a liquid phase displacement reaction, involving the displacement of Si from SiO 2 and its replacement by Al. A model for the formation mechanism is presented, based on the reaction thermodynamics and the associated experimentally determined transformation kinetics. It is shown that the process is essentially near-net shape, in which the features of the SiO 2 precursors are faithfully reproduced in the composite product. Various physical and mechanical properties that are exhibited by this composite have been determined and are presented.

The role of crystallographic and geometrical relationships between α and β phases in an α/β titanium alloy

The present study has examined for α/β-Ti alloys the relationship between the morphology and crystallography of Widmanstatten plates of α-Ti in colonies within a prior grain of β-Ti. Thus, optical metallography, scanning electron microscopy and transmission electron microscopy have been used to characterize the morphological features of the microstructure, whereas orientation-imaging microscopy (OM) and transmission electron microscopy (TEM) have been employed to reveal crystallographic information. It has been discovered that within a prior β-Ti grain, although the growth direction of the Widmanstatten plates in given colonies may differ by large angles from α-plates in other colonies, they may exhibit very close crystallographic relationships. For example, inclined α-plates may share common basal planes and be related by a rotation of ~10.5° about the c-axis of the crystals. This phenomenon has been interpreted on the basis of variant selection of the Burgers orientation relationship commonly adopted between the α and β phases in these alloys. Similar relationships have been observed in α-colonies growing either side of a given prior β grain boundary. These latter observations have been used to draw conclusions concerning the precipitation of α on prior β grain boundaries.

Deformation mechanisms in the intermetallic compound TiAl

Abstract Samples of a polycrystalline Ti-52at.°Al alloy have been deformed in compression at room temperature and 600°C to strains of ≈3°. The deformation microstructures have been characterized using transmission electron microscopy. It is found that, at room temperature, glide of dislocations with b = <101] has occurred, with some contribution from twinning. Dislocations with b=1/2<110] make up a negligible fraction of the dislocation density, and it is argued that these defects are in essence sessile at room temperature. However, in samples deformed at 600°C, the dislocation population is dominated by those with b=1/2<110], whereas those with b = <101] are most often involved in Kear-Wilsdorf configurations. These various results, including the line directions of the dislocations, are interpreted on the basis of the effect of covalent bonding on the anisotropy of the Peierls stresses (following the work of Greenberg, Anisimov, Gornostirev and Taluts) and therefore dislocation mobilities in this compoun...

The influence of second phase Ti3Al on the deformation mechanisms in TiAl

Abstract Dislocations in samples of a heat-treated and quenched two phase Ti-46at.% Al alloy have been characterized, and those with Burgers vectors, b, given by b=½<1l0] and b=½<112] have been observed. The microstructure of deformed samples is characterized mainly by dislocations with b=½<1l0]. These dislocations have high Peierls stresses in TiAl of nominal purity, because of the directionality of bonds between the Ti atoms. Therefore, the present observations have been interpreted on the basis that the phase Ti3Al getters the interstitial elements from the TiAl, since the solubility of these elements in the former phase is significantly larger than in the latter. It is proposed that the removal of interstitials from TiAl in this way decreases the degree of directionality of bonding between the Ti atoms, and so reduces the anisotropy in Peierls stresses caused by these directional bonds. A relatively large number of twins have also been observed in the lamellae of TiAl, and this may be interpreted on t...

Investigation of techniques for measuring lattice mismatch in a rhenium containing nickel base superalloy

Abstract Three techniques for measuring γ/γ′ lattice mismatch have been examined in a Ni-base superalloy subjected to two different aging heat treatments. The techniques used for measuring lattice mismatch were X-ray diffraction, convergent beam electron diffraction (CBED) and interface dislocation analysis. Additionally, a scanning transmission electron microscope equipped with an X-ray energy dispersive spectroscopy (EDS) system has been used to examine phase compositions. From this study, it has been determined that the X-ray diffraction and CBED yield similar results for room temperature lattice mismatch, although care must be taken in applying the CBED technique due to the complex strain fields present in high volume %γ′ alloys. The dislocation analysis technique gives larger negative values of mismatch. It is believed that these latter values represent those which exist at the aging temperature.

Direct laser deposition of alloys from elemental powder blends

Abstract The complexity in design of components used in advanced aerospace and automotive applications is continuously increasing. This has led to the development of near-net shape manufacturing techniques such as laser engineered net-shaping (LENSTM) which falls in the class of direct laser deposition processes from powder feedstock. Despite considerable advances in process optimization, there is a rather limited understanding of the role of metallurgical factors in laser deposition of alloys. This paper discusses the significant role played by the thermodynamic enthalpy of mixing in the deposition of alloys from elemental powder blends using LENSTM. This factor influences the homogeneity as well as the rate of solidification of the alloy and consequently the microstructure and properties of the deposit. The enthalpy of mixing could also serve as a very useful guideline in the design of novel alloys that are laser deposited from elemental powder blends.

Microstructural evolution in laser deposited compositionally graded α/β titanium-vanadium alloys

Abstract A graded binary Titanium-Vanadium alloy has been deposited using the laser engineered net-shaping (LENS™) process from a blend of elemental Ti and V powders. A compositional gradient in the alloy, from elemental Ti to Ti-25at%V, has been achieved within a length of ~25 mm. Subsequent to deposition, longitudinal sections of the deposit have been characterized in detail using scanning and transmission electron microscopy. Though the phases across the graded alloy correspond to those typically observed in α / β Ti alloys, the scale and morphology of the microstructural features varies substantially with composition. Several phase transformations, namely, β →Widmanstatten α , β → ω and martensitic β →hexagonal α ′, are encountered in the graded alloy sample during LENS™ deposition. The ability to achieve such substantial changes in composition across rather limited lengths make such graded alloys highly attractive candidates for investigating the influence of systematic compositional changes on phase transformations and concurrent microstructural evolution in these alloys.

Control and elimination of nucleation-related defects in GaP/Si(001) heteroepitaxy

GaP films were grown on offcut Si(001) substrates using migration enhanced epitaxy nucleation followed by molecular beam epitaxy, with the intent of controlling and eliminating the formation of heterovalent (III-V/IV) nucleation-related defects—antiphase domains, stacking faults, and microtwins. Analysis of these films via reflection high-energy electron diffraction, atomic force microscopy, and both cross-sectional and plan-view transmission electron microscopies indicate high-quality GaP layers on Si that portend a virtual GaP substrate technology, in which the aforementioned extended defects are simultaneously eliminated. The only prevalent remaining defects are the expected misfit dislocations due to the GaP–Si lattice mismatch.

Laser deposition of compositionally graded titanium–vanadium and titanium–molybdenum alloys

Compositionally graded binary titanium–vanadium and titanium–molybdenum alloys have been deposited using the laser engineered net-shaping (LENS™) process. A compositional gradient, from elemental Ti to Ti–25at.% V or Ti–25at.% Mo, has been achieved within a length of ∼25 mm. The feedstock used for depositing the graded alloy consists of elemental Ti and V (or Mo) powders. Though the microstructural features across the graded alloy correspond to those typically observed in α/β Ti alloys, the scale of the features is refined in a number of cases. Microhardness measurements across the graded samples exhibit an increase in hardness with increasing alloying content up to a composition of ∼12% in case of Ti–xV and up to a composition of ∼10% in case of the Ti–xMo alloys. Further increase in the alloying content resulted in a decrease in hardness for both the Ti–xV as well as the Ti–xMo alloys. A notable feature of these graded deposits is the large prior β grain size resulting from the directionally solidified nature of the microstructure. Thus, grains ∼10 mm in length grows in a direction perpendicular to the substrate. The ability to achieve such substantial changes in composition across rather limited length makes this process a highly attractive candidate for combinatorial materials science studies.

The influence of temperature and alloying additions on the mechanisms of plastic deformation of Ti3Al

Abstract Samples of polycrystalline Ti3Al and Ti3Al containing 4 at.°Nb in solution (substituted for Ti) have been deformed at room temperature and at 650°C and examined subsequently by transmission electron microscopy in order to determine the influence of temperature and alloy content on the relative activity of the various slip systems. It has been shown that at room temperature in Ti3Al the deformation mechanisms involve glide of coupled pairs of dislocations with b=1/6 on {1010} and, to a rather limited degree, glide of pairs of dislocations with b=1/6 on {112 1}, with the extent of the particular slip system which operates varying between grains of different orientations. At 650°C, glide of these two types of slip system occurs, but the morphology of slip of the dislocations with b =1/6 changes at the elevated temperature. Furthermore, evidence for the climb of dislocations has been obtained, which may produce strain at elevated temperatures. Only limited activity of the <11...