Development of ultrafine lamellar structures in two-phase γ-TiAl alloys

Processing of two-phase γ-TiAl alloys (Ti-47Al-2Cr-2Nb, or minor modifications thereof) above the α-transus temperature (T _α ) produced unique refined-colony/ultrafine lamellar structures in both powder-and ingot-metallurgy (PM and IM, respectively) alloys. These ultrafine lamellar structures consist of fine laths of the γ and α ₂ phases, with average interlamellar spacings (λ _L ) of 100 to 200 nm and α ₂-α ₂ spacings (λ _α ) of 200 to 500 nm, and are dominated by γ/α ₂ interfaces. This characteristic microstructure forms by extruding PM Ti-47Al-2Cr-2Nb alloys at 1400 °C and also forms with finer colony size but slightly coarser, fully lamellar structures by hot-extruding similar IM alloys. Alloying additions of B and W refine λ _L and λ _α in both IM Ti-47Al (cast and heat treated at 1400 °C) and IM Ti-47Al-2Cr-2Nb alloys (extruded at 1400 °C). The ultrafine lamellar structure in the PM alloy remains stable during heat treatment at 900 °C for 2 hours but becomes unstable after 4 hours at 982 °C; the ultrafine lamellar structure remains relatively stable after aging for >5000 hours at 800 °C. Additions of B+W dramatically improve the coarsening resistance of λ _L and λ _α in the IM Ti-47Al alloys aged for 168 hours at 1000 °C. In both the PM and IM Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine lamellar structures correlate with high strength and good ductility at room temperature, and very good strength at high temperatures. While refining the colony size improves the room-temperature ductility, alloys with finer λ _L are stronger at both room and high temperatures. Additions of B + W produce finer as-processed λ _L and λ _α in IM TiAl alloys and stabilize such structures during heat treatment or aging. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Grain boundary precipitation in aluminum alloys: Effect of boundary structure

A transmission electron microscope study of grain boundary precipitation in Al-Zn and Al-Zn-Mg alloys has been conducted with emphasis on the influence of localized boundary structure. Intrinsic grain boundary defects are found to have a significant effect on the precipitation sequence in that they assist the emerging precipitates in establishing a low energy habit plane relationship with at least one bordering grain. Under more extreme conditions of unavailable habits or unfavorable intrinsic structures, extrinsic defects dominate the precipitation reaction.     

Interaction of deformation shear bands with nanoparticles in amorphous-nanocrystalline alloys

The interaction of shear bands with crystalline nanoparticles in amorphous-nanocrystalline Fe-Ni-B alloys is analyzed theoretically and experimentally. The following five interaction mechanisms are revealed: absorption, bypassing, cutting, retardation, and accommodation. The nanocrystal size is shown to be the key factor of this interaction. The experimental results obtained are satisfactorily described by the proposed theoretical models.     

Compatibility of deformation in two-phase Ti-Al alloys: Dependence on microstructure and orientation relationships

A two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures. These consisted of lamellar and equiaxed distributions of y grains in which the α₂ phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between γ/γ and γ/α₂ grains have been measured, and these have been used to understand their effect on the compatibility of deformation across adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy (TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.     

Effects of plastic deformation on the structure of WC-Co alloys

Summary The structural picture is established of strain propagation in the carbide and cobalt phases in VK6 and VK50 hard alloys at loads approaching the yield point of the alloys. Electron-microscopical investigation reveals slip bands in the carbide phase at stresses close to, or equal to, the yield point of the alloys. This is observed both in the low-cobalt VK6 and the high-cobalt VK50 alloys, which may be attributed to their structures being similar in that they both exhibit continuous WC-WC grain contacts.Translated from Poroshkovaya Metallurgiya, No. 5 (77), pp. 63–68, May, 1969.     

Diffusion coarsening of the lamellar structure in two-phase Ti-47.5 at.% Al intermetallic alloy

Diffusion coarsening of the lamellar structure in two-phase γ-α₂ intermetallic Ti-47.5 at.% Al alloy is treated as caused by hole-like macro-defects growth. A model of unlimited diffusion-controlled growth of such defects and an approach to the experimental data processing are considered. Based on this model, experimental investigations of coarsening kinetics have been carried out in TiAl alloy in the temperature range 1073–1373 K. The low value of the activation enthalpy Q_m = 63.8 kJmol⁻¹ has been found. The stability of the γ-α₂ lamellar structure in the studied TiAl intermetallic alloy is compared with the stability of the γ-γ′ raft structure in Ni-base superalloy single crystals.     

Study of evolution of dislocation structure with the deformation in Zirconium alloys

Evolution of dislocation structure as a function of deformation, in case of a two phase zirconium alloy, was characterized by transmission electron microscopy as well as X-ray line profile analysis. Dislocation structures up to a deformation of 10% show that deformation is mitigated by a single active slip system. Dislocation cellular structures were observed at a deformation of 27%. Two different X-ray based techniques of evaluation of dislocation density were used in the present study. The methods though differed in the absolute values of dislocation densities (ρ), agreed with each other in terms of trend in variation of ρ with the amount of plastic strain.     

Convection in the two-phase zone of solidifying alloys

The analysis is applicable to alloy solidification which proceeds horizontally to the center of a mold. The model follows the growth of the solid-liquid zone adjacent to the chill face (the initial transient), the movement of the zone across the mold, and the region of final solidification adjacent to the centerline (the final transient). During solidification the density of the liquid varies across the twophase zone. Consequently, there is natural convection which is treated as flow through a porous medium. The equations for convection are coupled with the equation of solute redistribution between the phases in order to calculate macrosegregation after solidification is complete. Results were computed for alloys which show: (1) “inverse segregation≓ at a cooled-surface; (2) macrosegregation resulting from solidification with the initial transient, a period with a complete two-phase zone, and a final transient; and (3) macrosegregation when the width of the two-phase zone exceeds the semi-width of the mold.     

Effect of combined deformation on the structure and properties of copper and titanium alloys

The effect of a combination scheme of severe plastic deformation and subsequent cold rolling or electroplastic rolling on the deformability, microstructural evolution, and mechanical properties of copper, titanium of various purities, and a titanium alloy of an equiatomic composition is studied. The combined deformation method is shown to create a number of new nanostructured and ultrafine-grained states with a high strength and ductility.     

Redistribution of interstitial impurities in the structure of low-alloy powder metallurgy tungsten alloys in deformation

Translated from Poroshkovaya Metallurgiya, No. 12, pp. 65–69, December, 1990.     

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti₃Ni₄ precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti₃Ni₄ precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Lüders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Lüders band propagation and result in a more gradual transformation.     

Special features of structurization in powdered iron-manganese alloys in deformation and recrystallization. I. Formation of the deformation structure in hot extrusion

Translated from Porshkovaya Metallurgiya, No. 5(353), pp. 29–37, May, 1992.     

Thermotransport of carbon in two-phase V-C and Nb-C alloys

Thermotransport experiments were performed on two-phase V-C and Nb-C alloys for various times, temperature ranges, and compositions. A one-phase region develops in the hotter portion of the sample accompanied by an increase in carbide concentration in the colder region. A mathematical equation in which the direction of transport is described by the sum of the heat of solution,A―H, and the heat of transport,Q*, was fitted to the experimental data for all of the alloys studied. No evidence was found to support the physical motion of the carbide particles predicted by some models. On the assumption that local equilibrium exists between the particles and the matrix, points on the V-C solvus were determined from the temperature corresponding to the one-phase/two-phase interface in the different experiments. The solid solubility for carbon in vanadium was determined to be log C (at. pct C) = 2.918 - 4536/T with a heat of solution of 86.8 ± 2.9 kJ/mol. Formerly a Graduate Research Assistant, Ames Laboratory, Iowa State University     

Crystal structure of the GTPase-activating domain of human p120GAP and implications for the interaction with Ras

Ras-related GTP-binding proteins function as molecular switches which cycle between GTP-bound 'on'- and GDP-bound 'off'-states. GTP hydrolysis is the common timing mechanism that mediates the return from the 'on' to the 'off'-state. It is usually slow but can be accelerated by orders of magnitude upon interaction with GTPase-activating proteins (GAPs). In the case of Ras, a major regulator of cellular growth, point mutations are found in approximately 30% of human tumours which render the protein unable to hydrolyse GTP, even in the presence of Ras-GAPs. The first structure determination of a GTPase-activating protein reveals the catalytically active fragment of the Ras-specific p120GAP (ref. 2), GAP-334, as an elongated, exclusively helical protein which appears to represent a novel protein fold. The molecule consists of two domains, one of which contains all the residues conserved among different GAPs for Ras. From the location of conserved residues around a shallow groove in the central domain we can identify the site of interaction with Ras x GTP. This leads to a model for the interaction between Ras and GAP that satisfies numerous biochemical and genetic data on this important regulatory process.     

Evolution of the structure and phase composition of rapidly solidified Al-Mg-Mn-Sc-Zr alloys during deformation in a bridgman anvil

Metallography and electron microscopy are used to study the formation of submicrocrystalline and nano-structured states in Al-Mg-Mn alloys with scandium and zirconium during shear at a high hydrostatic pressure. The evolution of the structure and changes in the hardness of the alloys are discussed in relation to their composition and casting and heat treatment conditions.