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The energetics of interstitial oxygen (O) in β-Ti and β-Ti–X alloys (X = 3 d -, 4 d -, or 5 d -transition elements) was studied using first principles methods based on density functional theory (DFT). For small concentrations of X (=1.8 at.%), the insertion energy E ins X for interstitial oxygen along the three transition series follows a similar trend: it first decreases reaching a minimum between Groups 4 and 5 elements, then increases to reach a maximum for Group 8 elements (Fe, Ru, Os) before decreasing further. Except for the early transition elements, the interaction between X and O is repulsive. The lattice strain is found to be the dominant contribution to the insertion energy and can explain the trends along the three series. Using the calculated energetics and the lattice strain due to interstitial oxygen, a model is constructed to determine trends in maximum oxygen solubility C O, max in β-Ti–X alloys. The C O, max X decreases in the middle of each transition series.

The mixing enthalpy Δ H mix ( x ) of body-centered cubic (BCC) β-Ti with transition elements was calculated using first-principles methods based on density functional theory (DFT). The solid solution effect was treated by two different approaches, viz . special quasi-random structures (SQS) and the parametric method. The SQS- N method uses direct DFT to calculate energy of structures containing N atoms which approximate the correlation of an ideal solid solution up to some distance, whereas the parametric method employs a polynomial representation for Δ H mix ( x ) and the coefficients are calculated using DFT. Comparison of the two methods shows fair agreement for most alloys though differences as high as 40% can also be seen among some of the alloys. The trends in Δ F mix ( x ), obtained by adding entropy contribution from ideal solution model to Δ H mix ( x ) for 3d-, 4d- and 5d-transition series were analyzed in terms of e / a , the ratio of number of valence electrons to atoms. The early transition elements, between Group 4–7, was found to have very small Δ F mix ( x ) over a wide range of concentration. Stability of the alloys is analyzed by combining Δ F mix ( x ) with Hume-Rothery rules.

A technique to evaluate grain boundary fracture strength of metallic materials was developed. Coincident site lattice (CSL) boundaries were prepared by diffusion-bonding of oriented two single-crystal Ni–20Cr pieces. Microtensile specimens were fabricated across the CSL boundaries by femtosecond laser machining. Due to the highly ductile character of Ni–20Cr, notches were machined along the grain boundaries to initiate grain boundary fracture. The microtensile tests at room temperature demonstrated that Σ3(1 1 1) boundary was the strongest of all the CSL boundaries examined.

Using first-principles density functional calculations, we determine effects of environmental embrittlement on Ni/Ni 3 Al interfaces. We evaluate relative stability of sites for occupation of O, H and N and estimate their effects on cleavage energies. We find that O has the most deleterious effect on interface embrittlement, followed by N and H. Changes in bonding near dopant O arising from its strong ionic character, evident in electron localization function, are responsible for the corresponding large reduction in the cleavage energy.

The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

This article attempts to relate ultrasonic second harmonic generation to dislocation structures in fatigued materials. By focusing on the fact that asymmetric dislocation motion is required to generate the second harmonic, a theoretical derivation of the non-linearity parameter (ratio of the amplitude of second harmonic to square of the amplitude of the fundamental) from dislocation pile-ups is presented. In order to verify the theory, harmonic generation was measured along a failed fatigue sample of DA718, a nickel base superalloy that exhibits planar slip, and consequently dislocation pile-ups. TEM studies along the length of the failed fatigue sample revealed a banded dislocation structure, composed of pile-ups near the fracture zone and a sparse dislocation network closer to the grip region. The non-linearity parameter was found to increase by between 90% and 140% from the grip region to the fracture zone and this correlated well with the calculations based on theoretical expressions derived here. � 2007 Elsevier Ltd. All rights reserved.

This paper discusses the development of a novel processing route to produce ultra finegrain bulk alloy forgings; the microstructural response of these forgings to thermal exposure; and the comparison of mechanical properties to those from conventionally processed material. A Ni- 20Cr [wt%] alloy was processed by near-isothermal multi-axis forging to a grain size of approximately 1 μm. A heat-treatment study over the range 900 to 1200°C was conducted to determine the resultant grain size as a function of time and temperature. Tensile properties were measured at room temperature, 500°C, and 930°C. High-cycle fatigue properties were measured at room temperature. The room-temperature tensile strength was approximately 2.5 times greater than that of conventionally processed Ni-20Cr. Fatigue data showed that the room-temperature highcycle fatigue run-out stress was greater than 100% of the yield stress.

The mechanical behavior and microstructure evolution of a stable β, metastable β, and β-rich titanium alloys during hot deformation in both β and α/β phase fields were studied. The effects of thermomechanical processing and alloy content on final grain size and microstructure homogeneity in the alloys are given. The processing windows in both β and α/β phase fields for the formation of homogeneous microstructures with grain sizes down to submicron values are discussed. Isothermal multiple-step forging was used to produce the billets of a β-rich alloy with homogeneous fine-grained microstructure and low ultrasonic noise.

FEM analysis was used to study the interactions between die friction, backpressure and strain rate sensitivity during equal channel angular pressing. One of the key findings is that strain rate sensitivity causes strain redistribution, resulting in higher equivalent strains and a substantial component of compression in the workpiece. This compression gives rise to effects such as lift-off of the leading end of the workpiece from the die bottom and improved die fill. Friction has different effects at room and elevated temperatures. While a moderate amount is beneficial at room temperature, friction has a detrimental interaction with strain rate sensitivity at high temperature. Backpressure, though largely beneficial, is shown to have negative effects when combined with friction and a high strain rate sensitivity.

The thermal expansion behavior of three near-stoichiometric variants of ruthenium aluminide (RuAl) has been investigated. The coefficient of thermal expansion (CTE) varied between 5.5 × 10 −6 /K and 11 × 10 −6 /K in the temperature range from 400 to 1773 K. The behavior of RuAl is compared to that of other high temperature B2 intermetallics with emphasis on the use of RuAl as a bond coat material within a thermal barrier coating system.

Nitrogen-rich inclusions can lead to a reduction in fatigue life of highly stressed titanium-alloy components (found in the compressor sections of power generation gas turbines, turbine sections of power generation steam turbines, wing sections of aircraft air frames, and compressor and fan sections of aircraft jet engines, for example). It is the ”crack starting” nature of the nitrogen-rich inclusions that is the problem, not the presence of nitrogen in the system, per se. This paper covers the theory and laboratory experiments used to evaluate ESR (electro-slag refining) as an alternative technique for rapid dissolution of nitrogen-rich inclusions in titanium and titanium alloys. Five variations of melt rate, slag temperature and inclusion type were evaluated. One variant showed complete dissolution of the inclusions. Several of the other variants showed partial dissolution of the inclusions. The kinetics were observed to occur at a rate 100 to 1000 times faster than the rate of dissolution of nitrogen-rich inclusions by immersion in liquid titanium.