1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

Perspectives on Titanium Science and Technology

/pdf/effect-of-pore-size-on-bone-ingrowth-into-porous-titanium-e2dtit7a1r.pdf

Effect of pore size on bone ingrowth into porous titanium implants fabricated by additive manufacturing: An in vivo experiment.

Some issues in the application of cohesive zone models for metal–ceramic interfaces

https://recipp.ipp.pt/bitstream/10400.22/3445/7/ART_RaulCampilho_2013_DEM.pdf

Modelling adhesive joints with cohesive zone models: effect of the cohesive law shape of the adhesive layer

The thermo-mechanical characterization of interfaces in composite systems (PMC/MMC/IMC/CMC) is one of the challenging problems in composite mechanics and engineering. Each system has its own distinguishing features; however, in MMCs and IMCs the study is rendered more complex due to the evolving chemical species (both temporally and spatially), and the multi-axial state of residual stresses. Before MMCs or IMCs can be used in actual applications, the role of interfaces in not only the strengthening but also toughening mechanisms needs to be clearly understood. For evaluating the interfacial mechanical properties of interfaces, thin slice push-out test has emerged as the de-facto standard. Though, conceptually the testing procedure is simple, interpretation of the test results is not. It is essential to conduct very careful experiments, make precise meso- and macroscopic chemical/structural/mechanical observations and perform a thorough theoretical/numerical simulation before the test data can be used in a quantitative manner. In this paper, a comprehensive analysis of the push-out test is presented based on the theoretical/numerical and experimental research work of the authors’ group during the past few years. In this work, thin slice push-out tests were conducted primarily on Titanium Matrix Composites at various test temperatures (room and elevated) with different processing conditions (temperature and time). Different composite systems with Titanium based matrices (Ti–6Al–4V, Timetal 21S, Ti–15Nb–3Al) uniaxially reinforced with Silicon Carbide fibers (SCS-6) were chosen for the study. Effect of the evolution of interfacial chemistry and architecture (in matrix, coating and reaction zone) on both shear strength τs and frictional strength τf were studied. A novel finite element analysis based on nonlinear finite element method was implemented, in which not only the initiation but propagation of interfacial cracks are simulated. In the analysis, both shear stress and fracture energy based criteria are used to model the initiation of (closed) cracks. Quantitative values of τs, τf, GI and GII are then extracted based on the experimental data and the numerical simulation. A critical review of stress and energy based interface-modeling approaches and their applicability to various boundary value problems are made.

Interfacial mechanics of push-out tests: theory and experiments

This paper is concerned with the intergranular crack tip oxidation mechanism in alloy 718 at elevated temperatures. The basic concept is based on the ability of the oxygen partial pressure to control the preferential formation of oxide layers at the crack tip. The time required to complete the build-up of the protective oxide type at the metal-oxide interface is considered a measure of the limits of the oxidation process. Identification by transmission electron microscopy of oxide scale formed along fracture surfaces during a low frequency fatigue crack process in alloy 718 at 650°C supports the proposed model concepts. An experimental program was carried out to investigate the role of passivation time in controlling the progressive process of crack tip oxidation. This was achieved by testing the influence of oxide buil-up during hold time at minimum load, as well as the effect of a minor high frequency cycle imposed on the hold time period. It was established that an increase in fatigue crack growth rate accompanies the increase in passivation time period. These results were interpreted on the basis of the oxidation formation concepts.

Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Experimental study of the constant-probability crack growth curves under constant amplitude loading

The elevated-temperture fatigue crack growth behavior in alloy 718, when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed as fully environment dependent. In this process, the crack growth increment per loading cycle is assumed to be equal to the intergranular oxygen diffusion depth at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimens made of alloy 718 at 650 °C in which fatigue crack growth measurements were made for cyclic load conditions with and without hold time periods at minimum load level. This work resulted in establishing a relationship correlating the intergranular oxygen diffusion depth and the value of the stress intensity factor range ΔK. This relationship, when integrated over the cycle effective oxidation time, results in a closed-form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison shows good agreement between the two sets of results. Furthermore, by combining the parabolic rate law of diffusion and the equation for the intergranular oxygen diffusion depth, an explicit expression for the oxygen diffusivity of grain boundaries is derived. It is found that this diffusivity is both a ΔK- and a frequency-dependent parameter.

Hamouda Ghonem

Papers

Some issues in the application of cohesive zone models for metal–ceramic interfaces

Interfacial mechanics of push-out tests: theory and experiments

Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Experimental study of the constant-probability crack growth curves under constant amplitude loading

Depth of intergranular oxygen diffusion during environment-dependent fatigue crack growth in alloy 718