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

The MAX phases are a group of layered ternary compounds with the general formula Mn+1AXn (M: early transition metal; A: group A element; X: C and/or N; n = 1–3), which combine some properties of metals, such as good electrical and thermal conductivity, machinability, low hardness, thermal shock resistance and damage tolerance, with those of ceramics, such as high elastic moduli, high temperature strength, and oxidation and corrosion resistance. The publication of papers on the MAX phases has shown an almost exponential increase in the past decade. The existence of further MAX phases has been reported or proposed. In addition to surveying this activity, the synthesis of MAX phases in the forms of bulk, films and powders is reviewed, together with their physical, mechanical and corrosion/oxidation properties. Recent research and development has revealed potential for the practical application of the MAX phases (particularly using the pressureless sintering and physical vapour deposition coating rout...

Progress in research and development on MAX phases: a family of layered ternary compounds

Over the last 40 years, the commercial production of titanium and its alloys has increased steadily. Whilst these materials have some very attractive properties, enabling applications in many industries, they are seldom used in mechanical engineering applications because of their poor tribological properties. This paper starts with an introduction to the titanium material and a review of the different types of surface treatment. The processes of nitriding, oxidation and carburizing are among the most popular thermochemical treatments aiming at improving the surface properties of Ti-alloys. Different kinds of nitriding are investigated like plasma nitriding, ion nitriding, and laser and gas nitriding. The kinetics of nitriding and the conditions for the formation of nitrided layers are studied. The influence of the main processing parameters such as temperature, time on the microstructure and the formation of new phases during the processes of nitriding is discussed. Also based on investigations presented in the literature, the effects of nitriding on the microhardness and the corrosion resistance of titanium and titanium alloys are analyzed. The improved mechanical properties, which arise from these thermochemical treatments, are discussed in relation to the potential for applying these alloys to different industries.

Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods

Lightweight materials for aircraft applications

An overview of monolithic titanium aluminides based on Ti3Al and TiAl

Heat treatment of hot isostatically processed IN-738 investment castings

A cold-worked low-friction metallic shim or a low-friction nonmetallic composite shim is bonded onto the surface of a component with a layer of adhesive to form an anti-fretting barrier. This barrier is incorporated between the tightly fit components which are subject to high cyclic stresses, e.g. a blade and a disc slot in a gas turbine engine. In one form, the barrier is bonded to the surface of only one component. Bonding the shim significantly delays fretting damage on the surface of the component. The low friction nature of the shim significantly reduces the fretting damage on the contact surface of the other component. In another form, the barrier is bonded to the contact surfaces of both components, which eliminates or reduces fretting damage on the component surfaces. Reduced fretting action is confined to the consumable shim surfaces. Any fretting crack initiated in the barrier will not propagate directly into the component. Bonding the shim onto a component will significantly increase the fretting fatigue life of the component when compared to the fretting fatigue life of the component assembly obtained by using a freely inserted and unbonded shim.

Anti-fretting barrier

A new modified heat treatment has been developed for Inconel 718. This heat treatment leads to substantial improvements in elevated temperature crack propagation resistance with apparently limited loss in resistance to LCF crack initiation as compared to the conventional heat treatment for this alloy. This is a result of tailoring the microstructure to obtain the optimum combination of grain size, grain boundary structure and matrix precipitate morphology.

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Development of a damage tolerant microstructure for inconel 718 turbine disc material

A multi-variable test method, known as the Orthogonal Array, was used to optimize the parameters for the laser gas nitriding process (LGNP) to avoid surface cracking. Based on the fundamental requirement of a crack-free condition, the processing parameters were further optimized to improve the surface finish and to obtain a reasonable hardened depth. The effects of processing parameters have also been investigated with respect to the characteristics of the laser nitrided layer. Two types of lasers, i.e., CO2 and Nd:YAG lasers were used. The CO2 laser was operated in both the continuous as well as the pulse mode, while the Nd:YAG laser was used only in the pulse mode. A Nd:YAG laser in the pulse mode provided a better surface finish and lower cracking severity.

Laser Gas Nitriding of Ti-6Al-4V Part 1: Optimization of the Process

The characteristics of laser nitrided layers formed on Ti-6Al-4V are presented in this investigation. It has been determined that titanium nitride (TiN) is formed, which significantly increases the hardness of the nitrided surfaces. The amount of titanium nitride produced depends on the processing parameters such as laser pulse energy and nitrogen concentration. Nitrided layers are much smoother along the laser pass direction than perpendicular to this direction. The shrinkage effect in the laser melt zone produces surface residual tensile stresses in Ti-6Al-4V samples regardless of whether the processing environment is Ar, N2, or a mixture of these gases. Pre-heating or stress relieving after laser nitriding significantly reduces the residual tensile stress level.

W. Wallace

Papers

Heat treatment of hot isostatically processed IN-738 investment castings

Anti-fretting barrier

Development of a damage tolerant microstructure for inconel 718 turbine disc material

Laser Gas Nitriding of Ti-6Al-4V Part 1: Optimization of the Process

Laser Gas Nitriding of Ti-6Al-4V Part 2: Characteristics of Nitrided Layers