Plasma spray coating processes

Abstract

Plasma spray deposition is one of the most important technologies available for producing the high–performance surfaces required by modern industry. Over the past 25 years, there have been significant advances in the understanding of plasma physics and in the development of spraying equipment and techniques. This has enabled a range of materials including metals, alloys, ceramics, and cermets to be plasma sprayed on to a great variety of substrate types and geometries. During this period, the uniquely aggressive environment within the gas turbine engine has provided not only some of the greatest challenges to plasma spraying technology, but also some of its most successful applications. In this paper, the nature of plasma and plasma spray devices are discussed and factors affecting coating quality are considered. Practical aspects of plasma spraying are considered and finally the application of plasma spray coating processes to the protection of high–temperature gas turbine components is discussed using as examples turbine blade overlay coating, coatings for hot gas path seals, and ceramic thermal barrier systems.

MST/282     

金属间合物超塑性的研究进展

金属间化合物超塑性是近十几年开展的研究课题。超塑性加工技术是解决金属间化合物加工成型难题最可行的方法之一。综述了金属间化合物及其合金的超塑性研究进展,并着重介绍了热点研究的铝化物的情况。     

New materials for aerospace industry

Abstract

Two responses to the challenge of developing superior structural materials for the aerospace industry, Weldalite Al–Li alloys and XD technology, are briefly described.

MST/1319     

Influence of neodymium on microstructure and mechanical properties of AZ31B wrought magnesium alloy

Abstract

The influences of rare earth neodymium on microstructure and mechanical properties of as cast and hot rolled AZ31B wrought magnesium alloy were investigated. The results show that the mechanical properties of both as cast and hot rolled AZ31B alloys decrease due to Nd addition. Nd reacts with Al to form Al₂Nd phase when Nd is added. Bulky and brittle Al₂Nd intermetallic degrades the mechanical properties. Moreover, the addition of Nd weakens the grain refining effect of Al on as cast AZ31B alloy, resulting in grain coarsening. Coarse grains also cause the decline of the mechanical properties of as cast AZ31B–Nd alloy. The negative influence of the bulky and brittle intermetallics on mechanical properties of AZ31B alloy can be relieved by large deformation because the intermetallics can be sufficiently broken up during the deformation process.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Development of superalloys

Abstract

The development of what are now termed superalloys is reviewed from the early days of high-temperature alloys in the light of the demands of engineers for materials to serve under differing conditions of stress, temperature, and environment. Before the introduction of the aircraft gas turbine the main interest lay in the field of low-alloy creep resisting steels for steam raising plant or in heat resistant alloys for relatively low-stressed furnace components. Piston aeroengines posed two problems – the exhaust valve and the turbo supercharger rotor – and special materials for these components were introduced, largely on the basis of high-temperature tensile strength. The gas turbine demanded high creep resistance and two main lines of alloy development were followed: carbide-hardened iron or cobalt-base alloys and γ′-hardened nickel-base alloys. The latter type has surpassed the former and now, either in wrought form or, predominantly for the most severe conditions, as castings, is the mainstay of superalloy applications. In recent years, progress has mainly been sought by the improvement of macrostructure: in castings by directional solidification to give columnar crystals or single crystals and in wrought products by powder technology. Severe hot corrosion has been combated by the application of protective surface coatings and the development of a series of wrought and cast high-chromium nickel-base alloys. It is anticipated that future progress in high-temperature materials technology will be sought in the application of a variety of metallurgical and physical techniques to specific engineering components in much the same manner as was done with the aeroengine exhaust valve of 50 years ago.

MST/511     

Comparative viability of processing routes for intermetallic based materials

Abstract

Processing routes for intermetallic based materials are briefly surveyed and compared. For monolithic intermetallics the two main factors determining process routes both derive from the low room temperature ductility of most intermetallics. They are the need to maintain material cleanness, thus reducing fracture initiation sites, and the desire to achieve fine grain size to seek to improve ductility. For the titanium based aluminides there is also a need to minimise interstitial, particularly oxygen, pick-up during processing. For intermetallic based composites, a broad range of processes is already being investigated. In many of these, issues of cleanness may be more difficult to control than for the monolithic composites. With continuous fibre reinforced composites a further process impetus is the need to control interfacial interactions between the fibre and the matrix.

MST/1560     

Titanium in the power generation industry

Abstract

The role and construction of steam condensers are reviewed and the factors affecting materials selection are outlined. Comparisons are made between the behaviour of copper-base alloys and commercially pure titanium tubing under test and in service. The implications of a change in tube material on condenser design are reviewed. The potential for using the Ti–6Al–4V alloy for last stage low-pressure turbine blading is also reviewed.

MST/598     

Fine‐structure superplasticity in materials

Abstract

Superplastic forming is a viable net‐shape forming technology. It is an attractive manufacturing technique because it bypasses machining, joining and riveting, and reduces material waste. This paper reviews the history and progress in the field of superplasticity. Fundamental understandings, such as microstructural prerequisites and deformation mechanisms, and technological applications for superplasticity are discussed. A wide range of structural materials, including metals, intermetallics, and ceramics are presented. Some of the most recent advances, for example, high strain rate superplasticity, low temperature superplasticity, and the development of ultrafine‐grained materials for superplasticity are also described.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

Toughening and creep in multiphase intermetallics through microstructural control

The lack of engineering ductility in intermetallics has limited their structural applications, in spite of their attractive specific properties at high temperatures. Over the last decade, research in intermetallics has been stimulated by the discovery of remarkable ductilisation mechanisms in these materials. It has however often been the case that the process of ductilisation or toughening has also led to a decrease in high temperature properties, especially creep. In this paper we describe approaches to the ductilisation of two different classes of intermetallic alloys through alloying to introduce beneficial, second phase effects. The Ti₂AlNb based intermetallics in the Ti-Al-Nb system can be ductilised by stabilising thebcc phase of titanium into the structure. The principles of microstructural and compositional optimization developed to achieve adequate plasticity, while retaining creep properties of these alloys, are described. An entirely different approach has been successful in imparting plasticity to intermetallics based on Fe₃Al. The addition of carbon to form the Fe₃AlC_0.5 phase imparts ductility, while enhancing both tensile and creep strength.     

Laser-induced combustion synthesis of Zr–Ti–Al–Ni amorphous-based alloys

Bulk metallic glasses, like many crystalline intermetallics, have large negative enthalpy of mixing among the major constituent elements, and hence are potential candidates of self-propagating high-temperature reaction systems. Based on this characteristic, the laser-induced combustion synthesis (LCS) technique has been applied to fabricate amorphous-containing alloys. In the present paper, we report the LCS of the Zr–Ti–Al–Ni alloys. A series of Zr–Ti–Al–Ni alloys is designed and synthesized by LCS. The LCS products mainly consist of intermetallic phases, but in Zr₅₅Ti_10.8Al_17.1Ni_17.1 and Zr₅₀Ti_21.6Al_14.2Ni_14.2 amorphous phases are found. The hardness and tribology characteristics are closely related to the phase contents. The amorphous phases, ductile and soft, lower the hardness and increase the friction coefficient of the LCS samples.     

金属间化合物结构材料研究现状与发展

对有序金属间化合物结构材料的现状和发展做了简要回顾,特别对我国的发展情况做了评述。提出金属间化合物结构材料的研究成果要从新材料发展和金属材料学科新发展两方面来评估,并简述了这两方面取得的成果。认为我国的研究成果很大,但对国内外研究有重大影响的成果不多,在新世纪中,从提高国家国力的战略上看,加强金属间化合物结构材料的研究是必不可少的。但要选择重点,强化设计－－材料－－应用一体化研究,要优先源头创新项     

Superplastic microstructures

Abstract

The production of fine, stable equiaxed grains, having disordered high angle boundaries, is a prerequisite for superplastic behaviour in crystalline solids. The way that superplastic microstructures can be achieved in pseudo-single-phase and duplex materials by thermomechanical processing is discussed for a number of commercially significant materials. The resulting superplastic deformation characteristics are outlined, as are the factors that influence cavitation during superplastic flow. Alloys based on aluminium, titanium, copper, iron, and nickel are considered, and also aluminium based metal-matrix composites, intermetallic phases, and crystalline ceramic materials. Recent work on markedly enhanced superplastic behaviour in aluminium and copper alloys and in stainless steels is reported, and the similarities between superplasticity in crystalline ceramics and metallic materials is discussed. The development of superplastic microstructures in metal-matrix composites, intermetallic phases, and ceramics has enhanced their formability and their potential as high temperature structural materials.

MST/1298     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Some aspects of synthesis of nanocrystalline materials

Abstract

This paper reviews research work at the Institute for Materials and Advanced Processes, University of Idaho, on the synthesis of nanocrystalline materials and their consolidation. Nanocrystalline materials have been synthesised by a number of ‘far from equilibrium’ processes including mechanical alloying (MA), mechanochemical processing (MCP), supercritical fluid processing (SCFP), and severe plastic deformation (SPD). Examples of the materials include the TiAl based intermetallic compounds and composites produced by MA and SPD, Ti base alloys and metal carbides synthesised by MCP, thin film Cu produced by SCFP, and Al–Fe alloys produced by SPD. Details of the processes used and the enhancement of properties owing to the nanoscale structures in consolidated material will be presented. The potential of these processes to substitute for conventional methods of production will also be discussed.     

Other applications of superalloys

Abstract

It is the intention in this paper to put into context the development of high-temperature alloys to their present position in non-gas-turbine applications, and to identify new alloy systems capable of improving performance in hostile industrial environments. The rise of superalloys from ferritic steels to the current γ′-hardened nickel-base materials, the best of which experience strength limitations above l000°C, is traced. Desired increases in temperature capability are possible with oxide dispersion strengthened powder alloys, which were originally developed for gas turbine usage and are now being produced on a large tonnage basis by the mechanical alloying (MA) process. The MA technique and commercial alloys are described and examples given of the replacement of more conventional materials by fabricated MA components in a diversity of industries. MA alloys exhibit combinations of strength and corrosion resistance capable of meeting many industrial demands for economic improvements to processing capabilities and efficiencies.

MST/525     

Book review

Abstract

Magnesium based Mg–9Al–1Zn–5RE (RE=Y, La, Nd, Ce, or Pr) alloys with or without an addition of 1%Si were rapidly solidified by chill block melt spinning and splat quenching. The base alloy AZ91 (Mg–9Al–1Zn) was also rapidly solidified. Isochronal heat treatment for 1 h at 100–400°C showed that the microhardness of the ribbon maintained a similar level to that of the as spun alloy up to 300°C but decreased when heat treated at 400°C. Isothermal heat treatment for up to 24 h at 250–350°C showed that there were aging responses for the sample treated at 250°C while above this temperature, the microhardness decreased as the treatment time increased. The addition of 5% of RE elements to AZ91 displaced the Mg₁₇Al₁₂ phase in AZ91 with fine dispersoids of Al₂RE (RE=Y or Nd) or Al₁₁RE₃ (RE=La, Nd, Ce, or Pr) in Mg–9Al–1Zn–5RE alloys. These Al–RE intermetallics remained fine and precipitated at the grain boundaries so restraining grain growth during heat treatment at up to 400°C. Although Mg₂Si precipitates were found to be present in the silicon containing alloys after heat treatment at 400°C, their size was greater than those of Al–RE intermetallics, indicating that Mg₂Si has a lower thermal stability than these Al–RE intermetallics. The relationship between microhardness and grain size is discussed.

MST/3400     

A review of directionally solidified intermetallic composites for high-temperature structural applications

Alloys based on intermetallics have been considered for high temperature structural applications. However, many of these alloys suffer from intrinsic brittleness and low fracture toughness at ambient temperature. Therefore, ductile-phase-toughened intermetallic composites are being investigated as a means to improve the fracture toughness. A subset of this class of materials is in-situ composites produced by directional solidification of intermetallic eutectics. In this paper, we review recent developments related to the processing and properties of these composites.     

Investigation of ordering phenomenon in Me–Pt (Me=Fe,Ni) liquid alloys

Abstract

The phase diagrams of Fe–Pt and Ni–Pt liquid alloy systems show the existence of FePt and NiPt intermetallic compounds, respectively, in their solid intermediate states, and the associative tendency between unlike atoms in these liquid alloys has been analysed using the self-association model. The concentration dependences of mixing properties such as the free energy of mixing, G_M; the concentration fluctuations, S_cc(0), in the long-wavelength limits; the chemical short-range order (CSRO) parameter, α₁; as well as the chemical diffusion, enthalpy and entropy of the mixing of Fe–Pt and Ni–Pt liquid alloys have been investigated to determine the nature of ordering in the liquid alloys. The results show that heterocoordination occurs in the alloys at all concentrations. The effect of CSRO on S_cc(0), chemical diffusion, D, and the order parameter, α₁, has been considered. The ordering phenomenon in the liquid alloys is also related to the effect of the atomic size mismatch volume on S_cc(0).