Creep, fatigue and oxidation in crack growth in advanced nickel base superalloys

This paper presents some of our recent results from an ongoing collaborative research programme on creep-fatigue behaviour of two advanced nickel base superalloys for turbine disc applications. The role of creep, fatigue and oxidation in crack growth has been investigated at 650°C under typical loading waveforms at selected loading frequencies. Load-line deflections were monitored in selected tests under static and long dwell loading conditions. Scanning electron microscopy was adopted to identify the fracture mode and to facilitate the evaluation of oxidation.

The results show that mixed time and cycle dependent crack growth seems to be the predominant crack growth mode in the two PM nickel alloys studied. Whilst limited creep may be present at the crack tip, particularly under static and long dwell loading conditions, oxidation appears to be the predominant mechanism for crack growth under the test conditions examined.     

Some recent fatigue and fracture mechanics research in BIAM

The paper presents a brief review of some of the major research activities on fatigue and fracture mechanics in recent years at the Beijing Institute of Aeronautical Materials. Attention is mainly given to the studies on weight function methods for analyses of two- and three-dimensional crack problems, fatigue crack growth under variable amplitude loading, small crack effects and a fracture-mechanics-based total fatigue life prediction method.Abbreviations 2(3)D two- (three-) dimensional - BIAM Institute of Aeronautical Materials, Beijing - CCT center cracked tension - COD crack opening displacement - CTOD crack tip opening displacement - FEM finite element method - LEFM linear elastic fracture mechanics - SENT single edge notched tension - SIF stress intensity factor - WFM weight function method     

Review of rejuvenation process for nickel base superalloys

Abstract

Combustion turbine components exposed to elevated temperatures and high stresses are subject to periodic replacement as a result of mechanical property degeneration to levels below that required for continued safe operation. In view of the very significant cost associated with replacement components there has been much interest in the rejuvenation of parts. This report reviews published literature on the rejuvenation of nickel base superalloys. The restoration of microstructure and properties to levels equivalent to the original material have been achieved with the use of reheat treatment alone or recovery cycles incorporating both hot isostatic pressing and reheat treatment. The majority of the literature considers the restoration of creep properties, although some success in the rejuvenation of fatigue properties has also been reported.     

Developments of cast superalloys and technology for gas turbine blades in BIAM

Since 1960's many important subjects relating to cast turbine blades including alloy developments, directional solidification (DS) and single crystal (SC) technique and casting technology for blades have been performed with great successes in Beijing Institute of Aeronautical Materials (BIAM) in order to meet the requirements for developing new aero-engines in China.Abbreviations CC conventional cast - DS directional solidification - FC fine grain cast - SC single crystal     

Review of recent developments in advanced ejector technology

Previous reviews on ejectors for expansion work recovery have provided detailed discussions of operating characteristics and control of ejector cycles, zero-dimensional ejector modeling, ejector geometry effects, and alternate ejector cycles. However, important advances in the field of ejector technology have occurred since previous reviews were written. Several focuses of recent ejector research are the development of multi-dimensional CFD ejector models, investigation of alternate ejector cycles and uses of the work recovered by the ejector, implementation of effective control strategies for ejector cycles, and application of ejectors in real systems. The objective of this paper is to present a review of developments in the use of ejectors for expansion work recovery in vapor-compression systems focusing on the past several years. Although the first commercial applications are being introduced to the market, it is suggested that future works continue in these areas in order to make ejectors more suitable for additional applications.     

Some recent developments in computational modelling of concrete fracture

Some of the most important aspects of numerical modelling of cracking in concrete are reviewed. After a discussion of the three main lines in modelling cracking – discrete crack models, smeared representations and approaches using lattice models – a concise treatment including comparative studies is given of the various smeared crack approaches that exist to date. Next a discussion is presented of some issues pertaining to the sensitivity of numerical results on the fineness of the mesh and the direction of the mesh lines, and on size effects in concrete structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al-Si master alloy powders as conventionally recommended. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.     

Some recent developments in the theory of boundary layer instability

The stability of boundary-layer flow over a flat-plate is investigated after taking into account the effects of boundary-layer growth. A critical review and analysis of earlier work is presented, mainly for those works that use an inhomogeneous Orr-Sommerfeld equation and a solvability condition to obtain corrections for the growth-rate based on the quasi-parallel (qp) approximation. During the course of this review and analysis an important result is proven; viz., for a ray, the basic spatial periodicity of the disturbance wave is indimensional space. Thereafter the energy integral equation is invoked, and an optimal monitorable property is found that has the same growth-rate as given by the eigensolution of the associated homogeneous problem. This also leads to the optimal normalisation of the eigenfunction at different downstream stations along the plate. A surprising result found is that the past non-parallel results can be virtually totally reproduced based on theqp-approximation and using the present methodology. And, by using the present methodology for the full non-parallel problem, the results obtained are in very much better agreement with past experimental results, and with the results of Smith based on the triple-deck theory. The students and co-workers who have worked on this problem are Dr T K Vashist and Ms R Verma.     

Some recent developments in the theory of boundary layer instability

The online version of the original article can be found at     

Microstructural modifications in a dynamically consolidated microcrystalline nickel titanium alloy powder

Spherically shaped, microcrystalline NiTi alloy powder with both nonhomogeneous particle-size distribution (2 to 60m diameter) and chemical composition, was consolidated with a shock input energy of 316 kJ kg^–1. Upon shock compaction, the two-phase NiTi powder particles (containing 45wt% Ti and 65 wt% Ti) were bonded together, generally by interparticle melting and subsequent welding. The melted material at interparticle regions was observed to have rapidly solidified to largely amorphous and/or microcrystalline phases. Particle interiors were also subjected to extensive plastic deformation which resulted in deformation twinning, grain elongation and some recrystallization to defect-free grains. Unique microstructural modifications occurring due to inhomogeneous thermal and mechanical processing during the dynamic consolidation process, are reported here.     

Effect of residual elements on high performance nickel base superalloys for gas turbines and strategies for manufacture

The need for better gas turbine operating efficiency and reliability has resulted in tightening of specification and acceptance standards. It has been realized that some elements even at trace level, can have disastrous effect on high temperature properties. The present paper highlights the adverse effect of tramp elements and strategies that should be adopted to produce high purity superalloys.     

Some recent innovations in the Kroll process of titanium sponge production

Titanium and its alloys have emerged as cost-effective structural materials in many spheres of chemical and engineering industries including aerospace and power generation. Titanium in its pure form is invariably prepared starting from pure titanium tetrachloride. Titanium tetrachloride obtained by chlorination of the oxide mineral is purified and reduced with either liquid magnesium or sodium or electrolysed to obtain titanium in sponge form. The metal extraction processes are so complex that large scale production technology is limited so far only to a few countries in the world viz. the USA, Japan, CIS, UK and China. India is attempting to enter this arena shortly with a 1000 TPY commercial plant based entirely on home-grown technology. Among the extraction methods, the magnesium reduction of titanium tetrachloride, patented by W J Kroll in 1940, has received wider attention because of the inherent and relative merits of the process and its viability for economic production on an industrial scale. The original Kroll process, however, has undergone several modifications in the past few decades. The recent technological breakthroughs in the Kroll process as well as in the magnesium recycling technology has resulted in a significant reduction in the production cost of the metal. The paper describes these important innovations and also the efforts that are being put in for the establishment of a commercial plant for metal production in India based on indigenously developed technology.     

Cyclic deformation behaviour of a nickel base alloy at elevated temperature

Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204°C. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204°C, however, the cyclic stress-strain response shows a strain hardening first, followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed.     

透平膨胀机及发展动态

比较了国内外常规透平膨胀机的主要技术参数 ;重点介绍了国外带液透平膨胀机、全液体透平膨胀机、能量回收用透平膨胀机和磁悬浮轴承透平膨胀机的发展动态     

Some recent developments in stainless steels and their application to seawater cooled condensers

The adoption of steel refining processes such as argon oxygen (AOD) or vacuum oxygen (VOD) decarburisation has facilitated the manufacture of low carbon and low nitrogen content stainless steels. This, coupled with an improved understanding of the role of alloy elements has permitted the development of a range of grades suitable for service in aggressive chloride-condaining waters.The present status is illustrated by reference to the requirements for tubes in seawater cooled power station condensers. In this application the critical factor is the resistance to crevice corrosion attack. An accelerated laboratory test involving 60 days exposure in filtered seawater has been proposed. Ferritic steels with at least 25% chromium and $\text{3}\text{1}\text{2}\text{\%}$ molybdenum perform well in this test. The situation is more complex with austenitic steels and the effect of nitrogen is then significant. One major group of austenitic seawater resistant steels contains 19–20% chromium and a minimum of 6% molybdenum.Commercially available grades are listed and industrial installations indexed. In the majority of cases stainless steel has been used to replace copper base alloys which have failed in service, but there are an increasing number of instances in which stainless steel tubes are specified as original equipment.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.     

Some recent developments in numerical modelling of fracture toughness in brittle matrix composites

Finite element analysis was used to study the fracture toughening of a ceramic by a stress induced dilatant transformation of second phase particles. The finite element method was based on a continuum theory which modelled the composite as subcritical material. Transient crack growth was simulated in the finite element mesh by a nodal release technique. The crack's remote tensile opening load was adjusted to maintain the near-tip energy release rate at the level necessary for crack advance. The transformation zone surrounding the crack developed as the crack propagated through the composite. Resistance curves were computed from the analysis; and the results show that during crack advance maximum toughness is achieved before a steady state is reached. The toughening effect of a crack-bridging ductile phase in a brittle material may be predicted if ligament deformation is characterized. A plastically deforming ligament constrained by surrounding elastic matrix material is modelled using finite elements and the relevant toughness enhancement information extracted. Comparison is made to model experiments as well as to toughness measured for technologically important materials. The results suggest that debonding along the interface between the ligament and the matrix may enhance the toughening effect of a ductile phase.