Showing papers in "Materials Science and Technology in 1986"

Evaluation of principal wear mechanisms of cemented carbides and ceramics used for machining titanium alloy IMI 318

Abstract: Although significant advances in tool material development have been made to facilitate increased metal removal rates when turning steels and cast irons, no equivalent developments have been made for cutting titanium alloys. In this paper, part of a recent research programme is described in which the principal wear mechanisms of ceramic and cemented carbides (coated and uncoated) used for turning Ti–6AI–4V (IMI 318) and commercial purity titanium are investigated. A significant problem is the generation of rake face temperatures above 900°C at relatively low cutting speeds, i.e. ~75 m min−I, which promote rapid rake face crater wear by dissolution–diffusion. Of all the tool materials tested, it was the ‘straight‘ grade (WC–Co) cemented carbides which were most resistant to this type of wear. They also offered superior resistance to flank wear induced by attrition. Experiments in which cubic boron nitride was used and comparative tests with TiB2 and TiC coated carbides, in conjunction with solubili...

Properties of thermal barrier coatings

Abstract: A variety of mechanisms which degrade thermal barrier coatings are outlined. Some are specific to the bondcoat, others to the ceramic. Material properties which influence the rates of degradation, and so determine the useful life of the coating system, are outlined.MST/288

Pack cementation processes

Abstract: Pack cementation remains a widely used technique for producing high-temperature protective coatings for gas turbine components. The main limitations and advantages of pack cementation processes are...

Protective coatings – purpose, role, and design

Abstract: A wide range of coatings and coating processes is now available for protecting components in gas turbines operating in a variety of conditions. Coated aerofoils are less likely to fragment, and they keep their original shape – and hence their aerodynamic efficiency – for longer. Aluminide diffusion coatings perform well in relatively clean environments, especially on alloy bases which have a moderate resistance to the many forms of hot corrosion, and with additions of chromium or platinum they provide resistance to sulphate–induced hot corrosion. In hotter or more aggressive corrosive environments, overlay coatings based on MCrAlY (M = Fe,Co, Ni), CoCr, and MCrSi(Al), applied by electron beam evaporation or plasma spraying, are suitable for a wide variety of applications. It is not yet possible to design coatings from first principles for particular applications. Development still proceeds by repeated selection and testing, but modern technology has made this approach more effective than it was in...

Microscopic deformation behaviour of martensitic–ferritic dual-phase steels

Abstract: The deformation behaviour of the two phases of three plain carbon dual–phase steels after various treatments has been studied using a scanning electron microscope equipped with a tensile straining stage. The distribution of strains between the ferrite and martensite phases, as well as among the different grains of each phase, was observed to be inhomogeneous. The martensite/ferrite strain ratio, which defines the degree of uniformity of straining between the phases, depends on the microstructural parameters of the steels: it increases with increasing volume fraction of martensite, but decreases as the carbon content of the martensite increases. Tempering at various temperatures causes a decrease in the martensite/ferrite microhardness ratio and hence causes an increase in the strain ratio. The macroscopic strain of the specimen at which the martensite begins to deform was also found to be dependent on the microstructural parameters. Regions of applicability of the existing theories of the strength...

Microstructural changes in austenitic stainless steels during long-term aging

Abstract: The microstructural changes, precipitation behaviour, and mechanical properties of typical austenitic stainless steels (304 H, 316 H, 321 H, 347 H, and Tempaloy A–1) have been examined after long-term aging. The steels were aged statically in the temperature range 600–800°C for up to 50000 h. The microstructural changes were observed by optical and transmission electron microscopy, and the extracted residue was identified using X-ray analysis. Time–temperature precipitation diagrams were made for each steel. The amount of σ-phase was measured in samples aged at 700°C. The hardness and impact-value changes, and the tensile properties of aged samples were measured.MST/358

Quantitative microanalysis of carbide/carbide interfaces in WC–Co-base cemented carbides

Abstract: Carbide/carbide boundaries in WC–Co-base cemented carbides containing 6–20 wt-%Co were studied with two high resolution microanalytical techniques: atom probe field ion microscopy and analytical transmission electron microscopy. All boundaries studied, i.e. WC/WC boundaries and, in materials containing cubic carbides (γ-phase), WC/γ and γ/γ boundaries, were found to contain about half a monolayer of cobalt, localized to a zone of monolayer thickness. The carbide/carbide boundaries may thus be described as grain (phase) boundaries to which cobalt has segregated. The carbide skeleton model for WC–Co is thereby confirmed. In WC–Co materials which contain Cr3C2 as a grain growth inhibitor, chromium segregates to WC grain boundaries.MST/354

Numerical calculation of microsegregation in coarsened dendritic microstructures

Abstract: An earlier procedure for calculating homogenization during the solidification of binary alloys is extended to include more realistic assumptions for dendrite arm coarsening kinetics and the partition coefficient. Dendrite coarsening is shown to follow the usual exponential function which differs from that relating final dendrite arm spacing λf to local solidification time. This function is not sensitive to the assumption for the partition coefficient or the choice of initial spacing (between λ0 = 0 and λf/2). The homogenization parameter H (normalized difference between the minimum possible concentration and the actual minimum concentration in the centre of the dendrite arm) changes only slightly with the partition coefficient. With the average coarsening parameter used by earlier authors, homogenization is overestimated and a lower amount of the non-equilibrium phase is obtained. The experimental data available for Al–Cu alloys in the range 2–20 wt-% Cu are very closely predicted by the numerica...

Structure of metastable precipitate in some Al–Cu–Mg–Ag alloys

Abstract: The high strength of some Al–Cu–Mg–Ag alloys has been attributed to very thin (~2·5 nm), but broad, hexagonal-shaped precipitates. Previous work has shown that the precipitates have a hexagonal unit cell, but different lattice parameters have been reported. In the present paper, the intensities of X-ray diffraction reflections from the precipitates have been measured on Buerger precession photographs, and it is shown that the crystal structure is monoclinic (space group P2/m) with the parameters a = b = 0·496 nm, c = 0·848 nm, γ = 120°. The special values of these parameters confer a hexagonal symmetry on the lattice. This unusual structure is a slightly distorted form of θ-CuAl2, to which it appears to change after long aging times at 200°C.

Continuous γ′ precipitation in directionally solidified IN738 LC alloy

Abstract: A microstructural characterisation of continuously precipitated γ′ particles in IN738 LC alloy directionally solidified at various cooling rates has been carried out. With decreasing cooling rate there is a trend for the γ′ morphology to vary from rounded particles to irregular cuboids and, finally, a clusterlike formation. In the interdendritic regions, microsegregation leads to an increase in the γ′ solvus temperature, and γ′ precipitation begins at higher temperatures than in the dendrite cores, resulting in differences in particle size and morphology. Observations on the effects of solution treatment and aging on the development of γ′ dispersions are also reported.

From research to cost-effective directional solidification and single-crystal production – an integrated approach

Abstract: This approach has required work in the three broad areas of process development, material development, and quality assurance. The first has led to the development of an automated casting facility, and the adoption of the technically more difficult seeding technique, as a result of its greater process and design flexibility. The second has been met by the development of a series of three alloys to meet specific turbine applications and the economies necessary to make the complex shapes to the required precision. The third has been met by the development of SCORPIO, a real-time rapid-orientation measurement system for the production environment, and the understanding and development of the process controls necessary to control wall section measurements and heat treatment. By the involvement of multidisciplined teams, this approach has led to a demonstrated technology with little or no cost penalty.MST/244

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...

Nucleation of cube texture in aluminium

Abstract: Microstructures and textures have been examined in four different commercially pure aluminium alloys after cold rolling and annealing. A correlation has been established between the density of cube sites in the cold rolled structure and the strength of the cube texture component in the rolling ingots. Examination of the alloys by transmission electron microscopy revealed the presence of a {102} 〈201〉 texture component, i.e. the cube texture twisted 26° around the transverse axis. The cube crystallites frequently occurred either in bands together with {102} 〈201〉 oriented subgrains or in the periphery of larger areas of {102} 〈201〉. The effect of an inhomogeneous hot rolling texture on the recrystallization texture after cold rolling and annealing has been investigated and no correlation between the high fraction of cube texture in the hot rolled material and the strength of the cube in the sheet could be detected. The effect of dynamic recovery during cold rolling on the recrystallization texture ...

Influence of carbon on hot ductility of steels

Abstract: Hot ductility, measured by reduction in area, has been determined over the temperature range 550–950°C for a series of plain C–Mn steels having the same base composition except for the carbon content, which was in the range 0·04–0·65 wt-%. A ductility trough was obtained for all the steels and minimum ductility values were similar. Raising the carbon content from 0·04 to 0·28 wt–% caused the ductility trough to move to lower temperatures and this was in agreement with the observed changes in transformation temperature. Tensile fracture at the minimum ductility temperature was along thin films of ferrite which formed round the austenite grains – generally by deformation–induced transformation. The softer ferrite allowed strain concentration to cause ductile voiding at the MnS inclusions, and the voids eventually linked up to give intergranular failure. Raising the carbon content above the 0·28% level caused a change in the fracture mode. Instead of the ductility troughs moving to lower temperatures...

Determination of heat transfer coefficients during water cooling of metals

Abstract: The heat transfer coefficients for various cooling methods were investigated. Copper, aluminium, and nickel billets were heated and then subjected to cooling by water spray systems and immersion in water and mixtures of water and organic agents. The temperature close to the surface was measured and used for the calculation of the heat transfer coefficient by the implicit finite difference method. The results yielded a defined relation between the heat transfer coefficient and surface temperature, thermophysical material properties, and the intensity of spray cooling or the concentration of the organic agent in the bath. Using this data, a means for analytically formulating the heat transfer coefficient was obtained, enabling easy calculation to be made of the temperature field during quenching or cooling in continuous casting.MST/154

Mechanical behaviour of high-nitrogen stainless steels

Abstract: The application of previously published equations describing the dependence of proof stress on the nitrogen content of austenitic stainless steels has been validated and also extended to higher nit...

Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel

Abstract: The influence of prior austenite grain size dγ, and true interlamellar spacing of pearlite St on the strength and fracture toughness of a eutectoid rail steel has been investigated. Specimens were machined from rail sections and heat treated to produce a wide variation in dγ and st. Mechanical properties studied included 0·2% proof stress σ0·2, ultimate tensile strength σu, tensile ductility δ, cleavage fracture stress σf, and plane strain fracture toughness K1c. All tests were performed at a temperature of −80°C. The values of σ0·2 and σu increase as st decreases. The proof stress is related to the mean free distance λin the pearlitic ferrite by a Hall–Petch equation. A microstructural dependence similar to that of σ0·2 is shown by σf and for all but the finest pearlites σf is interpreted as a shear stress controlled cleavage nucleation stress. The value of K1c first decreases with decreasing st and then increases for the finest spacings. This behaviour is attributed to a change in the micromecha...

Performance of sialon cutting tools when machining nickel-base aerospace alloys

Abstract: A range of sialon compositions have been used to machine the nickel-base alloy Incoloy 901. Wear measurements, taken throughout the machining tests, have shown that tool life and flank wear resistance increase with α-sialon content. An examination of the wear mechanisms involved suggests that this can be ascribed to the increased resistance to dissolution into the work piece, afforded by the higher aluminium and oxygen levels in α-sialon compared with those in β–sialon. At higher cutting speeds one of the main mechanisms of tool wear was rake face flaking, and resistance to this mechanism was found to increase with tool material grain size. A model has been proposed for the initiation and propagation of the cracks which produce this type of failure. At lower cutting speeds depth-of-cut notch wear was of major importance, and resistance to this mechanism was found to decrease with increasing grain size.MST/249

Role of vacancies in coprecipitation of δ′ – and S–phasesin Al–Li–Cu–Mg alloys

Abstract: Precipitation reactions in a series of Al–Li–Cu–Mg alloys, encompassing the composition ranges of the commercial Lital A and Lital B materials, have been studied to determine the role of solute and vacancy concentrations on homogeneous S-phase (Al2CuMg) precipitation. Homogeneous precipitation of S-phase only occurs when a critical combination of free vacancy and copper and magnesium supersaturation is achieved. Lithium reduces the free-vacancy concentration by strong binding between vacancies and lithium atoms or atom clusters. Precipitation of δ′ (Al3 Li) releases the bound vacancies and thereby promotes S-phase formation and, in extreme cases, vacancy condensation, producing dislocation loops.MST/350

Effect of carbon content on hydrogen occlusivity and embrittlement of ferrite–pearlite steels

Abstract: Experiments on a series of pure Fe–C alloys consisting of ferrite and pearlite only have shown that the ferrite/pearlite and pearlite/pearlite interfaces are effective hydrogen trapping sites. The ferrite/cementite interfaces within the pearlite colonies, however, have little effect on the hydrogen occlusivity. With an increase in carbon content, more ferrite/pearlite interfaces are created and these increase the hydrogen occlusivity. Although the ferrite/cementite lamella interface has little effect on the hydrogen occlusivity, it does appear that the lamellae interfere with the hydrogen diffusion path across the pearlite colonies. Thus, the higher-carbon alloys in the pearlitic condition have a lower apparent hydrogen diffusivity. Hydrogen has little effect on the tensile strength, but significantly reduces the ductility. After hydrogen charging, high-carbon alloys suffer a lower ductility loss. However, in terms of absolute values, the low-carbon specimens are always more ductile than the high-...

Formation of intermetallic compounds during solidification of dilute Al–Fe–Si alloys

Abstract: Dilute Al–Fe–Si alloys with Fe/Si weight ratios of 2 and 4, which had been subjected to solidification rates of ~1 and ~10 Ks−1 were investigated by analytical transmission electron microscopy. The precipitation of ‘constituent’ intermetallic phases was characterized as a function of the Fe/Si ratio, alloy purity, rate of solidification, and heat treatment at 600°C. A range of phases, including three new ones (AlpFe, q1-AlFeSi, and q2-AlFeSi), were identified for each experimental condition. The crystal structures of the phases were determined by electron diffraction, and their compositions obtained by scanning transmission electron microscopy and energy dispersive X-ray analysis.MST/388

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract: The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a ...

Influence of grain size on hot ductility of plain C–Mn steels

Abstract: The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar3 temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the h...

Hot ductility of directly cast C–Mn–Nb–Al steel

Abstract: Tensile samples of a C–Mn–Nb–Al steel (BS 4360: 50D grade) have been cast in situ and either directly tested in the temperature range 850–1200°C, or were allowed to cool through the transformation, re–solution treated, and then tested in the same temperature range. The hot ductility of the directly tested cast material was found to be superior to that of the reheated material. Carbon extraction replicas taken close to the fracture surfaces showed large differences in the distribution of sulphide inclusions and NbCN precipitates along the γ boundaries. The directly cast material had sulphide inclusions and NbCN precipitates present in the form of coarse particles situated close to the interdendritic boundaries. A significant proportion of these coarse sulphide inclusions and NbCN eutectics, produced during solidification, redissolved on reheating at 1330°C, and subsequently precipitated in a much finer form at the γ grain boundaries, reducing hot ductility. It appears likely that the very marked se...

Use of protective coatings in aero gas turbine engines

Abstract: The various types of protective coating used in the turbine and combustor sections of gas turbine engines operating in aero environments are reviewed. Comments on experience in marine environments are included as appropriate. The factors affecting coating selection for turbine rotor blade applications are discussed, and service experience is reviewed. The influence of the blade operating environment is highlighted. Aluminide type coatings remain the most widely used and the improved performance arising from modifications in the chemical and physical characteristics of the coating is described. The most severe environments in either corrosive or mechanical terms have been found to require improved coatings, and experience with platinum aluminide and overlay coatings is discussed. Thermal barrier coatings have been standardized on some combustion liners for the past decade and component life has been significantly increased. More recent application of thermal barrier coats to stator vane platforms h...

Diffusion welding of Ti/Ti and Ti/stainless steel rods under phase transformation in air

Abstract: Titanium (Ti/Ti) rods and titanium and stainless steel (Ti/SS) rods have been joined by diffusion welding under phase transformation in an air atmosphere. Specimens were heated to above the transformation temperature of titanium and cooled below that temperature one, two, or three times by alternately applying and breaking an electric current. The welding process was finished in a few minutes. In Ti/Ti welds no joint interface was observed inside the specimen. With a maximum temperature of 1200–1300 K in the thermal cycling, the bulging ratio was less than 30%, the yield stress was 280 MN m−2 – exceeding 90% of that of the base metal, and the ultimate tensile strength was 380 MN m−2, exceeding 80% of that of the base metal. In Ti/SS welds a joint interface was observed inside the specimen, but there was no gross void at the interface. With a maximum temperature of 1150–1200 K in the thermal cycling, the bulging ratio was less than 30%, the yield stress was 250 MN m−2 – exceeding 90% of that of the...

Superplastic deformation characteristics of microduplex Ti–6Al–4V alloy

Abstract: A study has been made of the superplastic tensile behaviour of a microduplex Ti–6AI–4V alloy in the temperature range 760–940°C. Maximum superplastic elongations were obtained for a temperature of 880°C at which the volume fraction of β–phase was approximately 0·4. At temperatures below about 850°C, the main microstructural changes which occurred involved dynamic grain refinement and cavitation. The cavities, which occurred at α/α and α/β boundaries, interlinked parallel to the tensile axis at high strains. However, the volume fractions of cavities observed, although not quantitatively assessed, were always relatively small and had no significant effect on the fracture behaviour of tensile specimens, all of which pulled down to a fine point or chisel edge at failure. At temperatures greater than 850°C, no cavities were observed except near the fracture surface, and the most important structural change was grain growth. At all the temperatures examined, the structural changes which occurred during...

Investigation of structure and texture development during annealing of low-carbon steel

Abstract: The effect of carbon in different states of dispersion has been examined in relation to its effect on the texture formed during the annealing of cold-rolled steel sheet. Quantitative texture analyses using orientation distribution functions have been combined with detailed metallographic investigation of changes taking place during recrystallization. Calculations have also been carried out on the rate of dissolution of carbon from cementite particles for a range of relevant conditions. Dissolved carbon present before cold rolling greatly reduces the final annealed {111} texture strength and leads instead to a spread around the Goss orientation. Dissolved carbon present with manganese during annealing also reduces the final {111} intensity, tending to replace it with a number of relatively weak minor components; this is observed during conditions of slow heating, or when primary carbide particles are numerous, and seems to be associated with a significant nucleation rate of new grains during the la...

Thermodynamic evaluation of C–W system

Abstract: An evaluation of the C–W system has been made using a two–sublattice regular solution model for the interstitial solution phases and an ordinary subregular solution model for the liquid phase. A set of parameter values describing the Gibbs energy of each individual phase is given.MST/275

Mechanical alloying – the development of strong alloys

Abstract: Mechanical alloying is a solid-state process for making alloys by high-energy milling, under conditions such that constituent powders are repeatedly fractured and welded together and ever more intimately mixed. After subsequent consolidation at elevated temperature, the alloys can be shaped by rolling, forging, and machining. The process is used to incorporate a fine dispersion of ceramic particles. Mechanically alloyed nickel-base superalloys, combining a dispersion of yttria with conventional precipitation strengthening, have achieved higher strength at 900–1100°C than directionally solidified and single-crystal alloys, and are being used for gas-turbine vanes and blades. Mechanically alloyed ferritic stainless steel, with outstanding strength and corrosion resistance at temperatures as high as 1300°C, has been produced as sheet, tube, plate, rings, and forgings. Mechanically alloyed aluminium alloys also offer higher strength, e.g. in as-forged thick sections of Al–Mg–Li alloy.MST/567