Showing papers in "Materials Science and Technology in 1999"

Precipitation hardening in metals

Abstract: Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys. The theoretical treatments of precipitation hardening are briefly considered. The equations for strengthening by ‘hard’ indeformable particles and by ‘soft’ deformable particles are presented, and the implications are discussed. These lead to the concept of an optimum particle size for a given system, but the optimum can vary from system to system depending upon the particle characteristics. A broad comparison is made between the increments in strength that occur due to precipitation in commercial alloys and the predictions of the theories; an important contribution to these increments in strength is shown to derive from variations in the volume fraction of precipitated particles that can be employed in the various systems.

Evolution of dislocation structure induced by cyclic deformation in a directionally solidified cobalt base superalloy

Abstract: Cyclic deformation has been carried out on a directionally solidified cobalt base superalloy at room temperature in air under the control of different total strain amplitudes. Dislocation structures of different stages in the cyclic stress response curve were studied by TEM. Observations show that a large number of stacking faults and fault intersections are formed in the initial hardening stage, and this leads to the initial hardening of the alloy. However, formation of hexagonally close packed (hcp) zones and twins is proposed to be the cause of cyclic saturation. Models for both the transition from stacking faults to a hcp layer structure and to twins are proposed.

Grain refinement by Al-Ti-B alloys in aluminium melts: a study of the mechanisms of poisoning by zirconium

Abstract: Conventional grain refining tests confirm that the presence of zirconium in commercial purity aluminium melts dramatically reduces the effectiveness of Al–Ti–B grain refiners. Quantitative comparisons suggest that this poisoning action cannot be attributed solely to reduction in growth restriction arising from changed solute contents in the melt. Microscopic analysis of TiB2 particles in an aluminium based, zirconium containing metallic glass shows the potential for substitution of zirconium for titanium. Analysis of a conventional Al–5 wt-%Ti–1 wt-%B refiner doped with zirconium shows the potential for similar modification of Al3 Ti. Thus zirconium can affect the nucleation stage of grain refinement by substituting for titanium in the aluminide and/or the boride phase, consistent with earlier work suggesting that the key to the nucleation potency is the existence of a layer of Al3 Ti on TiB2 particles.

Friction welding process of 5052 aluminium alloy to 304 stainless steel

Abstract: Type 5052 aluminium alloy was joined to type 304 austenitic stainless steel via a continuous drive friction welding process. The joint strength increased, and then decreased after reaching a maximum value, with increasing friction time. Joint strength depended on the size and shape of the tensile testpiece. Friction weldability could be estimated by electrical resistmetry. The process of friction welding between the aluminium alloy and the stainless steel is proposed to evolve as follows: welding progresses from the outer to the inner region; an unbonded region is retained at the centre of the weld interface with shorter friction time; longer friction time causes the formation of an intermetallic reaction layer at the weld interface; and the reaction layer grows as the friction time increases. When the thickness of the reaction layer increased above a critical value, the joint was brittle and fractured at the weld interface. The joint was sound when there was no unbonded region and a thin reaction...

Static recrystallisation and hot strength of Al–1Mg during thermomechanical processing

Abstract: The kinetics of static recrystallisation of an Al–1Mg alloy has been investigated by rolling slabs of tapered and uniform thickness, and by plane strain compression (PSC) tests at 400°C for a wide ...

Structural characteristics of low temperature plasma carburised austenitic stainless steel

Abstract: A low temperature plasma carburising process has recently been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. The present paper discusses the structural characteristics of the carburised layers produced on AISI type 316 steel at temperatures between 400 and 600°C. It was found that at low temperatures (<520°C), the carburised layers produced were precipitation free and comprised a single phase, which had a face centred cubic structure and was identified as expanded austenite owing to the supersaturation of carbon in austenite. The carburised layer was in a deformed and distorted state. High densities of twins, stacking faults, and dislocations were found in the expanded austenite. The degree of lattice expansion was estimated and was found to vary with processing temperature and depth in the layer. Precipitation of carbides (mainly Cr7 C3 ) occurred when the carburising temperature was relatively high (for exampl...

Influence of processing parameters during disintegrated melt deposition processing on near net shape synthesis of aluminium based metal matrix composites

Abstract: This paper addresses the influence of processing parameters on the near net shape synthesis of Al–SiC particle reinforced metal matrix composites by the disintegrated melt deposition technique. The processing parameters that were investigated include stirring speed during reinforcement addition, holding and stirring times following reinforcement addition, and total flight distance. The effect of the weight fraction of SiC incorporated was also studied. Maximum incorporation of the reinforcement was achieved by dispersing SiC particles in liquid aluminium with an impeller at a stirring speed of 500 rev min-1. Composite porosity levels were minimised for holding and stirring times following SiC addition of 15 and 5 min, respectively, and for a total flight distance of 450 mm. Composite porosity levels were increased with increasing weight fractions of SiC incorporated. The results obtained clearly demonstrate the dependence of composite porosity levels on weight fraction of reinforcement incorporate...

After Concorde: evaluation of creep resistant Al–Cu–Mg–Ag alloys

Abstract: A European consortium is evaluating materials for the construction of a new supersonic transport aircraft that may replace Concorde. Current designs propose to use an aluminium alloy for the fusela...

Effect of austenite grain size and bainite morphology on overall kinetics of bainite transformation in steels

Abstract: An attempt is made to rationalise some contradictory observations on the effect of austenite grain size on the overall kinetics of the bainite transformation in steels. Experiments have been carried out on two steels which show opposite effects of austenite grain size on the reaction rate. General equations describing the reaction rate are derived by taking into account the morphology of the bainite in each steel. The equations derived can explain the contradictory effects of the austenite grain size on the overall reaction rate. The results are also found to be in good agreement with the published data.

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract: The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23 C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.

Influence of copper addition on precipitation kinetics and hardening in Al–Zn–Mg alloy

Abstract: The influence of the addition of copper on the precipitation behaviour of an Al–Zn–Mg alloy at 160°C has been investigated. Phase transformation has been investigated by differential scanning calorimetry, the precipitation kinetics by small angle X-ray scattering and transmission electron microscopy, and the mechanical properties by microhardness testing. The addition of copper results in an increase in the stability of Guinier–Preston zones formed at room temperature, and in a decrease in the nucleation temperature for the η′ phase. It also results in a decrease in the apparent diffusion coefficient for precipitation, and in an increase in the strengthening ability of precipitates. As a consequence, the copper bearing alloy shows a much higher maximum strengthening ability, and a lower sensitivity to the heating rate to the aging temperature. However, the higher supersaturation of the copper bearing alloy results in a higher quench sensitivity, and at slow quench rates the ternary alloy shows a ...

Some phase transformations in steels

Abstract: Many essential properties of iron alloys depend on the atomic mechanism of phase change. Following a description of the mechanisms established for solid state transformations in steels, a unique measure of the mechanism is proposed, one which can easily be accessed experimentally. The application of the mechanism to the design of novel steels is illustrated in the context of martensite and bainite. This is followed by an introduction to a new and quite general kinetic theory based on the Avrami approach. The application of the theory to creep resistant and structural steels is illustrated with example calculations.

Fatigue of precipitate strengthened Cu–Ni–Si alloy

Abstract: The fatigue resistance and tensile properties of a precipitate strengthened Cu–Ni–Si alloy have been investigated. Underaged material exhibited greater work hardening rates than either the peak aged or overaged material. It is proposed that this occurs because the δ-Ni2 Si precipitates can be cut by dislocations, but the significant difference in structure between the matrix and precipitate requires that interfacial dislocations are left behind which resist further cutting. At and beyond the peak aged condition, an Orowan type strengthening mechanism is believed to operate. Transmission electron microscopy studies of underaged material after both high and low cycle fatigue tests have revealed the presence of precipitate free bands. Such bands are usually attributed to the destruction of the integrity of the precipitates by repeated cutting. In spite of the complex structure of the strengthening precipitate δ-Ni2 Si and the higher work hardening rate observed in the underaged Cu–Ni–Si alloy, it is ...

Precipitation behaviour in heat affected zone of welded superaustenitic stainless steel

Abstract: The present work concerns the characterisation of intermetallic phases formed in the heat affected zone of a welded superaustenitic stainless steel of composition Fe–0·02C–3Mn–24Cr–7·3Mo–22No–0·5Cu–0·5N (wt-%). Grain boundary precipitates at various distances from the fusion line have been investigated regarding crystal structures, compositions, and particle morphologies. A correlation of precipitation with the temperature history recorded in the heat affected zone was also performed. Two different precipitates, σ and R, were detected in the heat affected zone. These precipitates were evenly distributed along grain boundaries and had platelike shapes with typical lengths of 200–900 nm and 30–300 nm for σ and R, respectively. Near the fusion line, coexistence of R and σ phases was observed but, at larger distances, only R phase was found, indicating a lower temperature of formation for this phase.

Effects of grain size on development of athermal and strain induced ɛ martensite in Co–Cr–Mo implant alloy

Abstract: In the present work, the development of athermal ɛ martensite during quenching of a low carbon Co–Cr–Mo alloy was investigated as a function of the grain size. In addition, a strain induced transformation (SIT) from fcc to hcp was exhibited during compressive plastic straining. It was found that grain size exerts a strong influence on the resultant volume fractions of athermal and strain induced ɛ martensite. In particular, fine grain sizes inhibit the formation of athermal martensite while promoting appreciable volume fractions of ɛ martensite through the SIT mechanism. Moreover, X-ray diffraction analyses indicated that in 10 μm grained structures the volume fraction of strain induced ɛ martensite reaches a saturation level of approximately 0·65 just before compressive fracture. In contrast, increasing grain sizes result in the formation of up to 0·9 volume fraction of SIT martensite. Moreover, the alloy yield strength was found to decrease down to 592 MPa (approximately half the yield strength...

Influence of Ti on hot ductility of C–Mn–Al steels

Abstract: The hot ductility of a series of C–Mn–Al steels with nominal compositions of Fe–0·1C–1·4Mn–0·3Si–0·006N with Ti additions of 0–0·04% (all compositions and percentages given in this paper are in wt-% unless otherwise stated) has been determined over the temperature range 1100–700°C. Tensile specimens were cast in situ and cooled at 25, 100, and 200 K min-1 to the test temperature and then strained to failure at a strain rate of 2 × 10-3 s-1. The cooling rates were chosen so that the range for conventional continuous casting (250 mm) and thin slab casting (40–60 mm) was covered. Titanium additions were generally found to impair ductility owing to the formation of fine TiN precipitates. Increasing the cooling rate to the usual rate for thin slab casting also resulted, in general, in finer AlN precipitation and finer MnS inclusions at the boundaries, leading to worse ductility. A regression equation was obtained which showed that the best ductility was obtained when the cooling rate was slow, the part...

Role of ferrite/pearlite banded structure and segregation on mechanical properties of microalloyed hot rolled steel

Abstract: Microalloying can significantly enhance the strength levels of low- and higher-carbon steels through grain refinement and precipitation. The role of niobium and titanium microalloying additions on the ferrite/pearlite banded microstructure and properties of low carbon hot rolled steel has been studied. A banded structure and microsegregation were observed which primarily contributed towards the anisotropic behaviour of the steel. The changes observed in the fracture morphology along and across the rolling direction of the impact specimens were consistent with banded microstructure. It was noted that microstructural and microchemical banding occurs primarily as a consequence of the rejection of substitutional alloying elements during solidification. However finer prior austenite grains may also be associated with the well defined banded microstructure observed in the steel containing microalloying additions. The grain refinement and precipitation strengthening in addition to change in the morpholog...

Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation

Abstract: Isothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2 O4 was also found in the case of the plain alumini...

Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

Abstract: A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 × 10-5. Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0·2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the tempe...

Theoretical design of ferritic creep resistant steels using neural network, kinetic, and thermodynamic models

Abstract: A neural network model has been developed on the basis of published experimental data. This allows the creep rupture strength of bainitic and martensitic electric power plant steels with compositions based on Fe–2·25Cr–1Mo and Fe–(9–12)Cr to be estimated as a function of chemical composition, heat treatment and time at temperature. This model, together with a variety of thermodynamic and kinetic calculations, has been used to propose two new alloys related to the 10CrMoW steel which in theory have outstanding creep rupture properties.

Damage mechanisms of coated systems under thermomechanical fatigue

Abstract: The damage mechanisms of several kinds of coatings on a single crystal nickel base superalloy under thermomechanical fatigue (TMF) are described. The systems investigated were diffusion platinum aluminide coatings, Co–Ni–Cr–Al–Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experiments were carried out on hollow specimens over a temperature range from 300 to 1050°C, at strain ranges Δɛ = 0·5 and 0·7%, and at a strain ratio R = -∞. No coating cracking was found for the platinum aluminide coating. Instead, specimens failed owing to oxidation induced crack initiation from the uncoated inner surface of the hollow testpieces, although coating surface roughening caused by non-homogeneous oxidation was observed. For the overlay coating, roughening in terms of coating rumpling and coating cracking occurred, resulting in reduced TMF life. For TBC specimens with a thin ceramic coating processed by electron beam–physical vapour deposition (EB–PVD), TMF life was comparable with that of speci...

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

Abstract: The effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more t...

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract: The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10-3 s-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ∼0·005 wt-%N ducti...

Slurry erosion of metallics, polymers, and ceramics: particle size effects

Abstract: One of the least well understood areas in the study of erosion by solid particles is the effect of particle size. Erosion is generally assumed to be independent of particle size above a critical value. However, there is evidence that this pattern is dependent on the process conditions. In the present study, the effect of particle size was investigated for different classes of materials, which included two pure metals, an alloy, a ceramic, and a polymer. The apparatus used was an impinging jet. Scanning electron microscopy was used to characterise the degradation following erosion. The results showed that the erosion rate peaked at intermediate particle sizes, for some of the materials studied. However, the particle size at which the peak occurred changed as a function of target and particle properties. Such observations were explained in terms of the combined effects of particle, target, and fluid flow parameters on the erosion mechanisms of the different materials.

Influence of silicon and magnesium on grain refinement in aluminium alloys

Abstract: Grain refinement in Al–Si alloys with silicon contents in the range of 0·2–30 wt-% has been studied in detail with conventional as well as higher level additions of a Al–5Ti–1B master alloy. A poisoning effect was observed with Al–Si alloys containing ≥7 wt-%Si and the extent of poisoning increased with an increase in the silicon content. Silicon improves the grain refining behaviour of aluminium when added in small quantities (0·2%). Magnesium can counteract the poisoning effect of silicon. The optimum level of magnesium required to overcome the poisoning effect depends on the silicon content of the alloy. Higher level additions of a grain refiner could overcome the poisoning effect of silicon and the level required to achieve good grain refinement is a function of the silicon and magnesium contents of the alloy. The present paper also reports the influence of degasser and melt temperature on the grain refining response of Al–Si alloys.

Rare earth metal induced modification of γ-M2 C, γ-M6 C, and γ-MC eutectics in as cast M2 high speed steel

Abstract: The effects of rare earth metals on the morphology of γ-M2 C, γ-M6 C, and γ-MC eutectics of as cast M2 high speed steel were investigated. The solidification of the specimens was monitored by conventional and differential thermal analysis and the characterisation of the microstructures was performed through optical and scanning electron microscopy. Rare earth metals favoured the formation of the M6 C eutectic, at the expense of the M2 C eutectic, and promoted the formation of the duplex M2 C/MC and M6 C/MC eutectics. The formation of these duplex eutectics was attributed to the decrease in the MC eutectic temperature.

Equiaxed solidification of Al–Si alloys

Abstract: An experimental programme has been undertaken to determine which of the grain formation mechanisms of equiaxed crystals are dominant in the solidification of Al-Si foundry alloys. Small ingots were cast from alloys of varying silicon concentration with and without gauze barriers, using different types of mould materials and different mould preheats. The results show that two mechanisms of grain nucleation are operating. The first is a wall mechanism where crystals are nucleated either on or near the mould wall owing to thermal undercooling. The second is a constitutional supercooling mechanism where nucleants are activated in the constitutionally undercooled zone ahead of the advancing interface. As a consequence, the grain size decreases with increasing silicon content. However a transition in the growth mode occurs once a critical degree of constitutional undercooling is exceeded. This change in growth is accompanied by an increase in grain size. The transition point can be shifted with respect to solute content by changing the casting conditions, and a mechanism is proposed to explain this effect. MST/4109

Gaussian process modelling of austenite formation in steel

Abstract: The present paper introduces the Gaussian process model for the empirical modelling of the formation of austenite during the continuous heating of steels. A previous paper has examined the application of neural networks to this problem, but the Gaussian process model is a more general probabilistic model which avoids some of the arbitrariness of neural networks, and is somewhat more amenable to interpretation. It is demonstrated that the model leads to an improvement in the significance of the trends of the Ac1 and Ac3 temperatures as a function of the chemical composition and heating rate. In some cases, these predicted trends are more plausible than those obtained with the neural network analysis. Additionally, it is shown that many of the trace alloying elements present in steels are irrelevant in determining the austenite formation temperatures.

Effect of aging on quality index of an Al–Cu casting alloy

Abstract: When the Al–Cu casting alloy A201 is aged, a plot of the tensile strength versus the elongation to fracture follows a circular pattern. This is in contrast with Al–Si–Mg alloys, which show a linear relationship between the two parameters when aged. The latter observation is the basis for the concept of quality index (Q) which is widely applied to Al–Si–Mg alloys but does not seem to be valid for alloy A201. As alloy A201 is aged up to peak aging, the yield strength increases while the ductility decreases. The quality index is high and remains nearly constant. When overaged, the yield strength decreases and the strain hardening rate at low strains is very high. At strains beyond 3–4% the strain hardening saturates. This limits the tensile strength and ductility, causing the quality index to fall. The circular pattern shown by the quality index results from the transition from the high Q value of the underaged condition to the low Q value of the overaged condition. An analytical model relating the ...

Interpretation of hot plane strain compression testing of aluminium specimens

Abstract: Plane strain compression (PSC) testing is now generally accepted as one of the most reliable methods for the generation of flow stress data and microstructural investigation of thermomechanical processing. It has been known for some time that extremely reproducible measurements may be made across different laboratories if a standardised procedure is used. However, particular care must be taken with both the experimental procedure and the interpretation of the measured force–displacement data. The present paper reports investigations that have built on previous work and looked further at the effects of spread of the specimen and friction. In deriving reliable flow stress data, the importance of tool and specimen geometry and consideration of the effects of lubrication and friction are clearly demonstrated. Furthermore, the paper demonstrates the current status of the work by presenting the algorithms behind new software that has been developed for interpretation of raw force–displacement data in a ...