摘要:

AbstractTest procedures for selected property measurements on metal matrix composite (MMC) materials are examined. The mechanical properties considered are associated with tensile, compressive, bend, impact, fracture toughness, fatigue, and high temperature testing. A brief review is also included of mathematical models designed to predict the mechanical properties of MMCs from a knowledge of the properties of the constituents and their geometry.MST/846

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.105

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractFrom experimental evidence it has previously been shown that heterogeneous nucleation is non-reciprocal and therefore that the balance of interfacial energies is a controlling factor. IfσNL>σNS+σSLthe nucleus S can grow from the liquid L on the substrate N. When nucleation takes place the contact angleθis small and there is epitaxy between the nucleant and the nucleus that reduces theσNSto very low values so that, as a first approximation, heterogeneous nucleation can be expected whenσNL>σSL. Interfacial energiesσSLare roughly proportional to the melting point, and high melting point phases usually nucleate lower melting phases. The contrary may happen if the low melting phase has a low symmetry crystal. Nucleation can take place on high index faces of this crystal because, although its averageσNLis lower than theσSLof a higher melting point, high symmetry crystal, theσNLof its high index faces may be higher than theσSL.MST/845

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.118

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractIn Al-Mo alloys, rapidly quenched from the melt at rates of approximately 106KS−1, it has been demonstrated that single phase solid solution alloys can be obtained in materials with up to 1·3 at.-%Mo. Above this concentration, two new metastable phases are observed in the form of small precipitates within the matrix of aluminium-rich solid solution. These have been identified as a diamond cubic phase, space group Fd3m, a =1·4–1·5 nm and a hexagonal phase, space group P6/mmm or P6/mmc, a = 0·45 nm, c = 0·27 nm. The maximum supersaturation of the solid solution matrix, obtained in alloys containing such precipitates, was found to be 2·45 at.-%Mo. Upon annealing, both the solid solutions and the metastable intermediate phases decompose directly to the equilibrium structure of aluminium-rich solid solution and Al12Mo. The Al12Mo forms either as precipitates on the grain boundaries or as a layer around the metastable intermediate phases. From a good correlation between the maximum supersaturation obtained and that predicted by consideration of thermodynamic and kinetic factors it is suggested that a similar theoretical treatment may be used to provide an indication of the maximum supersaturation achievable in other systems.MST/824

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.123

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractAn attempt has been made to model the kinetics of changes of composition in cementite during the aging of creep resistant low alloy steels of the type used in power plant. In the model a finite difference method is used for calculating the rate at which alloying elements redistribute between ferrite and cementite, subject to the thermodynamic constraints which determine the equilibrium compositions of these phases. It has also been possible to derive approximate analytical solutions which give good physical insight into the factors controlling the approach to equilibrium. Particle size has been found to have a strong influence on the rate at which the cementite composition is changed. A theoretical basis for the time dependence of cementite composition is also provided. The method is easily adapted to multiple or anisothermal heat treatments. Further work is suggested to account for simultaneous coarsening and changes in composition and also to allow for the formation of alloy carbides at long aging times.MST/874

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.131

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractTempered martensite embrittlement has been studied in a commercial purity AISI 4340 steel and the results are given in the present paper. In particular the role of carbides in the embrittlement process is examined. It is shown by the results that the standard tempered martensite embrittlement is observed in this steel. The fracture energy after 1 h tempering treatments decreases with increasing tempering temperature between 200 and 350°C. However, a significant amount of intergranular fracture was found only on the surface of the specimen that was tempered at 350°C. Initially, M3C carbides are precipitated after tempering at 250°C. As the tempering temperature increases, both the number density and thickness of the carbides increase. Also, the length of the grain boundary carbides increases significantly with increasing tempering temperature. The carbides are all ribbon-shaped and are essentially pure Fe3C. It is shown using Auger electron spectroscopy that the grain boundaries were enriched in phosphorus and sulphur. When these results are combined, it becomes obvious that the initial decrease in fracture energy after tempering at 250 and 300°C is caused by carbides aiding in ductile fracture. However, after tempering at 350°C for 1h, the carbide density and size on the grain boundaries become sufficiently large for intergranular fracture to occur along the boundaries already weakened by impurities. When these results are combined with others in the literature, it becomes obvious that carbides can decrease the fracture energy by aiding in ductile fracture, intergranular fracture, or cleavage. All these fracture modes can cause a decrease in fracture energy when specimens are tempered in the range of 200–350°C; which fracture mode occurs will depend on the total composition of the specimen, the matrix strength, and the test temperature.MST/815

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.138

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractEquilibrium segregation of phosphorus at grain boundaries of austenite was studied in Fe–P and Fe–10Mn–P (wt-%) alloys. The specimens were equilibrated at temperatures between 750 and 1200°C and after rapid quenching analysed using Auger electron spectroscopy. It was found that phosphorus segregated in both types of alloy and it is shown by the results that manganese increased the level of phosphorus segregation. There was no observation of segregation of manganese in austenite. Segregation of phosphorus in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. The equilibrium segregation of phosphorus in the Fe–P system and also in the Fe–10Mn–P system can be described using the Langmuir–McLean equation and a value for the free energy of phosphorus segregation∆GPo= 47 kJ mol−1. It is most probable that the level of phosphorus segregation is enhanced by manganese because manganese intensifies the additional segregation of phosphorus during quenching.MST/876

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.148

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractThe growth of grain boundary carbides has been followed during continuous cooling from the austenite using cooling rates of 7 and 150 K min−1and Mn levels of 0·5 and 1 wt-%. The isothermal growth of grain boundary carbides in the temperature range 600–650°C has also been examined and growth was found to obey a (time)1/4relation which is very slow. The activation energy for growth increased from 241 kJ mol−1for the lower Mn containing steel to 293 kJ mol−1for the higher Mn steel. For continuous cooling conditions growth was very rapid. Grain boundary carbides started to form at the time of the pearlite reaction and their initial thickness corresponded to the thickness of lath carbides in the pearlite colonies. The grain boundary carbides are believed to form the tails of the pearlite colonies and probably initiate the pearlite reaction. Increasing the cooling rate or Mn content reduced the lath carbide thickness within the pearlite colonies. Subsequent growth of the grain boundary carbides is believed to occur by the excess C rejected on cooling diffusing to the ferrite grain boundaries and the activation energy for growth in this case may well be that for C diffusion in ferrite (84 kJ mol−1). The amount of C available for precipitation in the form of grain boundary carbides is that present at the time of the pearlite reaction. The start of the pearlite reaction is suppressed by Mn and the amount of C available to form carbides is also reduced, hence producing finer carbides. By increasing the cooling rate, the pearlite transformation was suppressed to lower temperatures reducing the amount of C as well as increasing the number of nucleation sites for precipitation, both of which favour fine carbides. Except for very slow cooling rates‘isothermal’growth is not likely to be important.MST/868

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.155

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractThe microstructures of extruded high tensile brasses with compositions conforming to BS 2872/4 CZ114, CZ115, and CZ116 consist of copper rich fccα-phase, ordered B2β′-phase, and some iron rich particles; these particles areα-Fe in alloy CZ115 and Fe3Al in alloy CZ116. Alloy CZ114 contains mainlyα-Fe particles, although a few, small precipitates of Fe3Al have also been observed. It is shown in studies of the microstructural development that theα-Fe particles are formed in the liquid, while Fe3Al is precipitated from solid solution over a wide temperature range. All theα-Fe and Fe3Al particles/precipitates have the same orientation relationship with theβ′-matrix. Fine coherent precipitates of Fe3Al andα-Fe have been observed in quenched alloys. When the quenched alloys are tempered at temperatures between 400 and 500°C, theα-phase is precipitated fromβ′and has a Widmanstätten morphology. The rates of coarsening of Fe3Al andα-Fe precipitates appeared to be appreciably higher in the disorderedβthan in the orderedβ′-phase.MST/966

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.162

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractIn alloys such as 22Cr–33Ni–Fe (wt-%), titanium can improve creep strength, but may also give excessive hardening and low ductility in some regimes. Experimental casts having various Ti, Mn, Mo, and Si contents were solution treated at 1250°C and age hardening at 550–650°C was studied. Two commercial 800 type alloys were included for comparison. Slow strain rate tensile tests were carried out at 550°C to relate flow stress to creep ductility and age hardening. Age hardening was related to Ni3Ti (γ′) particle size and equilibrium volume fraction, and to titanium solute depletion. The critical effect of free titanium ([Ti]–3·4[N]) on the factors given above was apparent. Manganese suppressed an early stage of hardening at 600°C, apparently by hindering the clustering on cooling that nucleated a fineγ′precipitate. Aluminium enhanced the effect of titanium, but this could not be attributed solely to substitution inγ′. Intermediate treatments at 850–900°C improved the inherent creep ductility at 550°C by precipitating coarse (Cr, Fe)23C6on grain boundaries. These treatments also preprecipitated some of the titanium as TiC with some reduction in the subsequent creep flow stresses.MST/877

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.172

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor

摘要:

AbstractThe microstructure and hardness of Tribaloy 700 (T 700) of nominal composition (wt-%) 50Ni–32Mo–3Si–15Cr and two alloys of similar Ni, Mo, and Cr contents, but having no or 6%Si have been determined. The microstructure of the three alloys consisted of a primary Laves phase in a eutectic matrix, but the volume fraction and crystal structure of the primary Laves phase and the morphology of the eutectic matrix were different. The presence of Si increased the proportion of primary phase, stabilised the hexagonal form of the Laves phase, and produced an ill defined eutectic. The hardnesses of the as cast alloys were similar, but T 700 was the most stable at elevated temperatures.MST/881

ISSN:0267-0836    

DOI:10.1179/mst.1989.5.2.180

出版商:Taylor&Francis    

年代:1989

数据来源: Taylor