摘要:

AbstractExtensive tensile ductility during superplastic deformation in ceramics has been demonstrated experimentally in yttria stabilized tetragonal zirconia polycrystals, with the achievement of about 800% tensile elongation reported. It has not been possible to achieve such extensive elongations in other ceramics such as Si3N4/SiC composites and alumina. The authors consider that the extensive superplastic deformations obtained in ceramics could be associated with the fact that some ceramics can be classified as having a‘metallic’character based on the nature of the orbitals responsible for their bonding. These metallic ceramics show extensive superplastic deformation compared with other ceramics that can be classified as insulators based on the nature of their bonding orbitals. This observation would help to demarcate ceramic materials that can exhibit superplasticity from the brittle ceramics.MST/1911

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.669

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe microstructure, nature, distribution, and chemical composition of the different phases obtained inγ-TiAl alloys with small amounts of silicon additions have been investigated. Based on the microstructures observed and results obtained via energy dispersive X -ray microanalysis, the approximate partial liquidus projection of the ternary Ti–Al–Si system in the range of compositions over whichγalloys exist, has been established. Two four phase invariant points on which peritectic reactions occur are present. The eutectic valley betweenβand Ti5(Si, Al)3phases exhibits a maximum point, and the liquidus surface in this region is of saddle type. The results were found to be in good agreement with predictions based on thermodynamic modelling.MST/1937

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.674

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractArc melted and cast microstructures of Ti–Al–Valloys containing 10–57 wt-%Al and 4–46 wt-%V have been characterized using optical, scanning, and transmission electron microscopy, and room temperature X -ray diffraction. Chemical compositions of the phases present have been determined using energy dispersive X -ray, and these results have been utilised to plot a tentative non-equilibrium liquidus projection of the Ti–Al–V system. Five ternary peritectic reactions have been shown to be represented within the liquidus projection. These observations have been utilised to establish the solidification sequences of the Ti–Al–V alloys considered.MST/1939

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.681

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractPhase coexistence in the Fe–C–Si–Mn–Cr system, as the base system of a large number of low alloy steels, has been studied experimentally and theoretically. The vertical section of the phase diagram of the Fe–C–2Si–2Mn–1 Cr system near the eutectoid point (700–900°C; 0·14–0·80 wt-%C) and the chemical composition of the phases present were determined by means of scanning electron microscopy and energy dispersive X-ray analysis. Thermodynamic calculations of the Fe–C–Si–Mn–Cr system equilibria, based on the Hillert–Staffansson model, for low contents of silicon, manganese, and chromium were performed. These calculations gave results very close to both the experimentally determined phase boundaries and the composition of phases. A wide eutectoid zone (about 50 K) was found to exist in the system, for all carbon contents studied.MST/1940

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.691

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe effect of residual porosity on the flow stress of particulate composites was studied for a composite in which it is possible to create high porosity (>60%) by using particle coatings. The material was a eutectic composition Sn–Pb solder with additions of intermetaliic CU6Sn5particles. It was found that porosity affects the strength of these composites much less than for non-composites, an effect attributed to particle bonding during solidification. At the same time, residual porosity was found to inhibit strongly or even to prevent hardening of this material. A micromechanical model is developed which simulates this behaviour over a wide range of porosities.MST/1928

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.696

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractTwo extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. Atexture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towardshas been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lower temperature. Quenching experiments have shown that nucleation of secondary recrystallisation occurs at temperatures higher than that at which the recrystallisation interface grows at the zoning speed employed. It is suggested that the microstructure develops via the thermally activated unpinning of interfaces which have mobility advantages.MST/1948

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.703

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractAn unalloyed nodular cast iron, in which the conventional addition of silicon has been replaced by aluminium, has been studied after heat treating in the bainite temperature range. Specimens were austenitised at 950°C for 2 h and then austemperedfor times up to 5 h at either 300 or 400°C. At 400°C the microstructure producedfor austempering times up to 3 h was typical of austempered ductile iron, consisting of carbide free upper bainitic ferrite together with a stable, high carbon enriched retained, reacted, austenite. For longer times, transition carbides are precipitated, initiallyηcarbide in the ferrite, and after 5 h,χcarbide at the austenite/ferrite interfaces by decomposition of the high carbon austenite. Austempering at 300°C produced e carbide in a lower bainitic ferrite together with stable, high carbon retained austenite, for all heat treatment conditions:χcarbide precipitated at the ferrite/austenite interfaces after 5 h austempering. The fracture behaviour of samples austempered at both 300 and 400°C is dramatically influenced by theχcarbide formation, which results in a mostly brittle cleavage mode offailure.MST/1968

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.711

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractCompression testing was used to explore the influence of strain rate on the formation of deformation induced ferrite. Samples of a 0·4%C–1·4%Mn plain C–Mn steel were heated to 1225°C, cooled to test temperatures in the range 1100–610°C, and then given a true strain of 0·6, at strain rates of3×10−2, 3×10−3, and 3×10−4S−1. At the lowest strain rate it wasfound that the strain to peak stress decreased with decreasing temperature in the range 750–610°C. This behaviour is related to the formation of thin films of the softer deformation induced ferrite at theγgrain boundaries at the higher temperatures, and spheroidisation at the lower temperatures. More normal stress–strain curves were observed at the higher strain rates, as raising the strain rate prevents the formation of deformation induced ferrite and delays spheroidisation. The strain rate was also found to have an important influence on the extent of recovery in the deformation induced ferrite; the lowest strain rate enabling full recovery and or recrystallisation to occur, thus keeping the film soft. This behaviour is shown to account for the poor hot tensile ductility at the lowest strain rates. Raising the strain rate in this temperature range improves the ductility because work hardening takes place, raising the strength of the ferrite closer to that of the y, thus preventing strain concentration from occurring.MST/1934

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.721

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe influence of temperature and time of austempering on a nodular cast iron austenitised for 30 min at 900°C has been investigated using optical and transmission electron microscopy. The carbon content of the austenite has been evaluated by measuring the lattice parameter via X -ray diffraction. The structure consisted of bainite containing retained austenite, the amount of which increased, and the carbon content of which decreased, with increasing austempering temperature. No carbides were detected in the materials austempered at 300 and 370°C for up to 2 h, which contained only bainite and austenite. After austempering at 410°C and cooling to room temperature a certain amount of martensite was always detected. After long times at this temperature the austenite decomposed to ferrite and complex carbides. Martensite formation seems to be promoted by the decrease in the carbon content of austenite due to the precipitation ofɛcarbide homogeneously within it. During long austempering treatments at 410°C complex carbides are formed, probably from theɛcarbides precipitated in the austenite.MST/1926

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.728

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThree point bending test studies on 304L austenitic stainless steel at 293 and 77 K demonstrated that hydrogen charging promoted the formation of brittle fracture features at 293 K; promoted the formation of larger and shallower microvoids at 77 K; and reduced the energy absorbed by the material at both temperatures, although to a greater degree at 77 K. These observations suggest that although hydrogen redistribution during testing can affect the failure mode of this material, it is not a necessary requirement for hydrogen induced degradation. Furthermore, the observation that embrittlement is more severe at liquid nitrogen temperatures indicates that the effects of low temperatures and hydrogen may interact synergistically. The data presented in this paper are consistent with a model in which the embrittlement process is affected by local hydrogen concentrations.MST/1930

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.8.738

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor