摘要:

AbstractThe effect of plastic strain on the deformation microstructure has been investigated in polycrystalline copper rolled at room temperature to 5, 10, 20, and 30% reduction in thickness (equivalent strain 0·06–0·42). Results from transmission electron microscopy (TEM) observations show that dense dislocation walls (DDWs) and cells develop during the initial stages of cold rolling. Grains having a high density of DDWs are described as high wall density (HWD) structures, and grains having a low density of DDWs are described as low wall density (LWD) structures. These structures are characterised by cell size, misorientation across the cell walls, and the crystallographic orientation of the grains in which they appear. The DDWs in the HWD structures have special characteristics, extending along several cells and having a misorientation across them greater than that across ordinary cell boundaries at the same strain. The DDWs appear to have a macroscopically determined orientation. Analysis of their crystallographic orientation shows that they are not, as frequently reported in the literature, parallel to {111} planes. It is suggested that the DDWs separate regions of the crystal having different operating slip systems. Two generations of microbands are found to develop with increasing deformation. The first generation microbands are related to a continuous development of the structure according to the principle of grain subdivision, whereas the second generation microbands relate to localised shear on {111}. Finally, the evolution of the deformation microstructure in copper is compared with that observed in other face centred cubic metals, especially aluminium.MST/1273

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1069

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe kinetics of the phase transformation of thallium and its dilute alloys were investigated using X-ray diffraction and calorimetry. Pure thallium exhibits aβ(bcc)→α(hcp) phase transformation on cooling at 508 K. With alloying additions, the crystal structure for each phase does not change, although the size of the unit cell increases. The enthalpy and the temperature of phase transformation for each alloy have been determined. From these data, the chemical free energy change associated with the phase transformation of each alloy was calculated using an equation separated into volume dependent and volume independent contributions. The latter is interpreted on the basis of a change in the electronic chemical potential associated with the electrons occupying different energy states in the Brillouin zones of the two structural modifications. The valence electrons make an outstanding contribution to the chemical free energy change required for the phase change.MST/1420

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1076

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA new low Ni, Co free maraging steel containing 12Ni–3·2Cr–5·1Mo–ITi (wt-%) has been developed. The optimum heat treatment was found to be solution treatment at 1098 K for 60 min followed by air cooling then aging at 753 K for 180 min. Detailed optical and transmission electron microscopy revealed that the solution treated steel transformed to lath martensite. The optimised steel achieved an ultimate tensile strength of 1700 MN m−2and a yield strength of 1660 MN m−2. The toughness measured in terms of impact energy was found to be 38 J. Fractographic analysis carried out in the scanning electron microscope showed predominantly dimpled structures indicating ductile failure. The transformation temperatures for this new steel were established using dilatometry.MST/1398

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1082

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe kinetics of the equiaxed ferrite massive transformation in Fe and Fe alloys has been examined theoretically. The basis of the analysis is that the plateau temperature in a plot of transformation temperature versus cooling rate corresponds to the maximum rate of transformation. An approximate approach for the geometry of grain corner and grain edge nuclei having one or more coherent interfaces has been used. Neither Russell's treatment of incubation periods nor Cahn's analysis of overall reaction involving nucleation and growth predict the observed transformation plateaux temperatures. Instead, calculations assuming that growth kinetics is the rate controlling factor give good agreement for the plateaux temperatures in Fe, Fe–Ni, and Fe–Cr alloys. The theoretical calculations also show the existence of a plateau within±10 K over a range of cooling rates. The marked hardenability of Fe–Ni alloys is also discussed.MST/1393

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1089

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe influence of S and Nb on the hot ductility of as cast C–Mn–Al steel has been examined. Tensile test samples were cast in situ, cooled at 100 K min−1to test temperatures in the range 750–1000°C, and strained to failure at a strain rate of 2×10−3s−1. Both S and Nb caused the hot ductility trough to deepen and widen, an addition of 0·03%Nb giving a deterioration in ductility similar to that obtained by increasing the S content from 0·002 to 0·016%. Grain size was coarse and similar for all steels; thus, the changes in ductility could be related to changes in precipitate and inclusion particle distribution. The addition of Nb resulted in the presence of a coarse Nb(C,N) eutectic and a fine matrix and boundary precipitation of Nb(C,N). The addition of S resulted in the precipitation of coarse sulphides at the interdendritic boundaries, as well as the presence of a fine precipitation of Fe oxysulphides similar in size and distribution to the Nb(C,N) precipitation. The apparent similarities in precipitation patterns for S and Nb suggest similar mechanisms for intergranular failure. The coarser grain boundary precipitates probably encourage grain boundary sliding and void cavitation at the boundaries. The fine matrix precipitation, by increasing the strength of the matrix, concentrates the strain into the boundary region, further reducing ductility. Because precipitation patterns at the surface of continuously cast slabs and in tensile samples are different, the experimental results cannot be transferred directly to the continuous casting operation. However, it can be stated that reducing the S content will improve hot ductility and reduce transverse cracking.MST/1429

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1101

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effects of thermomechanical treatments on the reverse transformation behaviour from twinned plate martensite to austenite in Fe–31·1%Ni have been studied. The variation of both diffusion controlled and diffusionless reverse transformation with temperature and time was examined. Diffusional reversion was dominant at lower reheating temperatures and led to a fine martensite–austenite duplex microstructure with a grain size of 0·01–0·1μm, which caused a remarkable hardening∆Hvof 170–230 HV during aging. Cold working of the martensite promoted diffusional reversion and enhanced age hardening. X-ray analysis indicates that the age hardening is caused mainly by elastic strain resulting from coherent precipitation of austenite in martensite.MST/1414

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1108

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractIn Part 1 of this paper the microstructures generated in the Pb–Cu system at copper contents less than the eutectic content of 0·06 wt-% were studied for sheet produced both by the conventional method of rolling from heavy cast block and by a direct casting method without rolling. In Part 2 the mechanical properties associated with these microstructures are investigated and the mechanisms by which failure occurred are considered. Similar levels of tensile strength and thermal fatigue resistance are obtained but with lower tensile strain to fracture in the cast material. Both materials are considered satisfactory for use in flashing and roofing applications.MST/1465

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1116

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effects of preoxidation as a potential protective measure for alloys exposed to erosion–corrosion have been evaluated for Incoloy 800H in laboratory simulated fluidised bed conditions. The performance of the specimens after exposure to such environments was estimated from weight change data, electron microscopy, and X-ray diffraction patterns. The results showed that, in‘corrosion dominated’conditions, i.e. at relatively high temperatures and low velocities, preoxidation was successful in reducing the erosion–corrosion of the underlying alloy. However, when the velocity was increased and the temperature decreased, preoxidation afforded only some short term protection to the alloy. In these more‘erosion–corrosion dominated’regimes, preoxidation delayed the incubation period for erosion–corrosion of the alloy, but, once the scale was removed, the erosion–corrosion rates were similar to those of the non-preoxidised alloys. The effects of temperature, velocity, and exposure time are discussed for the preoxidised and non-preoxidised alloys. Comparisons are made between the results of the present and other erosion–corrosion studies, to explain the distinctive pattern of alloy wastage in these environments.MST/1432

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1128

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractApplication of coatings by plasma vapour deposition involving electron beam evaporation and ion plating onto green powder metal compacts has been studied as a potential method for encapsulating powder metal products before hot isostatic pressing. The deposition of defect free coatings is essential if this concept is to provide a reliable encapsulation technique. Coating structures are therefore discussed in terms of the plasma processing conditions and surface roughness of the powder substrate. It is shown that the most promising approach is a combined coating and sinter–hot isostatic pressing cycle, which enables defects within the coating to be removed by the formation of a transient liquid phase.MST/1455

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1138

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn examination is reported of the microstructure of joints between nickel substrates fabricated using Ni–Si–B filler metal (AWS classification BNi-3). This investigation was undertaken because the presence of residual boride and silicide phases after joining can affect the subsequent mechanical properties of the joint. The microstructures developed in the joints are compared with those formed in similarly heat treated filler metal. Various models of the transient liquid phase process are also considered and predicted times to complete isothermal solidification are compared with experimental values. An initial examination of the influence on filler metal microstructure of a ternary addition (chromium) to the substrate is included.MST/1506

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.12.1143

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor