摘要:

AbstractFor a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min−1through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the‘tails’to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be≦600°C to produce fine (≦0·2μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.MST/1858

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.89

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractAustenite reversion as a function of deformation processes has been investigated in an 18 wt-%Ni 350 grade maraging steel. The results reported show that the degree and type of deformation imparted to martensite influences the amount of reverted austenite obtained following aging. The validity of the X-ray diffraction technique in determining the reverted austenite content is discussed with reference to texture incorporated during cold forming. Transmission electron microscopy was carried out to study the partitioning of solute during austenite reversion.MST/1842

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.97

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe response to isothermal recrystallisation and zone recrystallisation of rods of MA 956 and MA 957 has been compared. Isothermal recrystallisation produced equiaxed millimetre sized grains in MA 957, whereas MA 956 had grains of high aspect ratio. This is attributed to an anisotropic distribution of oxide particles in MA 956. In both alloys the initial strong〈110〉fibre texture is transformed into textures lying on the〈001〉–〈111〉zone axis. A discrete zone recrystallisation interface could not be formed in MA 956 under the range of conditions investigated. This may be accounted for by an anisotropic boundary pinning force. This results in small transverse velocities and also a different zone recrystallisation texture in MA 956 from that in MA 957.MST/1860

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.102

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe transformation kinetics of ferrous weld metals has been investigated as a function of austenite grain size, alloy content, and inclusion characteristics. Two series of weld metals were studied, designated low and high B–Ti content. Aluminium and calcium additions were made to each weld series to change the inclusion characteristics. Austenite grain size variations were achieved by thermally cycling in a dilatometer to different peak temperatures. The transformation temperature and microstructural changes, and the soluble boron levels of the thermally cycled weld metals, have been quantified. The factors influencing the competing processes of grain boundary and intragranular ferrite nucleation at selected austenite grain sizes have been assessed. The results imply that different types of inclusion have different free energy barriers to ferrite nucleation. The balance of nucleation is shifted from the austenite grain boundaries to the intragranular regions and acicular ferrite formation is greatly increased above a critical austenite grain size, depending on the weld metal alloy content and inclusion type. In low B–Ti content deposits with predominantly galaxite inclusions, the critical austenite grain size is ~60μm. In high B–Ti content weld metals with extensive TiO and/or TiN on the surface of MnOSiO2or galaxite inclusions, the value is ~20μm. The work indicates that calcium bearing inclusions and MnOSiO2glassy inclusions have a high free energy barrier to ferrite nucleation. AlthoughγAl2O3inclusions may constitute an important catalyst phase for acicular ferrite formation, their presence results in a rapid increase in ferrite sideplate volume fraction. This is attributed to soluble aluminium retarding ferrite transformation during cooling to below the bainite start temperature. The growth of bainite laths then limits acicular ferrite formation.MST/1800

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.110

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe long term oxidation resistance of most high temperature alloys depends on the ability to form protective alumina or chromia surface scales during service. Scale growth and scale rehealing after spalling leads to depletion of the scale forming element, aluminium or chromium, in the bulk alloy. If the concentration of this element decreases below a critical level, oxidation of the base elements, e.g. iron, nickel, or cobalt, occurs leading to catastrophic oxidation and destruction of the metallic component. Using iron based, alumina forming, oxide dispersion strengthened alloys as an example, a model is presented which allows the calculation of the time at which the catastrophic breakaway oxidation for a given component occurs. The calculated data are presented as oxidation diagrams in which the time to breakaway is plotted as a function of component wall thickness. Experimental data for the oxidation of the oxide dispersion strengthened alloys MA 956, PM 2000 and ODM 751 in air in the temperature range 1200–1400°C showed excellent agreement with the calculated oxidation diagrams.MST/1839

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.126

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe mechanical properties and fracture mechanism of an aluminium alloy A356–20 vol.-%SiCPcast composite are presented and related to the treatments experienced during the casting procedure and to the aging evolution of the alloy. The treatments were aimed at investigating possible changes of the composite damage mechanism produced by modifying the properties of the matrix/reinforcement interface region. An A356–5%SiCPcomposite was also characterised to support the study on the conventional composite and to carry out calculations, based on the ductile fracture mechanism, concerning the interface stress acting in the material at fracture. Notwithstanding the approximation of the calculation method, the results obtained quantitatively confirmed the fractographic observations. It is suggested that there was a progressive weakening or embrittlement of the interfaces either when liquid phase reactions occurred or when the matrix alloy was overaged.MST/1864

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.132

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe initiation and propagation of hydrogen cracks in a pure iron single crystal have been investigated via notched tensile tests with simultaneous cathodic charging of hydrogen. The notch opening displacement for hydrogen crack growth when the tensile direction was parallel to [001] was much smaller than for [110]tension. Hydrogen cracks under [001] tension propagated in a straight line on the (001) plane from the notch root with only slight plastic deformation, whereas under [110]tension they propagated in a zigzag fashion with significantly larger plastic strain accompanying many slip bands. The slip band formation and the crack arrest patterns on the fracture surfaces indicate that hydrogen cracks were initiated and propagated by dislocation reactions on the slip systems in accordance with calculations for the maximum resolved shear stress distribution around the crack tip. The difference in hydrogen crack propagation between [001] and [110]oriented specimens has been explained by assuming that〈001〉Cottrell dislocations are stabilised by a high density of hydrogen atoms transported by dislocations which coalesce with each other.MST/1866

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.141

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractIt is shown via texture simulation that redundant shear deformation during rolling has an important influence on the amount of cube oriented material in cold rolled fcc metals and hence on the cube recrystallisation texture. A small amount of redundant shear enhances the development of the cube recrystallisation texture, whereas a large amount of shear suppresses it. The maximum volume fraction of cube oriented material is obtained in the deformed structure when the ratio of the shear to normal deformation is ~0·3. This corresponds to the typical condition found in laboratory rolling. It is further predicted that the density of cube oriented material is a complex function of position in the specimen thickness direction, and this function itself changes according to the rolling conditions.MST/1857

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.149

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractDeformed structures of cold rolled 70–30 brass have been studied using both textural and microstructural techniques to establish the twinning tendencies of different orientations. Extensive studies using transmission electron microscopy showed that there are significant volumes of material with near {110}〈112〉brass (B) and {110}〈001〉Goss ( G) orientations formed at intermediate to high strains (65–85% reduction). Neither twins nor shear bands are found in these grains. Two highly significant results are that non-twinned B and G volumes are associated with deformation banding and that there is a considerable volume of material spread between {123}〈634〉S and {135}〈712〉B–S. A modified deformation pathway is proposed for brass texture formation which, by including deformation banding, predicts the existence of B, S, and B–S. The failure to include S and B–S orientations in low stacking fault energy texture description is due to the low resolution of 111 pole figures.MST/1831

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.155

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe effect of additions of 0·08 and 0·14 wt-%Zr on the microstructure and mechanical properties of an Al–2·5Li–2Mg alloy has been studied. It has been found that the alloy having higher Zr content has a finer as cast microstructure as well as finer as extruded and solution treated microstructures with smaller subgrains. The alloy containing 0·14 wt-%Zr ages more rapidly and exhibits higher strength and fracture toughness, but a lower percentage elongation in the aged conditions. The effect of 2,3, and 4 wt-%Mg on the microstructure and properties of an Al–2·5Li–0·1Cu–0·14Zr alloy has also been studied. It has been found that the addition of Mg above 2 wt-% results in a deterioration in mechanical properties. The deterioration is attributed to the presence of Al2LiMg precipitates.MST/1861

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.2.162

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor