摘要:

AbstractRecently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of≥1400 MN m−2but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.MST/1413

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.481

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe microstructure and mechanical properties of hot pressed Al2O3–ZrO2ceramics prepared from ultrafine powders were studied by means of hardness and fracture toughness tests, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Experimental results show that Al2O3–ZrO2ceramics combine the high modulus of Al2O3and phase transformation toughening of ZrO2and give good mechanical properties. The fracture toughness of the samples increases monotonically with increasing ZrO2content. When the content of ZrO2is low, the ZrO2particles are surrounded by Al2O3grains and the matrix is thus strengthened, but when the content of ZrO2is high, the microcracks resulting from the tetragonal monoclinic phase transformation decrease the strength of the material. The relative density of the samples also increases with increasing ZrO2content, which is beneficial to both strength and toughness. Banded abnormal growth of the Al2O3grains parallel to the hot pressing plane was found in the samples. This phenomenon is considered to be a result of the priority of Al2O3grain growth and aggregation of ZrO2along the pressing direction in the later stages of hot pressing.MST/1359

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.490

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractIt has been found that the reaction rate between alumina and carbon in nitrogen can be considerably increased by using Al(OH)3–C mixtures instead ofαAl2O3–C mixtures. The C/Al(OH)3ratio in the starting mixtures should be greater than the stoichiometric ratio to complete the reaction. It was observed that alumina is reduced and nitrided to form aluminium nitride via spinel type aluminium oxynitride which is an intermediate compound. The overall reaction rate increased with decreasing Al(OH)3particle size and increasing nitrogen flowrate. It is thought that the faster reaction rates obtained from Al(OH)3–C mixtures were derivedfrom the high specific surface area of transition aluminas, such asγAl2O3, which formed during heating of the samples to the reaction temperature. The particle morphologies of the Al(OH)3powders did not change during the reduction and nitridation reaction, an important attribute to the process.MST/1346c

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.495

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractAn investigation has been made of the solid state constitution of the titanium rich portion of the Ti–Al–C system; partial isothermal sections have been established at 1250, 1050, and 750°C by means of electron microscopy (including energy dispersive X-ray analysis) and X-ray diffraction. In addition, a schematic liquidus projection has been deduced based on the solid state and as cast structures. The carbide phases present in the range studied are TiC, Ti3AlC, and Ti2AlC.MST/1305

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.505

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe 7475 Al–0·7Zr (wt-%) cold rolled alloy sheet fabricated by powder metallurgy exhibited remarkable superplasticity for strain rates between 2×10−3and 3×10−1s−1at 793 K. During the initial deformation stage, the strain rate sensitivitymincreased with increasing strain and attained a value greater than 0·5 after about 50% elongation. Thus, the microstructural evolution which occurred during this initial deformation stage was examined, especially at high strain rates. On heating to the tensile testing temperature of 793 K, the alloy developed a very fine subgrain structure, stabilised with finely dispersed Al3Zr precipitate particles. The average subgrain size was less than 1μm, and the misorientation angle of most subboundaries was less than 5°. In the initial deformation stage showing an increase inmvalue, the subboundary misorientations and, thus, the contribution of boundary sliding tended to increase with strain. Even the subboundaries with misorientation of 5–15°could play an important role through boundary sliding in the limited strain rate range showing superplasticity. Subgrains rotated as a group owing to the different sliding rates at different boundary misorientations, including those of original grain boundaries. The accommodation dislocations for the boundary sliding were absorbed into the subboundaries. Both subgrain rotation and dislocation absorption were considered to contribute greatly to the increase in subboundary misorientation.MST/1367

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.513

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractControlled amounts of sulphur were added to superferritic alloys based on a composition of Fe–40 wt-%Cr. The sulphur distribution was studied in relation to a series of isothermal annealing and aging cycles. The formation of a dispersion of CrS spheroids increases the tensile ductility and the impact resistance. Deformation studies suggest that the sulphide particles can act as local centres for plastic slip. However, overheating causes severe embrittlement by both precipitation hardening and intergranular precipitation. The results suggest that higher contents of residual sulphur can be tolerated without affecting the mechanical properties adversely, providing the correct thermomechanical treatment is followed.MST/1339

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.520

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effects of predeformation on the microstructure and mechanical properties of 1020 dual phase steel were investigated. It was found that predeformation showed a significant refinement effect on the microstructures of both ferrite and martensite. The amount of twin martensite was found to increase with increasing predeformation. Ferrite twins were found to form close to martensite/ferrite boundaries. The reasons for the formation oftheferrite twin and the increase of the martensite twin due to predeformation were analysed. Results of tensile tests have shown that both yield strength and tensile strength increase with increasing predeformation, and predeformation was also shown to have a complex influence on the uniform elongation and total elongation of this dual phase steel.MST/1181

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.527

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe transformation ofδferrite in type 316 stainless steel weld metal during creep deformation at 823, 873, and 923 K has been investigated. The extent of transformation, as measured using a Magne-gauge, has been found to be greater under the influence of applied stress. The rate of transformation follows the Johnson–Mehl equation both for stress assisted and for stress free conditions.MST/1274

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.533

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractCommercial purity titanium (IMI 125) and a Ti–Cu alloy (IMI 230) in sheet form have been deep drawn at room temperature and at elevated temperatures. Cylindrical cups drawn at room temperature and at temperatures up to about 550°C develop four ear peaks at about 45°to the rolling direction (RD). When drawing is carried out at temperatures above 600°C, two ear peaks are formed at 90 and 270°to RD. The change in anisotropy is attributed to the temperature dependence of crystallographic slip modes, the high temperature behaviour being associated with basal plane slip. Drawing with a temperature gradient (cold punch, heated die) enables high drawing ratios to be achieved.MST/1352

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.537

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractCreep curves of 0·5Cr–0·5Mo–0·25V steel have been analysed based on a simple power function of time. The long term creep properties, namely, the creep rate and the rupture lifetimes at low stress levels can be very well predicted by this simple power function. The Monkman–Grant relationship can also be obtained based on the predicted values.MST/1411

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.6.541

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor