Materials at High Temperature Vol 18, Issue 3, 2001
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Reduction in defect content of ODS alloys
J. Ritherdon and A.R. Jones
Materials Science and Engineering, University of Liverpool, Liverpool L69 3GH, UK
The current work has continued to develop cross-comparisons between the microstructures developed in an MA ODS Fe3Al intermetallic alloy and those in PM 2000 (a fully commercial ODS FeCrAl) at all stages between fully processed MA powders and the fully consolidated heat treated alloys. Principal concerns in the study have been to establish details of the nature of the secondary recrystallisation process in the MA ODS Fe3Al alloy and the way in which this is influenced by specific microstructural features, such as oxide stringers and other microstructural inhomogeneities. From the results it is clear that the final secondary recrystallised microstructures which can develop in the ODS Fe3Al alloy are distinguished from those in PM 2000 by factors which include the detailed size and spatial distribution of the coarser oxide particles and differences in the homogeneity of MA which exist between the two types of fully processed powders.
Keywords: defect content of ODS alloys
Evaluation of iron-aluminide CVD coatings for high temperature corrosion protection
B. A. Pint,Y. Zhang†, P. F.Tortorelli, J. A. Haynes and I. G.Wright
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6156, USA
†Center for Manufacturing Research, Tennessee Technological University, Cookeville, TN 38505-0001, USA
Chemical vapor deposited (CVD) Fe-Al coatings are being investigated to address fundamental issues concerning aluminide coating performance and lifetime. By using a well-controlled laboratory CVD procedure, the coatings are uniform in composition, purity and microstructure. A typical ferritic steel, Fe-9Cr-1Mo, and an austenitic stainless steel, 304L (nominally Fe-18Cr-9Ni), were coated to examine differences in the two types of substrates. For both substrates, the as-deposited coating consisted of a thin (<5 _m), Al-rich layer above a thicker (30–50 _m), lower Al content layer. To follow-up on initial results, which showed good coating performance in air+10vol.%H2O and H2S-H2O-H2-Ar, cyclic tests were performed in both environments at 800°C and more detailed characterization of the isothermally exposed coatings was conducted. During 2–5, 25h cycles at 800°C in H2S-H2O-H2-Ar, CVD coatings on both substrates showed progressively more attack during each cycle. However, in 1h cycles at 800°C in air + 10vol.%H2O, the coatings showed excellent performance, similar to cast Fe-(15–20at.%)Al specimens. The uncoated alloys were significantly attacked during all of these tests. Thermal expansion measurements show Al additions up to 20at% have little effect on the mean expansion of ferritic alloys but the higher thermal expansion of austenitic steels may be a better match with Fe3Al coatings.
Keywords: chemical vapor deposited Fe-Al coatings
Cyclic stress–strain behaviour of circumferentially notched cylindrical bars at high temperature
R. P. Skelton1, S.T.Wee2 and G. A.Webster1
1Department of Mechanical Engineering, Imperial College, Exhibition Road, London SW7 2BX, UK
2School of Artillery, 310 Sembawang Road, 01-01 Khatib Camp, Singapore 758542
Cyclic stress–strain tests were undertaken at temperatures between 550°C and 820°C on plain and doubly-notched specimens of three alloy steels: 20Cr/25Ni/Nb, 316 and 1CrMoV respectively. Integrated axial ‘strains’ were measured between the minimum sections of the (semi-circular) notches using a longitudinal extensometer, while surface hoop strains were measured on one notch by means of a diametral extensometer. Cyclic hardening occurred in plain specimens of 316 steel, cyclic softening occurred in the 1CrMoV steel and the 20Cr/25Ni/Nb alloy showed stable behaviour. These effects were also demonstrated in notched specimens, although to a lesser degree. From hysteresis loops determined during multiple step tests, the cyclic deformation response of the notched regions was expressed in terms of an ‘equivalent gauge length’. Comparison was also made between the equivalent (von Mises) stress–strain curves deduced at a representative (skeletal) point in the minimum section with stress–strain data obtained from uniaxial (plain) specimens. The equivalent curves calculated from standard relations were found to be lower than the uniaxial curves and possible reasons for this are suggested.
Keywords: axial/diametral strain, cyclic hardening/softening, effective Poisson’s ratio, equivalent gauge length, equivalent stress–strain, failure energy, low cycle fatigue, skeletal point
Simulating the cyclic mechanical response of titanium alloy 834 at 630°C
D.J. Smith
Department of Mechanical Engineering, University of Bristol, Bristol, BS8 1TR
This paper explores features of observed material response under cyclic loading of a creep resistant titanium alloy, IMI 834. Methods of describing the material response are examined, particularly in the context of material models available in the ABAQUS finite element (FE) code. Isotropic, kinematic and combined hardening models are used to describe material behaviour under cyclic loading. Results from an earlier test programme are examined in detail, and the results are used to identify material constants. The models are implemented external to the FE code, and used to simulate the material response. Results from simulations are then compared with experimental results. It is found that the current ABAQUS models are not sufficiently detailed to provide an accurate representation of material behaviour, and additional parameters are required. The simulations overestimate the decrease the peak stress as a function of the number of cycles and underestimate the cyclic plastic strain range.
Keywords: cyclic loading, material models, low cycle fatigue.
The effect of initial hot plastic deformation on creep behaviour of 12 Cr steel specimens in small punch tests
B. Ule1 and T.S˘ us˘tar2
1Institute of Metals and Technology, Ljubljana, Slovenia
2C3M, Centre for Computational Continuum Mechanics Ljubljana, Slovenia
The results of small punch creep tests carried out on flat and on sombrero-shaped disk test specimens made of high-alloy 12 Cr steel have shown that the time to rupture of the flat disk test specimens, which had been subject to rapid and large initial plastic deformation, was one order of magnitude shorter than that of the sombrero-shaped disk specimens whose contours had been shaped before the test to fit the shape of the punch. In the case of the flat disk test specimens, the strain energy density was increased by the initial hot plastic deformation, this probably accelerated the dynamic recovery and consequently reduced the time to rupture. However, the measured activation energy and the power to which the load is raised are practically the same at small punch creep tests with flat disk test specimens as well as at conventional constant load creep tests.
Keywords: Small punch creep test, geometry of specimens, 12 Cr steel, time to rupture
Investigation of the double shear creep specimens to study the creep behaviour of short fibre metal matrix composites
Z.F.Yue
Department of Applied Mechanics, Northwestern Polytechnical University, 710072 Xian, PR China
Institute of Materials, Ruhr University, D-44801 Bochum, Germany
Present Address: Engineering Department, University of Cambridge, Cambridge, CB2 1PZ, UK
E-mail: zy204@eng.cam.ac.uk
Double shear creep specimens have been investigated by the transverse anisotropic finite element method, in order to be applied to study the multiaxial creep behaviour of short fibre-reinforced metal matrix composites (MMCs). The creep behaviour was dependent on orientations of the specimens and the transverse anisotropic parameters. The detailed evolution of the stress and strain in the double shear creep specimens shows that there is a steady stress state in the shear zone. The shear stress is near to the applied average shear stress, and independent of orientations of the specimens and the transverse anisotropicic parameters. The double shear specimen can provide a simple multiaxial stress state for MMCs. A procedure has been presented to evolve the parameters for the transversely anisotropic MMC materials, so that the double shear specimens can be tested to obtain the transversely anisotropic material properties, some of which are difficult to get by the usual tensile specimens.
Keywords: double shear creep specimens, metal matrix composites, transversely anisotropic finite element method, stress and strain distribution