Creep Deformation Characteristics Research Articles

Long-term creep deformation characteristics of advanced ferritic steels for USC power plants

Creep rupture and deformation behavior of T122-type steels with different matrix phases such as α′ (martensite) and α ′ + δ (martensite and delta-ferrite) at different stress levels has been studied using creep testing and a detailed FE-STEM microscopy. Long-term creep rupture strength of the dual phase steel is found to be lower than that expected from the short-term creep testing. Fine grain microstructure enhances the creep deformation of the dual phase steel at lower stress region, but is not the major factor on the degradation. Decrease in fine MX and unequal distribution of MX in the ferrite matrix are found to be the major causes of the strength degradation in the dual phase steel with the higher Cr content. It is thus concluded that the heterogeneous creep deformation is much more pronounced at lower stress level in the dual phase steel, which is due to inhomogeneous microstructure consisting of α ′ + δ phase matrix and the relevant heterogeneous distribution of fine precipitates such as MX and M 23C 6 in the δ-ferrite matrix and near the interface between δ / γ .

Creep rupture life and variation of micro-structure according to aging time and creep test methods

To investigate the effect of defects on the mechanical property of 9Cr–0.5Mo–1.8W–V–Nb (ASME-P92) alloy, creep rupture and small punch creep (miniaturized disc band) tests were performed. The precipitates and micro-cavities generated during the test periods were investigated quantitatively. To evaluate the characteristics of creep rupture, the relationship between the precipitates and growth of cavities was derived. Also the creep deformation characteristics and rupture time according to different aging times, test temperatures, stresses, and loads were investigated. The growth of micro-cavities according to the different load application methods, or different test methods was investigated by uniaxial creep and small punch creep tests.

Effects of α 2 spacing on creep deformation characteristics of hard oriented PST crystals of TiAl alloy

Hard oriented PST crystals of a Ti-48 mol%Al alloy, in which lamellar boundaries are aligned parallel or perpendicular to stress axis, were creep tested. PST crystals of different α 2 spacing were used: one has the equilibrium amount of α 2 phase and the other contains an excess amount. Activation energy for creep rate of the former is 350 kJ/mol, being close to that of self-diffusion. The other crystal having fine α 2 spacing provides lower creep rate due to its more significant strain hardening in the primary creep stage. This crystal shows substantially high activation energy of 510 kJ/mol. The superior creep resistance of the fine α 2 spacing and its high activation energy is discussed in conjunction with the characteristics and degradation of lamellar structure in the material having the fine initial α 2 spacing.

Creep deformation characteristics of tin and tin-based electronic solder alloys

Creep deformation characteristics of pure tin, and Sn-3.5Ag and Sn-5Sb electronic solder alloys, have been studied at various temperatures between ambient and 473 K (homologous temperature 0.58 to 0.85). Power-law relationships between strain rate and stress were observed at most of the temperatures. The stress exponent (n=7.6, 5.0, and 5.0) and activation energy (Q c =60.3, 60.7, and 44.7 kJ/mol) values were obtained in the case of tin, Sn-3.5Ag, and Sn-5Sb respectively. Based on n and Q c values, it is suggested that the rate controlling creep-deformation mechanism is dislocation climb controlled by lattice diffusion in pure tin and Sn-3.5Ag alloy, and viscous glide controlled by pipe diffusion in Sn-5Sb alloy. The results on Sn-3.5Ag bulk material are compared with the initial results on solder bump arrays.

導電性セラミックスと金属の接合過程におけるクリープ変形の応力緩和効果

Residual stress in an E-Sialon/SKS3 joint with a brazed metal larger of BAg8 was analyzed by using an elasto-plastic-creep FEM model. The joint was brazed at 1113K and cooled under different cooling conditions to room temperature. Before the analysis, creep deformation characteristics of SKS3 and BAg8 were examined by means of tensile test at high temperatures. It was found that creep deformation occurs in SKS3 at above 673K and BAg8 at above 473K. These creep data were adopted for the analysis of residual stress in the E-Sialon/SKS3 joint bonded under different cooling conditions. Based on the analysis, cooling process for reducing the residual stress in the joint was discussed. In a cooling rate of 10K/s – 0.001K/s, it was found that the residual stress after cooling decreases as the cooling rate decreases. This means that a stress relaxation occurs during cooling due to the creep deformation of SKS3 and BAg8, in which the creep deformation of BAg8 was estimated to be especially large. Moreover, effect of maintaining at a constant temperature in the cooling process was discussed for reducing the residual stress. It was found that the maintaining for 3600s at 523K is quite effective for reducing the residual stress in the joint cooled to room temperature.

Physically-based model for creep in nickel-base superalloy C263 both above and below the gamma solvus

The polycrystalline nickel-base superalloy (C263) is used for stationary components in aero-engines such as combustion chambers, casings, liners, exhaust ducting and bearing housings. It is a fine-precipitate strengthened alloy at 800°C, with a precipitate solvus temperature of 925°C. Below the solvus, the precipitate coarsens at elevated temperature. A critical precipitate size exists below which particle cutting is the rate-controlling creep mechanism. Above the critical size, it is dislocation pinning and climb. Once the solvus temperature has been exceeded, however, the rate-controlling mechanism becomes pinning and climb within a dislocation network. This paper presents physically-based constitutive equations for creep deformation in C263. In the presence of the γ′ precipitate, populations of climbing and gliding dislocations are assumed to exist in a similar way to that proposed by Dyson and Osgerby [3]. Climbing dislocations are assumed to be pinned at precipitates. The dependence of steady state glide dislocation flux can then be obtained as a function of precipitate volume fraction. Above the γ′ solvus, the pinning process is different and results from the establishment of a dislocation network, the size of which determines the pinning distance. The physical constants arising in the equations have been determined by the conventional minimisation of errors between experimental and calculated creep curves. They have also been determined quite independently using fundamental data and experiments. Remarkable agreement between the two sets of physical constants is achieved. The constitutive equations have been shown to capture the material’s creep deformation characteristics over a broad range of temperature, both below and above the γ′ solvus. In addition, the effect of precipitate coarsening on creep rate is correctly captured, together with the effect of prior ageing on subsequent creep rate. The damage processes of cavitation and multiplication of mobile dislocation density have been coupled with the constitutive equations and used to predict failure in constant and variable stress creep.

Creep and shrinkage behavior of improved alternate substrates for magnetic tapes

The long-term creep deformation and shrinkage characteristics of improved alternate substrates for magnetic tapes are presented. The substrates include two polyesters (PET and PEN) and aromatic polyamide (ARAMID). PET film is currently the standard substrate used for magnetic tapes and thinner tensilized-type PET, PEN and ARAMID have recently been used as alternate substrates with improved material properties. The thickness of the alternate film ranges from 6.2 /spl mu/m to 4.8 /spl mu/m. The amount of creep deformation is considerably smaller for ARAMID and tensilized-type PET than for PEN. Shrinkage of ARAMID is lower than that of PET and PEN. Based on creep and shrinkage behavior, ARAMID and tensilized-type FET seem to have satisfactory dimensional stability.

Creep deformation characteristics of discontinuously reinforced aluminium-matrix composites

Recent developments in the study of creep behaviour of discontinuously reinforced aluminium-matrix composites (DRAMCs) at elevated temperatures are reviewed in this paper. These include the shapes of the creep curves, the origin and characteristics of the threshold stress, the creep strengthening of the DRAMCs, the nature of the rate-controlling processes, the effect of cyclic stress, and creep rupture. The DRAMCs exhibit high values of apparent stress exponent and apparent activation energy for creep. Incorporation of the threshold stress into analyses reduces the high and variable values of apparent stress exponent and activation energy to those anticipated from the creep of pure metals and solid-solution alloys. This indicates that the creep in the DRAMCs is controlled by the plastic flow in the matrix materials.

Deformation characteristics of polysynthetically twinned (PST) crystals during creep at 1150 K

The creep deformation characteristics of a lamellar polysynthetically twinned (PST) crystal of the composition Ti–48 mol% Al was investigated as a function of the lamellar orientation with respect to the compression axis and the applied stress. The creep resistance of hard PST orientation with the lamellar plates parallel or perpendicular to the compression axis was substantially higher than that of soft orientations with their lamellar plates oriented at intermediate angles to the compression axis. This fact could be associated with the predominant deformation of the hard orientations by deformation modes with the slip plane inclined to the lamellar interfaces in contrast to the predominant deformation of the soft orientations by deformation modes with the slip plane parallel to the lamellar plates. In the soft orientations, mainly straight ordinary dislocations with the Burgers vector b=1/2[1-10] aligned parallel to the lamellar interfaces were encountered in domains with a high resolved shear stress as well as in domains with no resolved shear stress. In the hard orientations, ordinary dislocations b=1/2[110] and superdislocations with a Burgers vector of the type b=1/2〈112] were observed. Despite a high resolved shear stress in certain oriented domains, superdislocations of the type b=〈101] were not found to play a considerable role during creep deformation under the investigated conditions. Cross twinning contributed to the deformation in favourably oriented variants, but twinning parallel to the lamellar interfaces was much more pronounced in the soft orientations leading to a substantial lamellar refinement. This microstructural hardening during creep results in the observed stress exponents of the soft orientations near unity at high stresses. A change in stress exponent from near unity at high stresses to about 9 at low stresses occurs at about 200 MPa. A critical stress of about 200 MPa for parallel twinning is proposed as a reason for the change in stress exponent.

Change of steels and alloys structure and properties during long operation at high temperature

When defining the stress-strain state (SSS) and evaluating safety factors for turbomashinery components operating at high temperatures under creep conditions, deformation and fracture characteristics obtained during tests of as-heat-treated materials are usually used. At the same time, if the change in material properties during long - term service is taken into account, then the SSS of companents and, consequently, the real safety factors can significantly differ from design values which have been calculated without accounting for these changes. In this report the changes in material structure and deformation characteristics, stress-rupture and creep-rupture properties, and thermal fatigue for a number of materials of different types (perlitic and austenitic steel, nickel-base alloys) are analyzed based on the results of long-standing studies were performed for the materials both in the original state and after long-term aging (to 10000 hour and in some cases up to 50.0000 hour and up) at high temperature (400-900 °C). The greatest changes in properties were found for 20Ch23N18 austenitic steel and EP99 nickel-base alloy (22%Cr) after long-term aging, resulting in topologically close-packed o- and μ-phases precipitations. Thus, EP99 alloy plasticity is decreased to about zero at 20 °C, number of cycles to failure is hundred times reduced thermal fatigue tests. The rate of creep for 20Ch23N18 steel 20X23H18 is 150 times increased.

Characteristics of creep deformation in discontinuously reinforced metal matrix composites

Recent developments in the creep characteristics of metal matrix composites (MMCs) at elevated temperatures are reviewed. Creep in MMCs is controlled by plastic flow in the matrix materials so that both the matrix alloys and the composites exhibit similar creep properties. When MMCs and their matrix alloys are fabricated using identical powder metallurgy procedures, both sets of material exhibit an increasing apparent stress exponent with decreasing applied stress which is indicative of the presence of a threshold stress. These threshold stresses arise from the presence of fine incoherent oxide particles introduced during atomisation when using powder metallurgy techniques or they may arise from the presence of precipitates in the crystalline lattice when using ingot metallurgy procedures. An additional creep resistance may be introduced in the MMCs by the occurrence of load transfer in which part of the external load is transferred to the reinforcement.

Comparative study of the estimation of creep crack growth behaviour of TiAI by using a precrack and a notch CT specimens

The fatigue crack growth behaviour of Ti-48.2%A1 intermetallics (full lamellar structure) strongly depends on the microstructure, that is lamellar alignment, and it grows along the lamellar direction. Therefore, it is difficult to introduce a straight fatigue precrack in a CT specimen to provide a standardized specimen for the estimation of the crack growth rate. Concerning high temperature creep brittle materials, since notch opening displacement at the time of creep crack initiation is small, a sharp notched specimen can keep similar stress singularity as a fatigue precracked specimen. In this paper, comparative research on the characteristics of creep deformation and crack extension was performed by using a fatigue precracked and a sharp notched CT specimens. Based on this result, a mechanical model was constructed and finite element stress analyses on the creep deformation for both specimens were performed. By these analytical results, the different behaviour of the characteristics of creep deformation and crack extension between a precrack and a sharp notch specimens were verified mechanically. Furthermore, the validity of using a sharped notched CT specimen to estimate creep crack growth rate was assured for creep brittle materials.

Creep crack growth in low alloy steel weldments

Developments in the determination and analytical representation of creep crack growth property data during the past 30 years are reviewed. The testing and data analysis of weldments involve additional complexities, and these are appraised with respect to low alloy steel weldments. Creep crack initiation and growth properties are dependent on creep deformation and rupture ductility characteristics. Consideration is given to the relationship between these properties using data determined for a ½Cr½ Mo¼V/2½CrMo pipe joint.

The role of grain boundary sliding and reinforcement morphology on the creep deformation behaviour of discontinuously reinforced composites

In this study, the role of grain boundary sliding behaviour on the creep deformation characteristics of discontinuously reinforced composites is investigated numerically together with the other influencing parameters: reinforcement aspect ratio, grain size and interfacial behaviour between the reinforcement and the matrix. The results obtained for the composites are compared with results obtained for a polycrystalline matrix material having identical grain size and morphology. The results indicate that, with sliding grain boundaries, the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behaviour between the reinforcement and the matrix. In the composites, the contribution of the grain boundary sliding to overall steady state creep rates occurs in a larger stress range in comparison to the matrix material. Experimentally observed higher creep exponent values or stress dependent creep exponent values for the composites could not be explained solely by the mechanism of grain boundary sliding. However, experimentally observed large scale triple point grain boundary cavitation in the composites could occur due to large grain rotations resulting from grain boundary sliding.

Creep of Cr 2Hf + Cr in situ intermetallic composites

Intermetallics are attractive for high temperature applications due to their high melting temperature and high temperature strength. One of the principal requirements for high temperature use is the creep strength. At present very little information on creep is available in intermetallics and it is even rare in the case of two phase intermetallic composites indicated above. In previous studies, the effects of microstructure and heat treatment on the mechanical behavior as a function of temperature, including flexure strength, ductility and fracture toughness of Cr{sub 2}Hf + Cr composites were investigated. The objective of the present study is to investigate the creep deformation characteristics of Cr{sub 2}Hf + cr composite. Creep data for one hypoeutectic microstructure and for the eutectic microstructure in this composite system were determined at 1,200 C and are compared with that of pure Cr. Estimates of composite creep behavior, made using known models, are also presented.

An analysis of creep deformation in nanocrystalline materials

The creep deformation characteristics of crystalline materials with grain sizes of > 1 pm have been studied for over four decades, from both the experimental and theoretical viewpoints. Recently there has been considerable interest in the development of nanocrystalline materials with grain sizes of $< 0.1 \mu m$ in which there is a significant increase in the volume fraction of a material occupied by grain boundaries and other intercrystalline defects such as triple lines and quadruple junctions [l-3].

Mechanisms and modelling of creep in superalloys

The characteristics of creep deformation of nickel-base superalloys are reviewed and the implications for the micromechanisms controlling the behaviour are considered. The development of a model of the creep deformation that is compatible with the physical mechanisms is traced, first in an isotropic form and later incorporating full crystallographic anisotropy. The validity of the model and its ability to be extrapolated to more complex loading conditions are evaluated against a wide range of experimental measurements.

Creep characteristics of wire-drawn Cu-20%Nb

In this study the creep deformation and rupture characteristics of a wire-drawn Cu-20%Nb composite are investigated at 500°C and compared with the results obtained from pure copper. The results show that the creep rupture strength of Cu-20%Nb is considerably higher than that for pure copper. The rupture strength of Cu-20%Nb increases up to a certain draw ratio, in contrast to the room temperature strengthening. The stress exponents n are in the range 6.6–8.1 for Cu-20%Nb, in comparison with 2.75 in pure copper. The increase in the creep strength and the higher stress exponent values in the Cu-20%Nb are associated with the reduction in the power law creep damage mechanism. This is due to the constraint introduced on the matrix creep flow by the niobium phase rather than the development of high threshold stress values. While the increase in the length of niobium filaments and reduction in interfilament spacing with increasing draw ratio increase the constraint on the creep flow of the matrix, they also enhance the creep damage caused by the diffusion mechanisms because of the easy diffusion paths along the niobium filaments and the reduction in the matrix gain size.

Characteristics of creep deformation in ceramics

The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.MST/1389

Characteristics of creep deformation in ceramics

The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.MST/1389