High Temperature Creep Behavior Research Articles

High-temperature creep of a bi-directional, continuous-SiC-fiber-reinforced glass-ceramic composite

The ‘off-axis’, high-temperature compression creep behavior of bidirectionally (2D, 0/90°) reinforced CAS–II/SiC (Nicalon® fiber) composites was studied experimentally in the stress–temperature regime of 1275–1325°C and 15–50 MPa. The results indicated that the overall, high- T rheologic response of the 2D composites was intermediate to the properties of 1D composites with fiber orientations corresponding to the constituent plies in the 2D material. This behavior strongly suggested that the 2D material behaved as an isostrain laminate during creep. A simple analysis, treating the 2D material as a three-phase laminate, where the constituent plies were assigned the viscoelastic properties of the corresponding 1D materials and separated by thin layers of unreinforced matrix, fit the experimental data. In the case of 2D composites with the plies misoriented at 20 and 70° to the applied stress (20/–70° composites), however, microstructural study suggested that growth of cracks in directions perpendicular to the applied stress due to the Poisson effect would have made a significant contribution to the bulk strain. Hence, such crack growth acts as a limitation to the universal applicability of the laminate model.

High temperature mechanical behavior of polycrystalline alumina from mixed nanometer and micrometer powders

Sintered aluminum oxide materials were formed using commercial methods from mechanically mixed powders of nano- and micrometer alumina. The powders were consolidated at 1500 and 1600°C with 3.2 and 7.2 ksi applied stress in argon. The conventional micrometer sized powders failed to consolidate. While 100% nanometer-sized alumina and its mixture with the micrometer powders achieved >99% density. Preliminary high temperature creep behavior indicates no super-plastic strains. However high strains (>0.65%) were generated in the nanometer powder, due to cracks and linked voids initiated by cavitation.

Creep properties of Ni3(AlTiTa) γ′ phase single crystals

In this investigation, we determine the anisotropy of the medium- and high-temperature creep behavior of single crystals of the γ′ phase in the stationary creep regime. Influence of orientation on activation energy for intermediate temperature creep (1123 K) and stress exponents at intermediate and high temperatures (1253 and 1423 K) could not be detected. On the other hand, absolute strain rates show a variation by a factor of up to 3, with the highest strain rate for [001] and lowest for [111] oriented crystals.

High temperature creep behavior of in-situ TiB 2 particulate reinforced copper-based composite

The tensile creep behavior of in-situ TiB 2 particulate reinforced copper-based (TiB 2/Cu) composite and unreinforced monolithic Cu fabricated by the powder metallurgy process was investigated at 773–873 K. The creep rate of monolithic Cu exhibited an exponential dependence on the applied stress, whereas the power-law relationship prevailed for the experimental data of the TiB 2/Cu composite. The composite exhibited a stress exponent of 25.5, 20.6 and 22.1 at 773, 823 and 873 K, respectively. Furthermore, the unreinforced Cu and in-situ TiB 2/Cu composite exhibited a creep activation energy of 187 and 444 kJ/mol, respectively. It is indicated, by incorporating a threshold stress into the analysis, that for the in-situ TiB 2/Cu composite, the true stress exponent was equal to 4.8 and the true activation energy was close to that for self-diffusion of copper, indicating that the creep of the composite is controlled by the lattice diffusion.

High temperature creep behavior of in-situ TiC particulate reinforced Fe–Cr–Ni matrix composite

The high temperature creep properties of the composites containing 5, 10, 16 vol% TiC particulate were investigated in the temperature range 973–1223 K and the stress range 40–160 MPa. The composites exhibit pronounced decrease of the creep strain rate and increase of the apparent activation energy and the threshold stress with increasing the volume fraction of TiC particulate. The creep life of the composites containing 5 and 10 vol% TiC particulate is longer than that of matrix alloy. By considering the threshold stress, all creep strain rates of the composites can be rationalized by a power-law with a stress exponent of 5. Transmission electron microscopy examination of the deformation microstructure reveals the local climb mechanism of dislocations acts as the predominant deformation mode in the composites. The dislocations are generally pinned by TiC particles and are bent to a high curvature on climbing over the particles. Thus a significant amount of new dislocation lines has to be created, leading to an increase of the threshold stress.

Creep behavior of an oxide dispersion strengthened Ni 3Al ordered intermetallic material

An alternative description of the high temperature creep behavior of oxide dispersion strengthened ordered Ni 3A1 material, to that recently given by Klotz et al., is presented in this paper. This material shows much higher values of apparent stress exponent and apparent activation energy than those of the oxide dispersion free material. Analysis of these data following the Arzt detachment model yield to good model predictions only at high temperature, whereas at low temperature the predictions deviate from the experimental data. In agreement with Klotz et al., a threshold stress approach is not appropriate to describe the creep behavior of the investigated material and it is concluded that the creep behavior is controlled by the interaction between dislocations and oxide particles. However, this interaction is best described by a temperature independent stress exponent, ñ. The ñ model, which considers that this interaction raises the oxide dispersion free material stress exponent by a value of ñ, inducing higher apparent activation energy values at high temperatures, satisfactorily describes the experimental data.

On the High Temperature Creep and Relaxation Behaviour of Zr-based Bulk Metallic Glasses

AbstractCreep tests under constant load as well as constant true strain rate were carried out at near the glass transition temperatures (Tg) to study the time dependent flow behaviour of a Zr-based bulk metallic glass (BMG). The strain rate - stress relation over a wide strain rate-range (10-7 to 10-2 s-1) was established for different temperatures. The high temperature deformation behaviour is explained on the basis of stress induced creation of free volume versus diffusion controlled annihilation processes. It was found that the creep kinetics near Tg is controlled by the mobility of atoms with an activation energy value Q =410kJ/mol.

Creep behavior of W-4Re-0.32HfC and its comparison with some creep models

The high-temperature creep behavior of tungsten-4 wt% rhenium-0.32 wt% hafnium carbide (W-4Re-0.32HfC) was evaluated at temperatures ranging from 2200 to 2400 K and stresses ranging from 40 to 70 MPa in a vacuum less than 1.33 × 10 −6 Pa (1.0 × 10 −8 torr). The effects of temperature and stress on the secondary creep rate were determined. The stress exponent for creep was 5.2, and the activation energy for creep was 594 kJ/mol. These creep parameters were compared with predictions made by three creep models: the Ansell–Weertman, Lagneborg and Roesler–Arzt models. The results showed that none of these models accurately predicted the creep behavior of the alloy. However, the Lagneborg recovery creep model qualitatively predicted the secondary creep rate of W-4Re-0.32HfC. The stress exponent for creep that was predicted by the Ansell–Weertman model did not agree with the values obtained in this research. The secondary creep rate that was predicted by the Roesler–Arzt model was approximately five orders of magnitude different from the rate obtained in this research.

High-temperature fatigue damage mechanisms in near-α titanium alloy IMI 834

The high-temperature fatigue and creep behaviour of near-α titanium alloy IMI 834 was studied for temperatures up to 650°C. In order to identify the relevant damage mechanisms, the test program involved continuous cycling, dwell tests with hold periods in tension and compression, and asymmetrical strain-time cycles. The dominant damage mechanism was found to change at a temperature of about 600°C. At low test temperatures fatigue life is largely dependent on the maximum stress of the fatigue cycle. At high temperatures a brittle oxygen-enriched subsurface layer forms, and thus, environmental degradation governs fatigue life in high-temperature low frequency tests. Lamella boundaries present in the bimodal microstructure were seen to deflect small fatigue cracks, and thus, this microstructure has superior fatigue properties as compared to an equiaxed one. During long-term high-temperature exposure, however, significant degradation of the lamella boundaries was observed, and then both microstructures display similar fatigue properties.

Joining of yttria-tetragonal stabilized zirconia polycrystals using nanocrystals

Superplastic flow has been successfully used to join fully dense 3 mol% Y{sub 2}O{sub 3}-tetragonal ZrO{sub 2} polycrystals (Y-TZP) at temperatures as low as 1,350 C, a temperature at which direct diffusional bonding would be unlikely to produce a strong, pore-free joint. The objective of the present work was to determine whether bonding temperatures could be further reduced. To achieve lower bonding temperatures, the authors have investigated the bonding of conventional Y-TZP in which a nanocrystalline Y-TZP with a 20-nm grain size is used as the interlayer between two pieces of 0.3 {micro}m grain sizes Y-TZP. Little is known about the deformation of fully dense nanocrystalline Y-TZP, but recent work indicates that above a threshold stress, the principal deformation mechanism would be grain boundary sliding that results in superplastic flow. Questions on the deformation mechanism in the nanocrystalline Y-TZP are being addressed as part of a larger investigation of high-temperature compressive creep behavior, currently in progress.

High-temperature creep behaviour of powder-metallurgy aluminium composites reinforced with SiC particles of various sizes

Tensile creep tests have been performed at 573–673 K on pure aluminium and aluminium-based composites reinforced with SiC particles of different sizes (3.5, 10 and 20 μm) fabricated by powder-metallurgy (PM) techniques. Both the pure aluminium and the composites exhibited high values of the apparent stress exponent (14.3–26.1) and creep activation energy (253–261 kJ/mol). The results showed that the creep rate of composites reinforced with small particles (3.5 μm) was two–three orders of magnitude lower than that of pure aluminium. However, the creep resistance of composites reinforced with large SiC particles (10 and 20 μm) was essentially identical to that of pure aluminium. Various creep models were attempted to rationalize the creep data of pure aluminium and particulate composites.

Creep strength of a tungsten–rhenium–hafnium carbide alloy from 2200 to 2400 K

The high–temperature creep behavior of tungsten–4 wt.% rhenium-0.32 wt.% hafnium carbide (W-4Re-0.32HfC) was evaluated at temperatures ranging from 2200 to 2400 K and stresses ranging from 40 to 70 MPa in a vacuum less than 1.33×10 –6 MPa (1.0×10 –8 torr). The effects of temperature and stress on the steady–state creep rate were determined. The stress exponent for creep was 5.2, and the activation energy for creep was 594 kJ mol −1. The creep strength of W-4Re-0.32HfC was compared with that of pure tungsten (W) and tungsten–5 wt.% rhenium (W–5Re) at a creep rate of 10 −6 s −1 as a function of temperature. The temperature-compensated creep rate of W–4Re–0.32HfC was approximately two orders of magnitude less than that of pure W and one order of magnitude less than that of W–5Re. The creep strength of W–4Re–0.32HfC was correlated with its microstructural development during the high-temperature deformation. Transmission electron microscope (TEM) study of the creep tested samples revealed that the high creep strength of this alloy resulted from finely dispersed submicron-sized HfC particles. The strengthening effect of the HfC particles could be attributed to direct and indirect particle strengthening. Direct particle strengthening was caused by the retardation of subboundary dislocation movement by direct particle/dislocation interaction. Indirect particle strengthening was caused by the formation of subgrains, which in turn reduces the distance that dislocations can move before being immobilized at subgrain or grain boundaries. The strengthening effect of HfC particles was reduced as HfC particle diameter increased.

The high-temperature creep behaviour of 2124 aluminium alloys with and without particulate and SiC-whisker reinforcement

Tensile creep behaviour of SiC-whisker and particle reinforced 2124Al-matrix composites and unreinforced 2124Al alloy was investigated at 573-673 K. The results showed that the unreinforced 2124Al alloy exhibited apparent stress exponents of 4.7–8.5 and an apparent activation of 288 kJ/mol, while the particulate and whisker-reinforced composites exhibited apparent exponents of 9.4–17.1 and 11.5–18.2, and apparent activation energies of 482 and 533 kJ/mol, respectively. Moreover, a critical strain rate was observed in both reinforced and unreinforced 2124Al alloys, below which the creep resistance of the composites was higher than that of the unreinforced 2124Al alloy. Above this value, the 2124Al alloy was more creep resistant than the composites. An increase in the critical strain rate with increasing temperature was observed. Several established models were used to rationalize the creep data for the composites. The creep data for both particulate and whisker-reinforced composites at 623 and 673 K can be satisfactorily interpreted by the substructure-invariance model with a stress exponent of 8, whereas those at 573 K cannot be reasonably rationalized by a stress exponent of 5 or 8.

The effect of Si 3N 4 whiskers on the high-temperature creep behavior of an Al-Fe-V-Si alloy matrix composite

The creep behavior of an Al–8.5Fe–1.3V–1.7Si composite reinforced with β-Si 3N 4 whiskers has been investigated at temperatures of 773 and 823 K. The results are characterized by a high stress exponent and high apparent creep activation energy. The creep data can be interpreted on the basis of the incorporation of a threshold stress and a load-transfer coefficient into the power-law creep equation. A good correlation exists between the normalized creep rate and normalized effective stress which demonstrates that the creep behavior of both the alloy and the composite is controlled by the matrix lattice self-diffusion in aluminum.

EFFECT OF SiC VOLUME FRACTION ON CREEP BEHAVIOR OF SiCp/2124Al METAL MATRIX COMPOSITE

The high temperature creep behavior of SiCp/2124Al metal matrix composites containing 10-30 vol.% of SiC particulate reinforcement was investigated to clarify the effect of the volume fraction of SiC particles on creep deformation. The SiCp/2124Al composites were fabricated by mixing 8μm SiC particles and 20μm 2124Al powders and were followed by hot pressing at 570°C and hot extrusion at 500°C with an extrusion ratio of 25:1. The high temperature creep behavior of SiCp/2124Al composites was investigated by constant stress creep tests at 300°C. The load transfer phenomena of a spherical particle in metal matrix composite were analyzed based on the shear-lag model. The minimum creep rate of SiCp/2124Al composite decreased with increasing the volume fraction of SiC particles. The increase in the volume fraction of SiC particles reduces the effective stress for creep deformation of the Al matrix by the load transfer from the matrix to SiC particles. The minimum creep rates of SiCp/2124Al composites with different volume fractions of SiC particles were found to be similar under an identical effective stress on the matrix, which is calculated by the modified shear-lag model. It is suggested that the role of SiC particles is to increase the creep resistance by reducing the effective stress acting on the matrix in metal matrix composites.

Separated contribution of particles and matrix on the creep behavior of dispersion strengthened materials

The high temperature creep behavior of an extruded Al–8.5Fe–1.3V–1.7Si dispersion strengthened material has been studied by means of tensile tests. The fine microstructure consisted of a 27% volume fraction of dispersoids, of about 50nm in diameter, homogeneously distributed in the matrix. The microstructure remained stable and fine after testing at high temperature. High apparent stress exponents, nap, of about 25 and high apparent activation energies, Qap, of about 400 kJ/mol were observed. These high values cannot satisfactorily be explained by the various models existent in the literature. A new creep equation has been developed which is a generalization of the conventional general slip creep equation but without the use of a threshold stress. This equation correctly describes the creep behavior of this material and the high values of Qap and nap.

Análisis del comportamiento en fluencia de un material intermetálico de base Ni<sub>3</sub>Al reforzado por dispersión de óxidos, mediante el modelo <i>ñ</i> de interacción dislocación dispersoide

This research work presents an alternative description of the high temperature creep behaviour of an ordered oxide dispersion strengthened Ni3Al intermetallic material to that recently presented by Klotz et al following an Arzt and Gohring model. An apparent stress exponent, nap, of 7.8 and an apparent activation energy, Qap, of 697 kJ/mole was obtained for this material. These values are approximately double than those for the non reinforced material. The dislocation-dispersoid interaction termed model, considers that this interaction merely raises the stress exponent associated to the non reinforced matrix by a value of n. Consequently, the apparent activation energy value is simultaneously raised. The n model predicts accurately the experimental data and presents advantages for analysing the creep behaviour of the dispersion strengthened materials.

High temperature creep behaviour of single crystal superalloy

Creep behaviour of a newly developed single crystal nickel base superalloy DMSX-l (Ni-7.7Co-6.4Cr-8.4W-0.3Ti-S.S7 Al-7.6Ta-Nf-Y-Nb-Re, wt-%) with 〈001〉 orientation has been analysed and compared with the reported data ofSRR99. It was concluded that the shear stress based model developed for SRR99 is also applicable for the new alloy. Although the material constants for octahedral slip for the new alloy are not exactly the same as those of SRR99, the estimated values of initial creep rate and softening coefficients are nearly of the same order. Therefore it is concluded that in the absence of a material database for cube slip for this new alloy, those available for SRR99 could be used to predict the orientation dependence of its creep behaviour.

High temperature creep behaviour of single crystal superalloy

Creep behaviour of a newly developed single crystal nickel base superalloy DMSX-l (Ni-7.7Co-6.4Cr-8.4W-0.3Ti-S.S7 Al-7.6Ta-Nf-Y-Nb-Re, wt-%) with 〈001〉 orientation has been analysed and compared with the reported data ofSRR99. It was concluded that the shear stress based model developed for SRR99 is also applicable for the new alloy. Although the material constants for octahedral slip for the new alloy are not exactly the same as those of SRR99, the estimated values of initial creep rate and softening coefficients are nearly of the same order. Therefore it is concluded that in the absence of a material database for cube slip for this new alloy, those available for SRR99 could be used to predict the orientation dependence of its creep behaviour.

High temperature creep behavior of oxide dispersion strengthened NiAl intermetallics

The intermetallic compound NiAl is a promising candidate material for high-temperature applications, provided its creep strength can be raised by some strengthening strategy. Oxide-dispersion strengthened (ODS) NiAl has been produced by powder-metallurgical methods and its creep behavior at temperatures up to 1723 K (1450°C) has been studied. Exceptional creep properties, with high stresses and stress sensitivities when compared to dispersoid-free NiAl, are found, which confirms the effectiveness of dispersion strengthening in this material. The creep behavior is interpreted in the light of creep models. Detachment-controlled models satisfactorily describe the deformation in coarse-grained ODS-NiAl, but modifications are necessary for fine-grained material. In technical terms, ODS-NiAl recommends itself as a strong, light and oxidation-resistant high-temperature alloy for applications up to 1700 K.