J. Čadek

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Abstract Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.

An analysis of a set of creep data for a 9Cr1Mo0.2V (P91 type) steel

Abstract Recently published creep data for a 9Cr 1Mo 0.2V steel are analysed and an attempt is made to interpret them applying the concept of thermally activated detachment of dislocations from carbide particles as the rate controlling process. For the data, very high and stress dependent apparent activation energy of creep, Q c , and very high and temperature dependent apparent stress exponent, m c , are characteristic. The modelling of creep behavior applying the above mentioned concept is shown to fail to account for this temperature and applied stress dependence of minimum creep strain rate and/or the values of Q c and m c following from the data analysis. It is suggested that the behavior of the dislocation substructure, which is affected by the presence of carbide particles, must be introduced into any model to adequately describe the unusual creep behavior of the steel of interest.

Notch effects on creep behaviour of CMSX-4 superalloy single crystals

Abstract Smooth and notched specimens of CMSX-4 superalloy single crystals of orientations 〈001〉, 〈011〉 and 〈111〉 were creep tested at 850°C. For the same net-section stress, the notched specimens exhibited a longer lifetime than the smooth specimens. Stress–strain analysis of notched specimens was carried out by an elastic–plastic finite elements procedure; the creep data for smooth specimens were used as input data. An excellent correlation was found between the lifetime of the notched specimens and the average value of the calculated steady-state creep strain rate. For all orientations the life curves of this type were found to be identical to the life curves for smooth specimens expressed in terms of lifetime versus steady-state creep rate. Thus a modified Monkman–Grant relationship is valid both for smooth and notched specimens.

Is creep in discontinuous metal matrix composites lattice diffusion controlled

Abstract Creep data for two discontinuous metal matrix composites, namely 3OSiC p -Al particulate and 26(Al 2 O 3 ) f -Al-5Mg short-fibre composites, were analysed to establish whether their creep is matrix lattice diffusion controlled. It was found that ϵ m 1 n ( ϵ m is the minimum creep rate) varies with applied stress σ linearly when the true stress exponent n is set equal to 5. For both composites, the threshold stress σ TH was found to decrease rather strongly with increasing temperature. Good correlation of ϵ m /D L with ( σ - σ TH )/ G D L is the coefficient of lattice diffusion and G is the shear modulus) for both composites strongly suggests matrix lattice diffusion as the creep-rate-controlling process. However, the origin of threshold stress has not been identified and thus its temperature dependence has not been accounted for. Knowing σ TH as a function of temperature and applying a proper consititutive equation of creep, the apparent activation energy of creep Q c and the stress sensitivity parameter of creep rate m were calculated as functions of applied stress and temperature. These Q c calc and m calc were then compared with Q c exp and m exp following from ϵ = ϵ m (σ, T) relations as determined experimentally. While for the 3OSiC p -Al composite the agreement was found to be good, this was not the case for the 26(Al 2 O 3 ) f -Al-5Mg composite.

High temperature creep behaviour of an Al-8.5Fe-1.3V-1.7Si alloy reinforced with silicon carbide particulates

Abstract The creep behaviour of an Al-8.5Fe-1.3V-1.7Si (the 8009Al type, all numbers indicate wt.%) alloy reinforced with 15 vol.% silicon carbide particulates — the Al-8.5Fe-1.3V-1.7Si-15SiCp composite — is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behaviour is observed to be associated with the true threshold stress that decreases more strongly with increasing temperature than the shear modulus of aluminium. The minimum creep strain rate is controlled by the lattice diffusion in the composite matrix, and the true stress exponent is close to 5. The results are compared with those obtained investigating the creep behaviour of an unreinforced Al-8.5Fe-1.3V-1.7Si alloy in the same temperature range. The creep strength of the composite as characterised by the minimum creep strain rate is found to be up to six orders of magnitude higher in the composite than in the alloy. This creep strengthening is attributed to a much higher true threshold stress in the composite than in the alloy, which is primarily due to finely dispersed alumina particles appearing in the composite matrix as a result of composite fabrication. The creep behaviour is interpreted in terms of athermal detachment of dislocations from interacting particles admitting a temperature dependence of the relaxation factor that characterises the strength of dislocation/particle interaction.

Creep behaviour of an oxide dispersion strengthened Al-5mg alloy reinforced by silicon carbide particulates : an oxide dispersion strengthened Al-5Mg-30SiCp composite

Abstract The creep behaviour of oxide dispersion strengthened (ODS) Al–5%Mg alloy reinforced by 30vol.% silicon carbide particulates – an ODS Al–5Mg–30SiC p composite — is investigated at three temperatures ranging from 623 to 723 K. The creep is associated with a true threshold stress depending on temperature more strongly than the shear modulus of the matrix solid solution. At applied stresses only slightly higher than the true threshold stresses, the apparent stress exponent m c reaches values as high as ∼85 and the apparent activation energy Q c values as high as ∼2000 kJ mol −1 at 673 K. However, the true stress exponent is close to five and the minimum creep strain rate is controlled by the lattice diffusion in the composite matrix. The difference between the apparent and the true stress exponents as well as between the apparent and the true activation energies of creep is attributed to the strong temperature dependence of the threshold stress. The origin of the true threshold stress and its temperature dependence is discussed. Alloying the aluminium matrix with 5% Mg is shown to have only a slight effect on the creep strength of the ODS Al–30SiC p composite.

Threshold creep behaviour of aluminium dispersion strengthened by fine alumina particles

Abstract Results of an investigation of creep in aluminium strengthened by 2.6 vol.% fine alumina particles—ODS–2.6Al alloy—are presented with an emphasis on threshold creep behaviour. At temperatures ranging from 623 to 723 K the measured minimum creep strain rates covered six orders of magnitude. The true threshold stress σ TH decreases with increasing temperature approximately linearly and the same holds for σ TH / G ratio, where G is the shear modulus of the matrix metal—aluminium. The threshold stress is suggested to originate from attractive dislocation/particle interaction although the detachment stress σ d scales the temperature dependence of the shear modulus. The minimum creep strain rate ϵ m is controlled by matrix lattice diffusion and the true stress exponent n of the minimum creep strain rate is close to 5. The results for ODS–2.6Al are compared with similar ones for ODS–Al–30SiC p composite. From the comparison it follows that the load transfer effect is absent in this latter composite with heavily dispersion strengthened matrix and that the threshold stress effect plays the dominant role, similar to that in ODS–2.6Al alloy.

Creep behaviour at high stresses of a Mg-Zn-Ca-Ce-La alloy processed by rapid solidification

Abstract In the present paper it was shown that under the above conditions of temperature and applied stress given, the creep strain rate is controlled by the matrix lattice diffusion and is not associated with a threshold stress. The true stress exponent n of the creep strain rate was found close to 7. These findings were compared to those obtained [1] for the same temperature interval and applied stresses lower than about 85 MPa. Under these “lower stress” conditions the creep is dislocation glide controlled (Peierls or quasi-Peierls mechanism) and is associated with temperature dependent threshold stress. The true stress exponent n is much lower than that for the “higher stress” region and decreases with increasing temperature. The differences in the creep behaviour of the Mg-alloy B in the high stress region and low stress region are discussed. The absence of threshold stress in the high stress region has not been accounted for finally. Some important creep characteristics (ΔHC, n, σTH) are summarized in the form of a “diagram of creep” of the alloy. Also, the creep behaviour of the alloy investigated is compared with that of pure magnesium.

Creep behaviour of ODS aluminium reinforced by silicon carbide particulates : ODS Al-30SiCp composite

Abstract Results of an investigation of creep behaviour in ODS aluminium reinforced by silicon carbide particulates—an ODS Al–30SiC p composite—are reported. The minimum tensile creep strain rates were measured at temperatures of 623, 673 and 723 K; applied stresses ranged from 2.77×10 −3 to 7.74×10 −3 G, where G is the shear modulus of aluminium. The creep in the composite is associated with a relatively high true threshold stress which decreases with increasing temperature more strongly than the shear modulus. The true threshold stress is suggested to originate predominantly from an attractive dislocation–fine alumina particle interaction; the presence of SiC particulates does not seem to contribute to it significantly. The minimum creep strain rate is matrix lattice diffusion controlled and the true stress exponent of this strain rate is close to 5. In this respect, the creep behaviour of the ODS Al–30SiC p composite is similar to that of an Al–30SiC p composite. Depending on the conditions of applied stress and temperature, the minimum creep strain rate in the ODS Al–30SiC p composite is up to 8 orders of magnitude lower than that in the Al–30SiC p composite. This effect of strengthening of aluminium matrix by fine alumina particles is largely, but not entirely, due to the higher threshold stress in the ODS Al–30SiC p composite. Some other possible contributions to this effect are discussed, but the load transfer is not considered to play any significant role in the heavily alumina particle strengthened Al–30SiC p composite.

Disappearance of the true threshold creep behaviour of an ODS Al–30SiCp composite at high temperatures

Abstract The creep behaviour of an oxide dispersion strengthened (ODS) Al–30SiC p composite has been investigated in a broad temperature interval ranging from 623 to 798 K. At temperatures up to 723 K true threshold creep behaviour occurs, while at temperatures above 723 K no true threshold stress has been found. Accepting the creep model of Rosler and Arzt (Acta Met. Mater. A150 (1992) 21), the creep behaviour of this composite at temperatures ranging from 748 to 798 K is interpreted in terms of the thermally activated detachment of dislocations from small alumina particles in the composite matrix. The disappearance of the true threshold stress at temperatures above 723 K is then due to a transition from athermal to thermally activated detachment of dislocations from alumina particles.