S.J. Zhu

High temperature creep deformation of Al18B4O33 whisker-reinforced 8009 Al composite

The compression creep deformation of an Al18B4O33 whisker-reinforced Al-Fe-V-Si (8009 Al alloy) composite has been investigated in the temperatures range from 573 to 723 K. The addition of 15 vol.% Al18B4O33 whiskers to the 8009 alloy decreases the creep rate by two orders of magnitude compared to the 8009 alloy. The stress and temperature dependence of the creep rate of the composite is similar to that of the 8009 alloy. The threshold stress for creep was used to analyze the experimental data for the composite. The creep rates of the composite are shown to be controlled by lattice diffusion of aluminum. The contribution of the whiskers on creep rate of the composite is interpreted by a modified load-transfer model

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 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.

High temperature creep deformation of an Al-Fe-V-Si alloy

The creep behavior of dispersion-strengthened Al-8.5Fe-1.3V-1.7Si (8009 Al) has been investigated in the temperature range of 573-723 K. The results exhibit a high apparent stress exponent (14-18) and a high apparent activation energy for creep (296 kJ mol(-1)). The creep data were analyzed by incorporating a threshold stress into the power-law creep equation, which leads to a stress exponent of 5 indicating dislocation-climb mechanism. Traditional dislocation creep theories neither lead to correct values nor explain the origin or temperature dependence of the threshold stress, which is much stronger than that attributable to the shear modulus. A good correlation between experimental and theoretical prediction using the dislocation detachment model was obtained by adjusting the structure factor in the creep equation because the dislocation density is not well known. Dislocation/particle interactions as well as dislocation networks and subgrains are frequently observed after creep tests at different conditions

Creep behavior in an Al-Fe-V-Si alloy and SiC whisker-reinforced Al-Fe-V-Si composite

High temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 15vol. pct SiCw/8009Al and that of its matrix alloy, 8009Al. Both the alloy and composite exhibit a single-slope behavior with anomalously high values of apparent stress exponent and high apparent activation energy. The presence of SiC whiskers does not remarkably influence these two kinds of dependence of creep rates but reduces the creep rates by about two orders of magnitude. Transmission electron microscopy examination of the deformation microstructure reveals the occurrence of attractive dislocation/particle interaction. The creep data were analyzed by the threshold stress approach and by the dislocation-climb theories based on attractive interaction between dislocations and dispersoids. All data can be rationalized by a power-law with a stress exponent of 5 and a creep activation energy close to that for the self-diffusion in aluminum. The threshold stress decreases linearly with increasing temperature. General climb together with the attractive but not strong interactions between the dislocations and dispersoids is suggested to be the operative deformation mechanism. The contribution of SiC whiskers to the creep strength of 8009 Al composite can be evaluated quantitatively when the shear-lag model is applied. However, the effects of whisker length and whisker orientation distributions must be considered. Two probability density functions are used for modelling the distribution of whisker length and whisker orientation.

The effect of prior aging on the creep crack growth behaviour of austenitic HK40 steel

Abstract The influence of prior aging on the microstructure, creep properties and creep crack growth rate has been studied in HK40 austenitic steel. The creep ductility and the creep strength increase on increase in the prior aging temperature or aging time. The creep rupture life is the longest in specimens aged for 3000 h at 871 °C or those aged at 800 °C for 200 h. However, the resistance to creep crack growth is optimized in specimens aged for 5000 h at 871 °C and at 871 °C for 200 h. It is suggested that the resistance to creep crack growth is related to both creep strength and creep ductility.

The effect of Si3N4 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 beta-Si3N4 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

Creep deformation behavior of SiC particulate-reinforced Al–C–O composite

Creep behaviour of 10 vol% SiC particulate-reinforced Al–C–O– composite has been investigated at the temperatures of 623 and 723 K. The addition of SiC particulates in Al–C–O alloy decreases creep rates by two to four orders of magnitude, compared with Al–C–O matrix alloy. The stress and temperature dependences of creep rates of the composite are similar to those of the Al–C–O matrix alloy. The threshold stress for creep was used to analyse the experimental data of the composite. The creep rates of the composite are concluded to be controlled by lattice diffusion of aluminium.

The effects of carbides and cavities on creep crack growth in HK40 steel

ABSTRACT Creep orack growth tests at 871°C have been performed on compact tension specimens of HK40 steel in different microstructures and cavity densities conditions. The skeleton shape carbide on grain boundaries has a higher resistance to crack growth than the blocky carbide. The cavities increase the creep crack growth rate by reducing grain boundary strength. With the increase of aging temperature, the difference of crack growth rate caused by the carbide shape and by the cavity density on grain boundaries becomes small. The secondary carbide size and distribution explicitly affect the crack growth behavior.