Creep strengthening in a discontinuous SiC-Al composite

High-temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al. Both materials were prepared by powder metallurgy techniques. The experimental data show that the creep behavior of the composite is similar to that of the alloy in regard to the high apparent stress exponent and its variation with the applied stress and the strong temperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy. Analysis of the experimental data indicates that these similarities and differences in creep behavior can be explained in terms of two independent strengthening processes that are related to (a) the existence of a temperature-dependent threshold stress for creep, τ₀, in both materials and (b) the occurrence of temperature dependent load transfer from the creeping matrix (6061 Al) to the reinforcement (SiC). This finding is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mole) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporated in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent,n, equals 5, and the true activation energy for creep in the composite,Q _c , is equal to that in the alloy. formerly Research Associate, Materials Section, Department of Mechanical and Aerospace Engineering, University of California This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Analysis of steady-state creep and creep fracture of directionally solidified eutectic γ/γ′-α alloy

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-l.6V-l.2Re (wt pct) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress isn = 7.5 ± 0.3, and the apparent activation energy for creep of the eutectic γ/γ′-α composite is determined to beQ = 517 ± 11 kJ mol₋₁. When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creepQ _c is calculated to be between 412 and 424 kJ mol₋₁ and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.     

High temperature creep of silicon carbide particulate reinforced aluminum

The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC_p-6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n, is high (n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q_a, is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC_p-6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC_p-6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC_p-6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.     

Elevated temperature creep and fracture properties of the 62Cu-35Au-3Ni braze alloy

The Cu-Au-Ni braze alloys are used for metal/ceramic brazes in electronic assemblies because of their good wetting characteristics and low vapor pressure. We have studied the tensile creep properties of annealed 62Cu-35Au-3Ni alloy over the temperature range 250 °C to 750 °C. Two power-law equations have been developed for the minimum creep rate as a function of true stress and temperature. At the highest temperatures studied (650 °C and 750 °C), the minimum creep rate is well described with a stress exponent of 3.0, which can be rationalized in the context of Class I solid solution strengthening. The inverted shape of the creep curves observed at these temperatures is also consistent with Class I alloy behavior. At lower temperatures, power-law creep is well described with a stress exponent of 7.5, and normal three-stage creep curves are observed. Intergranular creep damage, along with minimum values of strain to fracture, is most apparent at 450 °C and 550 °C. The lower stress exponent in the Class I alloy regime helps to increase the strain to fracture at higher temperatures (650 °C and 750 °C). The minimum creep rate behavior of the 62Cu-35Au-3Ni alloy is also compared with those of the 74.2Cu-25. 8Au alloy and pure Cu. This comparison indicates that the 62Cu-35Au-3Ni has considerably higher creep strength than pure Cu. This fact suggests that the 62Cu-35Au-3Ni braze alloy can be used in low mismatch metal-to-ceramic braze joints such as Mo to metallized alumina ceramic with few problems. However, careful joint design may be essential for the use of this alloy in high thermal mismatch metal-to-ceramic braze joints.     

Creep behavior of an AZ91 magnesium alloy reinforced with alumina fibers

Creep tests were conducted at elevated temperatures on an AZ91 alloy reinforced with 20 vol pct Al₂O₃ fibers. When the creep data are interpreted by incorporating a threshold stress into the analysis, it is shown that the true stress exponent, n, is ∼3 at the lower stress levels and increases to >3 at the higher stresses. The true activation energy for creep is close to the value anticipated for interdiffusion of aluminum in magnesium. This behavior is interpreted in terms of a viscous glide process with n =3 and a breakaway of the dislocations from their solute atom atmospheres at the higher stress levels. The threshold stresses in this composite appear to arise from an attractive interaction between mobile dislocations in the matrix alloy and Mg₁₇Al₁₂ precipitates. The experimental results reveal several important similarities between the creep behavior of this magnesium-based composite and the well-documented creep properties of aluminum-based composites.     

The compressive creep behavior of a Pu-1 wt pct Ga δ-stabilized alloy at elevated temperatures

Constant stress compression creep tests were performed in vacuum on a high-purity Pu-1 wt pct Ga ö-stabilized alloy over the temperature range from 252° to 382°C for stresses from 700 to 2500 psi. Although the primary creep behavior could not be correlated by established techniques, the creep rates developed after true creep strains of about 0.15 provided good agreement with the temperature and stress dependence of creep for pure metals and dilute alloys. A power stress law for steady-state creep of the alloy was found forδ/E values less than 5 x 10′4, with the stress exponent being 4.0, and it was concluded that the alloy exhibits Class I solid solution behavior. For higher stress, exponential stress dependence was observed. The true activation energy for creep was found to be 38,900 cal per mole which is in good agreement with the value for self-diffus ion of plutonium in the ô-stabilized alloy. The primary creep behavior could be divided into three types: 1) at low strain rates, the creep rate gradually increases to a nearly steady-state; 2) at intermediate strain rates, the creep rate first decreases and then increases to steady-state; and 3) at high strain rates, the creep rate decreases gradually to steady-state. It was concluded that the failure of established creep correlations for primary creep of Pu-1 wt pct Ga was the result of some temperature-dependent component of creep structure, possibly resulting from radiation damage byα particles.     

Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite

The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an Al-10 vol pct SiC_p composite has been studied. The Al-4 wt pct Mg-10 vol pct SiC_p composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol^-1. These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol^-1, at a constant stress, σ = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the Al-Mg-A1₂O_3f composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the Al-Mg-SiC_p, composite are compared with those of the Al-Mg-Al₂O_3f and 6061 Al-SiC_p.w, composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the Al-SiC_p composite.     

High-temperature deformation of 6061 Al

The creep behavior of powder metallurgy (PM) 6061 Al, which has been used as a metal matrix alloy in the development of discontinuous silicon carbide reinforced aluminum (SiCAl) composites, has been studied over six orders of magnitude of strain rate. The experimental data show that the steady-state stage of the creep curve is of short duration; that the stress dependence of creep rate is high and variable; and that the temperature dependence of creep rate is much higher than that for self-diffusion in aluminum. The above creep characteristics are different from those documented for aluminum based solid-solution alloys but are similar to those reported for discontinuous SiCAl composites and dispersion-strengthened (DS) alloys. Analysis of the experimental data shows that while the high stress dependence of creep rate in 6061 Al, like that in DS alloys, can be explained in terms of a threshold stress for creep, the strong temperature dependence of creep rate in the alloy is incompatible with the predictions of available threshold stress models and theoretical treatments proposed for DS alloys.     

Tension creep of wrought single phase γ TiAl

The minimum strain rate, tertiary creep and damage behavior of a single phase gamma (γ) TiAl alloy over the temperature range 760–900°C at initial applied stress levels ranging from 32 to 345 MPa are reported. Two regions of creep deformation are identified. These consist of a region having a stress exponent of 6 and an activation energy of 560 kJ/mol and a region having a stress exponent of 1 and an activation energy of 192 kJ/mol. These are postulated to represent dislocation and boundary diffusion dominated creep respectively. The activation energy for dislocation creep is suggested to represent the energy to generate an appreciable density of dislocations in the minimum strain rate region. In the diffusional regime the minimum strain rates at 760°C lie well below the predicted minimum strain rates when compared to the Coble creep equation. In addition, a natural transition from diffusional creep to glide dominated deformation occurs at 760°C with increasing stress level. Tertiary creep of this material is found to correlate well with a two state variable approach. The initial stage of tertiary creep is dominated by an increase in the mobile dislocation density with increasing creep strain. Tertiary creep is found to obey a power law relationship with a stress exponent of 3 and an activation energy of 304 kJ/mol and is explained by the coupling of an increasing mobile dislocation density in the early stage of tertiary with constrained cavity growth in the late stage which leads to specimen failure.     

Creep characteristics of a diecast AM50 magnesium alloy

Tensile creep tests were conducted to examine the creep behavior of a diecast AM50 magnesium alloy in the temperature range from 423 to 498 K. A normal transient creep stage is followed by a minimum creep rate stage and finally by an accelerating stage at each creep condition. The stress exponent of the minimum creep rate, n, increases from ∼5 at lower stresses to ∼10 at higher stresses at each temperature, and the value of n changes at the yield stress of the alloy. The activation energies for the creep, Q _c , are determined to be 121±10 and 162 kJ/mol, at lower and higher stresses, respectively. It is concluded that the creep of the diecast AM50 alloy is controlled by the high-temperature climb of dislocations, whereas the instantaneous plastic strain introduced by the higher stress of the creep test is assumed to cause the increased values of n and Q _c .     

Cyclic creep and anelastic relaxation analysis of an ODS superalloy

This paper documents the effect of stress and temperature on the cyclic minimum strain rate at two different loading frequencies for the oxide dispersion strengthened (ODS) superalloy, INCONEL* MA 6000. The apparent stress exponent and activation energy for cyclic creep at both frequencies studied are shown to be greater than values observed for static creep. The large values of the stress exponent and activation energy for cyclic creep are proposed to result from anelastic strain storage delaying nonrecoverable creep during the on-load portion of the cyclic creep loading, such that the “effective stress” driving nonrecoverable creep is only a small fraction of the applied stress. In addition, the temperature dependence of the anelastic relaxation that occurs during the off-load portion of the cyclic creep loading is determined. The activation energy found for the relaxation process is equal to about one-half that for self-diffusion in nickel. A mechanism of localized climb of dislocations over the oxide dispersoids present in INCONEL MA 6000 is postulated to account for the observed activation energy of the relaxation process. VINCENT C. NARDONE, formerly Associate Research Scientist, Henry Krumb School of Mines, Columbia University     

Creep and low-cycle fatigue behavior of ferritic Fe-24Cr-4Al alloy in the dynamic strain aging regime: Effect of aluminum addition

Creep and low-cycle fatigue behavior of ferritic Fe-24Cr-4Al alloy was studied in the temperature range of 673 to 873 K, where dynamic strain aging (DSA) occurrence was found. The DSA of the alloy manifested in the form of serrated flow, negative strain rate sensitivity, and the peak or plateau in the variations of yield strength (YS) and ultimate tensile strength (UTS) with temperature. The characteristic creep behavior of the alloy was experimentally verified as that for a class I solid solution. However, this ferritic alloy showed an anomalous high stress exponent (n=5.7) and high activation energy (Q _c =285 kJ/mol) of the secondary creep, which were commonly exhibited by class II solid solutions. During cyclic deformation, the alloy displayed serration in the stress-strain hysteresis loops, increased cyclic hardening, and enhanced planarity of dislocations. On the basis of the observed experimental results and proper analysis, it was proposed that there was strong elastic interaction between solute aluminum atoms and dislocations in the DSA temperature domain. The anomalous creep and fatigue features were interpreted in terms of the interaction of aluminum with the dislocations.     

High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s⁻¹ is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.     

Analysis of the creep behavior of silicon carbide whisker reinforced 2124 Al(T4)

The effect of stress and temperature on the steady state creep rate of SiC_w/2124 Al (T4) has been determined. The stress exponent for steady state creep of the composite is shown to increase from a value of 8.4 at 177 °C to a value of 21 at 288 °C. The activation energy for creep was determined to be 277 kJ/mol for testing in the temperature range from 149 to 204 °C and 431 kJ/mol for testing from 274 to 302 °C. These values are much greater than that for self-diffusion in aluminum. Such a severe temperature and stress dependence of the steady state creep rate is characteristic of precipitation and oxide dispersion strengthened nickel-base superalloys, where the creep behavior is explained by the particle strengthening contribution being a significant fraction of the applied creep stress. In contrast, the estimated particle strengthening for the composite is much less than the applied creep stresses. Alternate strengthening mechanisms are proposed to account for the observed creep behavior of the composite material, including the effect of temperature on the measured values of the stress exponent and activation energy for creep.     

Creep and stress rupture of oxide dispersion strengthened mechanically alloyed inconel alloy MA 754

The creep and stress rupture behavior of the mechanically alloyed oxide dispersion strengthened nickel-base alloy MA 754 was studied at 760, 982 and 1093 °C. Using material with a fine, highly elongated grain structure, tensile specimens oriented parallel and perpendicular to the longitudinal grain direction were tested at various stresses in air under constant load. It was found that the apparent stress dependence was large, with power law exponents ranging from 19 to 33 over the temperature range studied. The creep activation energy, after correction for the temperature dependence of the elastic modulus, was close to but slightly larger than the activation energy for self diffusion. Rupture was intergranular and the rupture ductility as measured by percentage elongation was generally low, with values ranging from 0.5 to 16 pct. The creep properties are rationalized by describing the creep rates in terms of an effective stress which is the applied stress minus a resisting stress consistent with the alloy microstructure. Values of the resisting stress obtained through a curve fitting procedure are found to be close to the values of the particle by-pass stress for this ODS alloy, as calculated from the measured oxide particle distribution. .nt]mis|Formerly at Columbia University     

Steady state creep in two Ni-Al alloys

Creep of two Ni-AI alloys containing 4.8 and 7.0 wt pct Al was studied in the temperature range 873 to 1073 K and stress range 30 to 400 MPa. The former alloy represents the solid solution of aluminum in nickel, the latter a solid solution strengthened by NI3AI particles. As to its creep behavior the solid solution alloy belongs to the Class n of solid solu-tions,i.e. the creep controlling mechanism is the same as in pure nickel. From the analy-sis of an effective stress dependence of steady state creep rate it follows that the mo-tion of jogged screw dislocations can be considered as the most probable creep control-ling mechanism. The apparent activation energy of creep in the two phase alloy increases with tempera-ture. This effect is caused by changes in the volume fraction of second phase particles and by the onset of climb around particles at high temperatures. At lower temperatures particles are cut by dislocation pairs.     

Steady state creep behaviour of a rapidly solidified and further processed Al-5 wt% Ti alloy

The steady state creep behaviour of rapidly solidified and further processed Al-5 wt% Ti alloy has been studied in the temperature range 573–673 K. The experiememtald ata exhibited apparent stress exponents of 7–8 and an apparent activation energy of 240 kJ mol⁻¹. The results are analyzed using the semi-empirical power law, the substructure invariant model and an exponential law. The semi-empirical power law with a threshold stress and a stress exponent of 5 is found to be the best representation for steady state creep of such alloys. By analyzing literature data on the metallic/ intermetallic dispersion strengthened aluminium alloys, a modification in the dimensionless constant, A = 8.3 × 10³ exp[−104√(b/L)], is suggested to account for the influence of dispersion on creep kinetics. It is proposed that the attractive dislocation-particle interaction originates from the dissociation of lattice dislocations into interfacial dislocations when they enter the matrix-particle interface at high temperatures for climb by-pass.     

Creep rupture of a silicon carbide reinforced aluminum composite

The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.