Stress corrosion behaviour of Al-Zn-Mg alloys based upon microchemical surface reactions

Divergences between the chemical compositions of the natural oxide layers on pure aluminium and of alloys containing magnesium have been shown with the aid of SIMS measurements. Scanning and transmission electron micrographs and electron microprobe measurements indicate the sensitizing effect of the thermal oxide layer to water vapour and this is substantiated by SIMS studies. From published data and this investigation a further elaboration of the possible explanation for stress corrosion is proposed.     

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Hydrogen bubbles in embrittled Al-Zn-Mg alloys

Al-Zn-Mg alloys become embrittled during exposure to moist environments due to hydrogen penetration of grain boundaries. The result of this hydrogen penetration due to surface reaction with water vapour of both bulk specimens and electron-transparent thin foils, has been studied at high resolution in the JEM 100 C transmission electron microscope as a function of alloy composition and ageing treatment. In bulk specimens of alloys solution-heated, water-quenched, and aged in water-vapour-saturated air at 70° C, the hydrogen is in the form of a mobile atomic species which is transformed to bubbles of molecular hydrogen under the action of the electron beam. However, in electron-transparent specimens of aged alloys after exposure to water vapour the accumulated hydrogen is observed directly as bubbles. These bubbles take the form of hexagonal lenses bounded by {111} planes, and are associated with grain-boundary precipitates, particularly in over-aged microstructures, and with primary intermetallic particles in alloys containing sparingly soluble transition elements. The consequence of the observed hydrogen penetration of grain boundaries in promoting environmental debilitation of mechanical properties and stress-corrosion cracking of Al-Zn-Mg alloys is discussed.     

Fracture behaviour of Al-Zn-Mg/SiCp composites

Al-Zn-Mg alloys reinforced with different volume fractions of SiC particulates were prepared by a liquid-metallurgy technique. The mechanical properties in uniaxial tension and compression were evaluated, and fractographic observations were made on the fracture surfaces. The distribution of SiC was quite uniform in the extruded condition, and the mechanical-property data show that the composite properties were inferior to those of the control alloy; they essentially showed a decreasing trend with increasing volume fractions. These observations can be explained in terms of the particle distribution, the porosity and the interfacial characteristics.     

Effect of grain-structure topology in modelling aggregate irradiation creep behaviour of two-phase alloys

The two-phase alloy Zr–2.5Nb, a high-strength alloy for nuclear applications, has a grain structure in which the hcp -phase is surrounded by the bcc β-phase. A satisfactory method for the macroscopic overall properties of this type of materials has yet to be developed. Based on the small volume fraction of the β-phase, some existent models of polycrystalline aggregates of Zr–2.5Nb argued to completely ignore the presence of the β-phase, and hope that errors cancel by fitting to experimental data. This may not introduce significant errors if the β-phase is randomly distributed, and if the deformation mechanisms are not an important part of the investigation. However, for highly correlated distributions of the two phases, the dependence of the overall polycrystalline properties in the topology of the distribution of the β-phase is intuitively feasible, or at least, the contrary has not been established. In this paper, we are interested in ascertaining the effect, if any, of the grain structure topology on the irradiation-induced deformation of this type of alloys. We compare the behaviours of three model polycrystalline aggregates with topologically different grain structures, one made up of “grains” in which the -phase is embedded periodically in a β-phase, and another in which the β-phase is embedded periodically in the -phase, and the third in which the β-phase is neglected. All have the same texture, and the first two have the same :β volume ratio, and therefore cannot be distinguished within the existent models. The double interaction method (DIM) is introduced for the treatment of this type of quasi-periodic textured polycrystalline aggregates. In this method, the overall properties of the constituent grains with a periodic structure, are first calculated. The interaction direction derivation (IDD) method is used to consider the interactions among the “intra-granular” periodic elements. The overall properties of the textured polycrystalline aggregate can then be calculated, by considering the inter-granular interaction using the usual self-consistent method (SCM). It is found that, the β-phase may not be neglected in the usual self-consistent model treatment, unless the magnitudes of the creep compliances of the two phases are not very different.     

Static and cyclic creep behaviour of 2024/SiCp and its matrix alloy at high temperature

Abstract

The tensile creep and tensile-tensile cyclic creep behaviour of 2024/SiC_p composite and its matrix alloy have been investigated and analysed at high temperature. It was found that the creep threshold stress of the composite may not be caused by SiC_p alone: the matrix alloy also contributes to the threshold stress. The higher threshold stress of the composite compared with that of the matrix alloy can be explained in terms of load transfer in the composite and the value of threshold stress for the matrix alloy. A direct comparison between the composite and its matrix alloy indicates that only below a critical stress does the composite show a creep resistance higher than that of its matrix alloy. The two materials shown cyclic creep retardation in the tested stress range, cyclic creep showing a higher stress exponent and higher apparent activation energies in comparison with static creep. An analysis based on anelasticity is introduced to explain this result. The relationships between rupture lifetimes and applied stress, creep rate, and unloading amount show that the creep fracture mechanism is dominant in the present test condition.     

Mechanical properties of Al-Zn-Mg alloys investigated by microhardness measurements

One of the most simple and economic methods of testing the mechanical properties of alloys is the microhardness measurement. In the present paper we report on the results and the interpretation of experiments carried out on a series of AI-Zn-Mg alloys prepared from high purity base materials. The following results were obtained: (a) the incremental microhardness,HV, was related to the microhardness,HV, of high purity Al and can be given asHV=264(C_mg–0.25C_zn) wherec _mg andC _zn are the concentrations of Mg and Zn respectively, in the as-quenched state after solution heattreatment, (b) the ultimate tensile strength and the microhardness were correlated by the approximation:HV3_u.HV was investigated in the light of the average radius and the volume fraction of zones forming at room temperature. On the basis of the micromechanism of plastic deformation further evidence was found to show that the shearing mechanism is responsible for strengthening by GP zones in AI-Zn-Mg alloys.     

Transitions in creep behaviour

Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate. The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted. The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted.At Dept. of Metallurgy, University of British Columbia, Vancouver 8, BC, Canada until 31 March 1970.     

Indentation creep of lead and lead-copper alloys

Stress exponent values have been determined in Pb and Pb-Cu alloys with small Sn, Se and Pd additions by indentation methods (long time hardness tests) to evaluate their applicability as compared with tensile tests. Homogeneous, fine grained alloys were obtained by induction melting and thermo-mechanical treatments. Grain size was 38–60 m in alloys and 183 m in pure lead. Stress exponent values, i.e. of 11–12 agree between different methods of derivation and, in fine grained material, with tensile methods. The largest differences in pure lead, i.e. 10–11 versus 7–8 are attributed to high strain rates when indentation size is comparable to grain size. In all cases indentation and tensile tests indicate the same deformation mechanism, namely slip creep. The indentation test is thus considered useful, within limits, to acquire information on the deformation mechanism.     

Flux creep in niobium-titanium alloys

The voltage-current characteristics of a cylindrical sample of Nb-15 at % Ti were investigated in the presence of a perpendicular applied magnetic field. The flux creep rate is dependent upon the applied magnetic field and the sample temperature, as suggested by Anderson's theory of thermally activated flux creep.     

Flux creep in lead-indium alloys

The voltage-current characteristics of a cylindrical sample of lead-indium alloy Pb₈₃In₁₇ were investigated in the presence of a longitudinally applied magnetic field. The flux creep voltage is dependent upon the applied magnetic field and independent of the sample temperature.     

Factors leading to grain-boundary fatigue crack propagation in Al-Zn-Mg alloys

Fatigue crack propagation experiments have been carried out at low load amplitudes with a high purity and a corresponding commercial purity Al-Zn-Mg alloy. When the high purity alloy was tested in laboratory air, cracks were often seen to propagate along the grain boundaries. Particularly in the peak aged condition, this alloy is highly susceptible to failure by intercrystalline cracking. However, with dry nitrogen as the test environment, the crack was observed to propagate preferentially along shear bands within individual grains. In the commercial purity alloy, grain-boundary crack propagation was not observed for either laboratory air or dry nitrogen atmospheres. The proportion of intercrystalline cracking in laboratory air could be lowered for the high purity alloy by a thermomechanical treatment.     

Non-linear creep behaviour of PET

The non-linear creep behaviour has been studied on PET films at room temperature. A particular value of the stress, _c, was used to characterize the change between the linear to the non-linear domain. The variations of the elastic modulus, the relaxed modulus and _c revealed great sensitivity to the morphology of the crystallization. A molecular model of non elastic deformation, assuming (i) hierarchical correlated molecular motion, and (ii) nucleation and expansion of sheared-microdomains, was used to analyse the role of stress on anelasticity. To take into account the two-phase structure of semicrystalline films, a phenomenological series/parallel model was applied to express the mechanical coupling between amorphous and crystalline phases. Quantitative agreement was found between theoretical predictions and experimental data for low and high stresses. However, there was a discrepancy in the rate of recovery because the model predicts a strain recovery slower than the experimental behaviour. Consequently, it is proposed to develop further the molecular model mentioned above by specifying the energy profile of a sheared-microdomain and its stress dependence. Then, the difference between creep and recovery strain rate could be explained.Nomenclature A Anelastic equilibrium compliance - A Parameter proportional to the relaxation strength - b Shear vector - Correlation parameter - _i Particular value in the distribution - _e Average value in the material ( = 0.27) - Correlation parameter characterizing the ability of chain orientation - d _a Amorphous density - d _c Crystalline density - Parameter of the mechanical mixing law - E Tensile modulus - E _c Crystalline Young's modulus - g _i Statistical weight in the distribution - G ₀ Shear modulus at 0 K - J _max Creep compliance at the end of the creep time - J _max(0) Value ofJ _max for low stresses - J _u Unrelaxed compliance - J _i( _i;A _i) Calculated compliance for a couple ( _i;A _i) - J _exp Experimental creep compliance - J _a Compliance of the amorphous part - J _c Compliance of the crystalline part - J _sc Compliance of the semi-crystalline material - k Boltzmann's constant - Parameter of the mechanical coupling law - R Radius of a shear micro domain - ₀ Stress necessary to cross the energy barrier only by mechanical activation - T _c Crystallization temperature - t _c Creep time - _mol Time for a translational motion of a structural unit over a distance comparable to its size - Particular value of _mol in the time distribution - Characteristic time for the secondary relaxation - ₀ Time proportional to the Debye time - t ₀ Scaling time parameter determined by the experimental value of _mol - U Activation energy for an elementary molecular motion - X _c Crystallinity ratio - V _a Volume fraction of the amorphous part - V _c Volume fraction of the crystalline part