Observations of the effect of an applied stress on the morphology of discontinuous precipitation in a CuCd alloy

The influence of an applied tensile stress on the morphology and growth rate of the discontinuous precipitation product in a Cu−3.8 wt% Cd alloy was first reported by Sulonen. In this contribution, we investigate and further quantify a number of factors associated with the role of applied stress in this alloy, including absolute and relative growth rates of interfaces with normals parallel to and perpendicular to the tensile axis, and the morphological stability of discontinuous precipitation front as a function of the applied stress. Once morphological instability has occurred, the amplitude and wavelength of the protuberences formed depends on the value of the applied stress, and on the angle between the average interface normal and the tensile axis. It is suggested that the process is best viewed as one of transformation-assisted viscoelastic deformation.     

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.     

Effect of cyclic stress on the high temperature creep behavior of AlMg alloys

The effect of cyclic stress on high temperature creep and also the dislocation structures developed during cyclic creep for three AlMg solid solutions have been studied. When the steady state creep rate is the range of 10⁻⁸ s⁻¹, cyclic creep with complete unloading (R = 0) exhibited only the transition from class M to class A creep instead of the two transitions from class M to class A and then to class M again ihe corresponding static creep. The threshold stress for the transition in cyclic creep was found to be close to or larger than that for the class A to class M transition in the static creep. With increase in magnesium content, cyclic creep retardation became stronger at lower stresses. However the transition from retardation to acceleration in the cyclic creep has been observed at higher stress levels only in the high magnesium alloys. Also found were two types of relationships between the unloading amount and steady state creep rate. The micromechanisms of cyclic creep, especially the interaction of dislocations with solute atoms, have also been discussed.     

Elastic constants of the monoclinic 18R martensite of a CuZnAl alloy

The sound velocities of 15 propagation modes in an 18R single crystal of a CuZnAl alloy have been measured by the pulse-echo method. The elasticity equations have been solved using a numerical selfconsistent procedure, and the values of the 13 elastic constants of this monoclinic structure have been determined.     

On the creep deformation of a cast near gamma TiAl alloy Ti48Al1 Nb

The steady-state creep deformation behavior of a cast two phase gamma TiAl alloy having the composition Ti48Al1Nb (at.%) has been studied. Tension creep tests using the stress increment technique (θθ₂θ₃) were conducted over the temperature range of 704–850°C at constant initial applied stress level of 103.4–241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied 317.5 kJ/mol (137.8 MPa) up to 341.0 kJ/mol (206.8 MPa) with an average value of 326.4 kJ/mol. This is well within the range of values previously measured for gamma TiAl alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents meaured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against long stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiAl alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking γ/α₂ laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti48Al1Nb alloy to that of a Tiz.sbnd;50.3Al binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiAl alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies.     

On the formation of 2H stacking sequence in 18R martensite plates in a precipitate containing CuAlNiTiMn alloy

Basal planes stacking sequence variation in M18R martensite of CuAlNiTiMn alloy have been studied by means of electron diffraction and lattice imaging. Samples subjected to two different homogenisation temperatures (1173 and 973K) and two different cooling rates—water quenched and air cooled were analysed. The observed changes in stacking sequence have been attributed mainly to stress accomodation around X_L and X_S precipitates. Very small X_SS precipitates of the same phase have been observed and their high influence on the stacking sequence of martensite plates have been explained by means of a new “dislocation” mechanism related to their coherency loss into the matrix.     

Investigation of thermoelastic martensitic transformation in a CuZnAl alloy

Thermoelastic martensitic transformation in a Cu-29%Zn-3%Al alloy was investigated experimentally and theoretically. The phase structures and morphological changes occurring during transformation were studied using optical microscopy and high voltage TEM (1000–1200 kV), both equipped with combination heating and cooling stages. A “single crystal pure shear” experiment was designed to measure the relationship between M_s and shear stress, and from these data changes of enthalpy and entropy of the transformation were calculated through the Clausius-Clapeyron equation. The changes of enthalpy and entropy were also obtained by using calorimetric measurement. Both results were in good agreement. The kinetic behavior of the transformation in polycrystalline alloy as a function of applied stress was followed using electric resistance measurement. The slope of the transformation rate was constant over the range of 20–70% transformed, which corresponds physically to transformation occurring by plate growth in the unpartitioned parent phase or equivalently, the interphase boundary “moving freely.” A phenomenological theory was suggested to describe the constant slope portion of the transformation. Here, phase boundary motion is related to thermal hysteresis and quantitatively described the effect of applied stress on the transformation behavior.     

Ageing kinetics of a silicon carbide reinforced AlZnMgCu alloy

The ageing kinetics of a composite of an AlZnMgCu powder alloy (“CW67”) combined with a varied volume fraction of particulate silicon carbide were investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). DSC revealed that the maximum rate of precipitation of the metastable η′ phase was substantially lower for CW67/SiC/20p than for the unreinforced alloy or CW67/SiC/10p. TEM of isothermally aged material revealed differences between the unreinforced alloy and composites in respect of precipitate size and morphology. We conclude that SiC additions, by dint of additional dislocations generated during quenching, can affect the ageing of CW67 either by accelerating the nucleation of precipitates or by accelerating precipitate growth. The ageing rate in CW67/SiC/20p was increased by accelerating both the nucleation of precipitates and growth, whereas the ageing in CW67/SiC/10p was enhanced by accelerating precipitate growth only.     

Microstructures and mechanical properties of rapidly solidified CuCr alloys

Rapidly-quenched CuCr alloys have been prepared by melt-spinning and the microstructures and mechanical properties examined as a function of alloy content and subsequent annealing treatment. Mechanical properties have been successfully measured by tensile testing on the ribbon samples and it is shown that this method is more suitable than hardness testing for a proper evaluation of the materials. An alloy containing 2% Cr has been prepared in the totally solid-solution state, whilst a 5% Cr alloy contains large chromium particles distributed uniformly throughout the ribbon and fine-grain-sized CuCr solution. These large chromium particles are deduced to arise by a uniform, primary-solidification mode prior to, and independent of, the nucleation and growth of the copper from the lower ribbon surface. The increased solid solubility obtained allows extensive precipitation and strengthening following ageing: the primary chromium particles play an important role both in strengthening and in restricting grain coarsening.     

The influence of zirconium on the early stages of aging of a ternary AlZnMg alloy

The commercial 7000 series aluminum alloys are based on AlZnMg and AlZnMgCu systems. These alloys commonly contain trace amounts of zirconium for grain refining purposes. In this paper, the influence of zirconium on the early stages of aging an AlZnMg alloy is considered. It is shown that two different kinds of microstructure are developed for materials with and without zirconium when heat treated slightly above the GP (Guinir Preston) zone solvus. For the ternary alloy, the microstructure consists of both metastable MgZn₂(ν′)phase and “stable GP zones(/clusters)”; the stable GP zones act as the nucleation sites for the ν′ phase. Whilst, in the case of the zirconium bearing alloy, the microstructure consists only of a heterogeneous distribution of under developed ν′ precipitates. These observations are discussed in relation to the critical role of vacancies in the formation and stability of GP zones and precipitate nuclei, and of zirconium in reducing the total numbers of available excess vacancies in these materials.     

The effects of Zr and Ti on the microstructure and tensile properties of Al–Cu–Mg aluminium alloy

The effects of Al–5Ti–1B and Al–15Zr master alloys on the structural characteristics and tensile properties of Al–4.5Cu–0.3Mg aluminium alloy were studied. Al–5Ti–1B was found to be more effective than Al–15Zr in grain refining of the alloy. It could be seen that average grain size reduces from 570 to 260?μm when 0.01?wt-% Ti addition; additionally, while different amounts of Ti and Zr were added to the alloy, the dendritic structure changes from long dendrite to rather rosette-like morphology. Furthermore, tensile testing of cast specimens revealed that ultimate tensile strength (UTS) of the cast alloy increases from 241 to 283 and 260?MPa after adding the optimum amount of Ti and Zr containing master alloys, respectively. Moreover, UTS values of T6 heat-treated specimens also showed 73 and 61% improvement after adding 0.05?wt-% Ti and 0.3?wt-% Zr to the alloy. Fracture surface examinations exhibited a transition from brittle fracture mode in as-cast to ductile fracture in refined and T6 heat-treated specimens.