The influence of MN additions on the deformation behaviour of an AlMgSi alloy

The deformation behaviour at low plastic strains of three AlMgSi alloys in the fully aged condition has been studied. The MN-free alloy (BD3) plastically deforms to form narrow slip bands whose spacing decreases slightly with increasing strain. This alloy fails intergranularly at low strain because the high stress concentrations at the head of the slip bands nucleate voids at grain boundary precipitates which then propagate by coalescence within the PFZ.The two Mn-bearing alloys (BD3 and BDS) contain volume fractions of 0.004 and 0.011 respectively of 0.1 μm dia. incoherent particles of α-Al₁₂Mn₃Si. as well as the age-hardening phase. At small strains these alloys develop slip bands of slightly smaller spacing than those in BD3. out with increasing strain the slip band spacing decreases until at 9% strain are no longer resolvable in BD8.The Mn-phase thus causes lateral spreading of the slip bands, giving rise to significantly smaller stress concentrations at the head of the slip bands, and whose magnitude is further reduced by the smaller grain size of these alloys. A semi-quantitative assessment of these two contributions to the local grain boundary stress has been made. It is concluded that intergranular failure in BD6 and BD8 is suppressed at low strains because the associated stress concentrations are insufficient to produce decohesion at the grain boundary precipitates.     

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.     

Decomposition of an AlLi alloy—The early stages observed by HREM

Using HREM the decomposition of an Al7 at.% Li alloy during isothermal aging at 463 K was studied. Ordered precipitates with radii down to 0.5 nm were resolved. The size distributions of the precipitates and the decomposition parameters such as mean radius, particle density, and precipitated volume fraction were measured. The time dependence of these parameters is well described by a modified Langer-Schwartz model. Thus, at the aging conditions chosen the decomposition follows a classical nucleation and growth mechanism. The analysis of the decomposition process yields some hints for a congruent ordering reaction, which takes place during quenching.     

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.     

The effect of temperature on strain rate sensitivity in an AlMgSi alloy

The effect of temperature on strain rate sensitivity has been studied in an AlMgSi alloy in the range of dynamic strain ageing. Two regimes of behaviour were observed which depended on the temperature. At low temperatures (T <- 303 K) it was possible to correlate all strain rate sensitivity measurements with a single strain/strain rate/temperature parameter. In the high temperature regime the temperature and strain rate dependence of the strain rate sensitivity deviated significantly from that measured at low temperatures. The two regimes of behaviour were found to correspond with the normal and inverse Portevin-Le Chatelier effect.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

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.     

The influence of Li on the nucleation of defects of quenched AlLi alloys

The nucleation and kinetics of defects formed by quenching in two AlLi alloys having Li content of 1.7 and 3.74 at.%Li have been studied by positron annihilation spectroscopy. It has been found that the defect formation is sensitive to the aging time at the temperature from which the samples are quenched. This fact has been related to the Li loss experienced by the alloys aged at high temperature. The quenched-in defects have been identified as vacancy-Li clusters and dislocation loops. The latter are formed by the collapse of the Li-rich vacancy complexes and are very sensitive to the Li content; as a consequence, the loops are decorated by Li-rich zones and are revealed as very effective positron traps in comparison to the vacancy-Li complexes, giving rise to an enhanced trapping.     

The structure of G.P. zones in an AlCuZn alloy

An Al-46 at.% Cu-2.20 at.% Zn crystal was quenched and aged at 353 K for 24 h. Anomalous dispersion was used to separate the effects of Cu and Zn in the diffuse scattering by measuring the diffuse scattering in absolute units at two different energies for each respective absorption edge. The scattering contrast between these two conditions near each edge was analyzed to get the short range order parameters for AlCu and AlZn pairs. Via computer simulation with these parameters the Cu was found to lie in small {100} plates and the Zn in short 〈110〉 strings that probably lie on planes adjacent to the Cu plates.     

Microstructure and microchemistry of an AlZnMgCu alloy matrix-20 vol.% SiC composite

The microstructure and microchemistry of an underaged and overaged AlZnMgCu alloy matrix composite reinforced with 20 vol.% SiC was examined uusing analytical electron microscopy. The presence of MgO particles at the Al/SiC interfaces and Mg₂Si in the matrix of the composites suggests that Mg could be depleted in the matrix. The precipitates in the unreinforced control alloys were found to be consistently larger than those of the composites following the same heat treatment conditions, suggesting that the aging kinetics of the unreinforced control alloy are faster than those in the composite. Data on the aging acceleration in other Al-alloy matrix composites has been analyzed to study aging kinetics. Aging acceleration was found to be more pronounced with increasing average absolute values of the atomic size misfit parameters of the constituent elements of the major precipitates. This result suggests that the preferential segregation of solute atoms and easier nucleation of precipitates from segregated solutes is responsible for aging acceleration in other composites. On the other hand, the depletion of Mg atoms in the matrix and the lack of interaction between Zn atoms and dislocations seems to be responsible for the aging deceleration in the reinforced AlZnMgCu alloy composites.