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.     

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.     

Kinetics of cellular dissolution in an AlZn alloy

Dissolution of the lamellar precipitate by cell boundary migration has been studied in an Al-18.9at.% Zn alloy in the temperature range 554–637 K. Microstructural observations have revealed that the process of dissolution in this alloy is cellular mode of transformation in the early stages. The boundary diffusivities were calculated by using the theory of Petermann and Hornbogen modified for cellular dissolution. The diffusivities calculated from the experimental data are four orders of magnitude higher than the volume diffusivities. From the Arrhenius plots, activation energies of 51.5 ± 2 and 47.5 ± kJ mol⁻¹ were obtained from the temperature dependence of diffusivity and mobility respectively. These values are about half the activation energy required for volume diffusion of Zn in Al and compare very well with the activation energy of 60 kJ mol⁻¹ for the cellular precipitation in this alloy system. There exists a range of temperature between 502 and 532 K where the two kinetic processes, cellular precipitation and dissolution, are equally probable. The forward migration of the grain boundary during cellular precipitation is acted upon by the back pull of dissolution and the migrating grain boundary remains motionless.     

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.     

An AP-FIM study on metastable phases in AlAg binary alloy

The morphology and chemical composition of metastable phases in an Al-5.72at.% Ag alloy have been investigated by means of atom-probe field ion microscopy. It was found that the η G.P. zones formed by aging at 413 K have an average solute concentration of 45.8 ± 0.7 at.% Ag. the present data on zone size and interparticle distance were found to be consistent with data determined by small-angle X-ray scattering. When the specimen was aged at 436 K for 60 ks, both ϵ G.P. zones and γ′ metastable phase were observed simultaneously. The solute concentration of ϵ zones was determined to be 35.7 ± 0.2 at.% Ag. The γ′ phase was a plate-like precipitate lying on the {111} matrix plane, and its solute concentration was 33.3 ± 1.5 at.% Ag and its thickness was in the range of 2.3–7.0 nm. By combining the present data with other results certain pending questions on the structures of metastable phases in this alloy have been successfully solved.     

Residual resistivity during clustering in AlZn solid solutions

In the alloys Al 8 to 25 wt,% Zn, the isothermal kinetics of clustering has been followed at different ageing temperatures by resistivity measurements. At low temperatures, the classical “anomaly” (increase, maximum and decrease) is observed. It is associated with a large density (~10¹⁹ cm⁻³) of Zn clusters of 1–2 nm of diameter. The resistivity kinetics have been calculated with the help of Hillel's diffraction model and of transmission electron microscopy and neutron small-angle-scattering structural results: the experimental behaviour is fairly well reproduced. At high ageing temperatures, a metastable state sets during an incubation period of clustering and the corresponding resistivity plateau is higher for lower temperature. A binding energy between zinc atoms of 0.04 eV has been deduced from a simulation of the equilibrium between clusters containing less than 5 atoms. At intermediate temperatures, the behaviour is composite and is not fully understood.     

Fatigue in CuZnAl single crystals

Fatigue properties associated with the β-18R martensitic transformation are analyzed in CuZnAl single crystals. The fatigue induced changes are studied by optical, scanning and electron microscopy. It is found that in the interior of the crystals dislocation defect arrays are created which act as local obstacles for the martensitic transformation and lead to the formation of extrusions and holes at the surface, which with increasing numbers of cycles join to form continuous cracks of about 1 μm width. Crack propagation seems to be little affected by composition or orientation.     

Hydride formation in an AlLiCu alloy

A hydride phase, LiAlH₄, has been identified in Al-2.0%Li-2.2%Cu alloy electrochemically charged with hydrogen. This is a hydride having the composition of LiAlH₄. The orientation relationship of the hydride and the matrix has been determined and rationalized with the O lattice theory. The thermodynamic stability of the hydride is discussed and possible formation mechanisms explained. The hydride forms from the grain boundary phase, AlLi, as the maximum amount of available Li is already present in the AlLi (δ) phase.     

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.     

Structure-property relationships in AlLiCuMgAgZr alloy X2095

Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al₃Li) particles and reduced subgrain boundary T₁ precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength.