Mahoto Takeda

Stability of metastable phases and microstructures in the ageing process of Al–Mg–Si ternary alloys

Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si=2), excess silicon (Mg/Si<2) and excess magnesium (Mg/Si>2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, β″, β′ and stable β phases. The peaks corresponding to β″ and β′ were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that β″ precipitates played a major role in improving the hardness, but not β′ precipitates.

Discontinuity of G.P.(I) zone and θ′-phase in an Al-Cu alloy

Al-Cu is a precipitation-hardening Al-base alloy with several metastable phases formed in its phase-decomposition process. It is widely accepted that the phase decomposition in the Al-Cu alloy is as follows: supersaturated solid solution {r_arrow} G.P.(I) {r_arrow} {theta}{double_prime}-phase {r_arrow} metastable {theta}{prime}-phase {r_arrow} stable {theta}-phase. Based on this model, valuable information has been obtained through experiments since studies on the Al-Cu alloy began. The present work aimed at examining the thermal stabilities of G.P.(I) and {theta}{double_prime} in order to determine the relation between these precipitates by taking the copper concentration into account. To consider the precipitation behavior in an Al-Cu alloy, differential scanning calorimetry (DSC), TEM observations and Vickers hardness tests were used in combination in this work.

TEM Study on Precipitation Behavior in Cu–Co Alloys

The precipitation behavior of Cu-Co alloys has been investigated by means of TEM, Octahedral f.c.c. Co-rich precipitates were observed in isothermal aging processes at 873 and 1073 K as in previous works. However, it has been revealed in this work that the octahedron which was coherent to the Cu-matrix rather drastically changed in shape at the transition condition whereby the precipitate lost its coherency. That is, the coherent precipitate had an octahedral shape limited with facets which were nearly {110} planes, whereas the incoherent f.c.c. Co-rich precipitate had an octahedral shape with {111} facet planes. The size distributions and growth rates of the two types of precipitates were examined. This result suggests that two different precipitation mechanisms occur in the coherent and incoherent precipitation stages.

The influence of Mn on precipitation behavior in Al-Cu

The precipitation-strengthened 2000-series Al-Cu alloys are one of the most important high-strength Aluminum alloys. For practical purposes, some amount of the third element is normally introduced to improve the strength at high temperature, creep resistance, etc. Manganese is an important element which is often added to obtain fine-grained polycrystalline 2000-series alloys. It is also known that Mn addition improves the strength at high temperature. To identify the influence of the Mn addition, Heimendahl and Willing investigated the composition of precipitates occurring in an Al-Cu-Mn alloy and reported a refinement of the intermediate phase {theta}{prime} and an apparent suppression of the Ostwald ripening of the phase. There are, however, other works presenting different results or alternative interpretations such that connect Mn addition with the kinetics of precipitation based on vacancy diffusion. Thus the influence of Mn addition is not completely understood. The present work aimed at examining the influence of Mn additions to an Al-Cu alloy, based on Differential Scanning Calorimetry (DSC), Transmission Electron Microscopy (TEM) observations, together with energy dispersive X-ray (EDX) analysis and Vickers microhardness tests.

Copper concentration inside guinier-preston I zones formed in an Al-Cu alloy

Abstract The copper concentration inside a Guinier-Preston (GPI) zone was studied by means of an extended Huckel molecular orbital (EHMO) calculation. The present EHMO calculation revealed that the Cu concentration inside a GPI zone depends on the size of the zone. The GPI zone comprises both Cu and Al in a small GPI zone, whereas the Cu concentration inside the GPI zone increases to nearly 100 at.% Cu in a large GPI zone. A compressive lattice distortion, which is associated with the zone, increases the stability, but when a GPI zone is 5 nm in diameter, the stabilizing effect of the zone is estimated to be not higher than 10% of the chemical energy due to Cu coalescence.

The microstructure and magnetic properties of nano-scale Fe magnetic particles precipitated in a Cu-Fe alloy

Abstract The evolution of nano-scale Fe particles precipitated in a Cu – Fe alloy has been examined from the viewpoint of the relationship between the microstructure and magnetic properties, using conventional, high-resolution and Lorentz electron microscopy, and magnetic measurements. It has been revealed that spherical Fe particles have a tendency to align along the <001> direction in a Cu matrix at the early stage of precipitation. A twin-like structure developed when spherical Fe particles grew to approximately 40 to 60 nm in size. Lorentz microscopy was successfully applied to determine the direction and magnitude of the magnetic momentum of Fe particles.

The Metastable Phase Responsible for Peak Hardness and Its Morphology in an Al-Mg-Si Alloy

Precipitation behaviour in an Al-Mg-Si alloy aged at 403 K to 483 K was studied with respect to thermal stability and morphology of the metastable precipitates, using high resolution transmission electron microscopy (HRTEM), Vickers microhardness tests and differential scanning calorimetry (DSC) measurements. The quantitative analysis of the DSC measurements revealed that the change in the first exothermic peak (the peak P) of the metastable phase is proportional to the increases in the Vickers hardness. The HRTEM observations showed the four types of the precipitates in morphology during the isothermal ageing and the change in the peak P was mainly caused by the formation of the precipitates with irregular contrast and the ones with the network interior angle between 65°and 80°

Atomic composition of the metastable β″ phase precipitate in an Al–Mg–Si alloy

Abstract The atomic composition of a metastable β″ phase precipitate, which is responsible for strengthening of Al–Mg–Si alloys in a precipitation-hardening process, was studied by means of Vickers microhardness tests, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) analyses. On the basis of the present TEM-EDX experiments and DSC measurements, our study revealed that the β″ phase precipitate formed in Al–Mg–Si alloys has a high silicon concentration. It was also concluded that the precipitation behavior of the β″ phase coincides with the precipitation occurring in the binary Al–Si alloy.

Magnetic and magnetoresistive properties related to microstructure in Cu75–Fe5–Ni20 alloys

The electromagnetic properties and microstructures of a Cu75?Fe5?Ni20 alloy have been investigated on isothermal annealing at 1073?K, using a superconducting quantum interference device magnetometer, quantum design, physical property measurement system and transmission electron microscopy. Nanoscale magnetic particles were formed randomly in the Cu-rich matrix after receiving a short annealing due to the phase decomposition in the alloy. With increasing isothermal annealing time, however, rod-type precipitates aligned along the 1?0?0 directions were observed in the matrix, on isothermal annealing at 1073?K. Although the size of the precipitates became larger (from ~10?nm to >300?nm) after further annealing, no significant change (less than 2%) was detected in the MR value. The largest MR value (MR ~16% at H = 7?T and T = 10?K) was attained, in particular, for the as-quenched specimen. This study revealed that several significant influences were introduced into the magnetic and magnetoresistive properties during the phase decomposition process in the Cu?Fe?Ni alloy.

Microstructures and thermal stability of metastable-phase precipitates formed in an Al–Cu alloy at 463 K

Abstract The precipitation behavior in an Al–Cu alloy isothermally annealed at 463 K was studied using Vickers microhardness tests, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). An additional endothermic peak found in the DSC measurements may be attributed to the θ”-phase, independent of the dissolution of the G. P. (II) and the θ-phase at the aging temperatures. High-resolution transmission electron microscopy (HRTEM) revealed that the G. P. (I) is formed at a very early stage of isothermal aging, even at 463 K. Comparing the Vickers microhardness and TEM images, it was concluded that the stage at which the G. P. (II) and θ-phases precipitated simultaneously is mainly responsible for the peak hardness.