Akihiko Kamio

Precipitation sequence of various kinds of metastable phases in Al-1.0mass% Mg2Si-0.4mass% Si alloy

The precipitation of three new types of metastable phases, i.e., TYPE-A, TYPE-B and TYPE-C, with different crystal structures from the β′ phase is proposed from our research on the change in crystal structures and formation sequence of metastable phases during the aging of the Al-1.0mass% Mg2Si-0.4mass% Si alloy by a combination of analytical high resolution electron microscopy and energy dispersive X-ray spectroscopy. The sequence of their formation is explained as follows. First, precipitation of the β′ phase and TYPE-B precipitate, then β′ dissolution into the matrix and degradation of the TYPE-B precipitate. Finally, predominant precipitation of the metastable TYPE-A precipitate. The TYPE-C precipitate appeared heterogeneously in the over-aged condition.

Classification of the role of microalloying elements in phase decomposition of Al based alloys

Abstract The fundamental role of microalloying elements in several aluminium alloys such as Al–Cu, Al–Li–Cu and Al–Cu–Mg has been investigated using a Monte Carlo computer simulation. All the utilized simulation parameters, e.g. pair interactions between same atoms species, between different atom species and between an atom and a vacancy, were derived from known thermodynamic or kinetic quantities. A small addition of Mg to Al–Cu alloys exhibits a strong tendency to form Mg/Cu/Vacancy complexes in the atom configurations, which is more remarkably revealed in Al–Li–Cu alloys. The combined addition of Ag or Si with Mg significantly increases the number of Mg/Cu/Vacancy complexes in Al–Cu–Mg alloys. From the comparison with experimentally reported results, these complexes are reasonably regarded as an effective heterogeneous nucleation site for GP zones, GPB zones and/or the Ω phase. The utilized simulation model, furthermore, permits the role of microalloying elements to be well classified in terms of the characteristic features of each element.

Solidification structure of monotectic alloys

The formation manner of the monotectic structure is shown from the observation of the evolution of crystallization and growth of CuPb and some aluminium-based monotectic alloys solidified in a free direction and unidirectionally. The morphological change of the monotectic composite structure is discussed in relation to the solid-liquid interfacial morphology at the monotectic growth front as well as the interfacial energy balance between the solid and two liquids in the monotectic reaction. In free-directional solidification, a characteristic monotectic cell with a spherical shape is formed which is unlike dendritic and lamella morphologies. The monotectic cell consists of a spherical solid and a separated L2 liquid lying along the radii of the solid sphere. In unidirectional solidification, the monotectic structure of AlPb, AlBi and AlIn alloys changes in the following sequence with decreasing growth rate under a constant temperature gradient; random dispersion of L2 droplets in the aluminium solid matrix → periodic regular array of L2 droplets → fibrous L2 composite → aluminium single-phase region without the L2 phase. In CuPb monotectic alloy the monotectic structure changes with decreasing growth rates as follows: irregularly shaped rod-like L2 composite → coalesced coarsened discontinuous L2 composite → periodic banded structure consisting of L2-rich regions and L2-poor regions. These morphological transformations of monotectic structure are strongly affected by the ratio of the temperature gradient to the growth rate, the volume fraction of liquid L2 separated through the monotectic reaction and the interfacial energies between the solid and two liquids at the monotectic growth front.

Crystal system of rod-shaped precipitates in an Al-1.0mass%Mg2Si-0.4mass%Si alloy

The crystal system of a rod-shaped precipitate (metastable phase) in an Al-Mg{sub 2}Si pseudo-binary alloy had been presumed to have a face centered cubic or hexagonal unit cell. Recently, the crystal system of the precipitate was confirmed to be a H.C.P. unit cell through the high resolution transmission electron microscopy (HRTEM) and electron diffraction. The crystal system of the metastable phase in the Al-Mg{sub 2}Si alloy containing excess silicon is not yet precisely determined, although the crystal structure is assumed to be the same as the metastable phase formed in an Al-Mg{sub 2}Si pseudo-binary alloy. In this study, the crystal system of the metastable phase in an Al-1.0mass% Mg{sub 2}Si alloy containing 0.4mass% silicon in excess was investigated using transmission electron microscopy (TEM). A new phase with a hexagonal lattice was found in the present alloy. Both the lattice parameter and the orientation relationship with the matrix are different from those of the metastable phase formed in a pseudo-binary Al-Mg{sub 2}Si alloy.

High resolution electron microscopy of phase decomposition microstructures in aluminium-based alloys

High resolution electron microscopy (HREM) recently applied to phase decomposition and microstructure analyses was demonstrated for aluminium-based alloys such as the AlCu, AlMg and AlLi systems. The HREM has provided direct information on local structures on an atomic-scale resolution. In AlCu alloys, typical Guinier-Preston (GP) zones of type GP (1) and GP (2) with a single copper-rich layer and two copper-rich layers separated by three aluminium layers were clearly observed in the HREM images. The HREM images also revealed the coexistence of multiple-layer zones composed of several copper-rich layers. Lattice distortion around a single-layer GP (1) zone was not necessarily symmetric owing to the neighbouring-zone effect. A fine periodic structure similar to that of a GP (2) zone was found in rapidly solidified AlCu alloys. This structure was proposed to be closely connected with the multiple-layer GP zones. In AlMg alloys the supersaturated solid solution decomposed to form an initially modulated structure and subsequently spherical GP zones with the L12-type ordered structure. The periodic lattice distortion was introduced into the modulated structure associated with the magnesium atom fluctuation. Rapidly solidified AlMg alloys suggested the possibility that the homogeneous ordering process takes place. The uniformly ordered structure of the L12 type was more clearly detected in the AlLi alloys in the initial stage of decomposition. Both the interface and the morphology of this structure were quite different from those of the discrete δ′ phase. Thus the initial ordered structure was regarded as the precursor to the δ′ phase. The decomposition process was also discussed on the basis of experimentally obtained microstructures.

Fabrication of Al/Al3Fe composites by plasma synthesis method

Abstract The present work was undertaken to highlight a novel in-situ process in which plasma spraying techniques plus electromagnetic stirring were used to produce Al/Al 3 Fe composites consist of angular and needle-like morphology of Al 3 Fe intermetallic compounds of about 10–20 μm in size. The microstructures of fabricated materials consisted of α-Al and Al 3 Fe intermetallic compounds, and the size, volume fraction and shape of Al 3 Fe intermetallic compounds depended mainly on the temperature of the melt and the plasma spraying conditions such as input current, gas flow rate of plasma and spraying distance. The microstructural difference arose from the difference of instantaneous temperature and velocity of the iron particle on the melt surface. The temperature and velocity of the iron particles in the plasma arc were calculated and the results were compared with the fabricated microstructures. In addition, the evolution of microstructures of angular and needle-like Al 3 Fe intermetallic compounds was analyzed morphologically by spherical shell model incorporating of diffusion kinetics.

Effects of Mg addition on the kinetics of low-temperature precipitation in Al–Li–Cu–Ag–Zr alloys

Abstract The thermal stability at around 343 K is one important problem of Al–Li alloys related to the microstructural changes during the low temperature exposure. The present work aims to investigate the effects of Mg addition on the age-hardening behavior and precipitation kinetics of an Al–Li–Cu–Mg–Ag–Zr alloy in the low temperature range from 278 to 373 K. The microstructures observed with transmission electron microscopy (TEM) and the hardness changes indicate that a small amount of Mg markedly accelerates the formation of GP(1) zones, not the δ ′ (Al 3 Li) phase, resulting in an enhanced age- hardening. Quantitative analysis of the precipitation kinetics determined by the electrical resistivity changes also elucidates the more detailed effects of Mg addition on the characteristic precipitation phenomena in the Al–Li–Cu alloys, i.e. a decreased activation energy for the GP(1) zone nucleation and a suppressed growth of both GP(1) zones and the δ ′ phase (and/or its precursory structures). The complicated effects of Mg addition are well explained in terms of both the enhanced nucleation rate of GP(1) zones with the aid of Mg/Cu/Vacancy complexes and the pronounced decrease in free-vacancies available for Cu and Li diffusion due to the preferential vacancy trapping by Mg atoms.

Evolution of iron aluminide in Al/Fe in situ composites fabricated by plasma synthesis method

Abstract The evolution of intermetallic compounds for Al/Fe in situ composites fabricated by the plasma synthesis method (PSM) was investigated through the microstructural analyses for the partially reacted particle region. The partially reacted particle is composed of Al/Al 13 Fe 4 /Al 5 Fe 2 /Fe layer. Based on the concept of effective free energy of formation, the first phase that nucleates on the interfacial layer of Al/Fe is determined as Al 13 Fe 4 . By the reaction of Al 13 Fe 4 and Fe, Al 5 Fe 2 forms and grows inward. Due to heat extraction by the reaction, the edge of Al 13 Fe 4 dissolves to form angular Al 13 Fe 4 in the matrix. By the reaction and decomposition of Al/Fe through the intermediate phase of Al 5 Fe 2 , the Fe particles transform to Al 13 Fe 4 in the plasma synthesis method. In addition, the differences of main intermetallic compounds between by the hot dipping process and by the plasma synthesis method were discussed.

A metastable phase having the orthorhombic crystal lattice in an Al-1.0mass% Mg2Si-0.4mass% Si alloy

The properties of Al-Mg-Si alloys are known to be drastically changed depending on the alloy compositions and heat treatments. These behaviors of the alloys originate in the complicated precipitate microstructures. In this work, the authors focused on the Type-B precipitates which are the largest in number among three types of precipitates in the excel Si alloy. The crystal system of the Type-C precipitate will be discussed elsewhere. The authors propose the crystal lattice of the Type-B precipitate based on the detailed electron diffraction patterns and HRTEM images.

Alloys Design of PdTi-Based Shape Memory Alloys Based on Defect Structures and Site Preference of Ternary Elements

Defect structures and site preference of ternary elements in B2-type PdTi alloys are evaluated in order to design PdTi-based smart materials using the pseudo-ground state analysis (or analysis performed at near zero Kelvin). It has been found that this analysis can predict types of defect structures in these alloys which are of antistructure (substitution) type regardless of alloy compositions. Substitution behavior predicted is as follows: (1) most elements belonging to groups 1A through 5A occupy Ti sites only; (2) most elements belonging to groups 6A through 8A occupy Pd sites only; and (3) the others studied occupy either sites when these sites are unfilled with the corresponding constituent elements (Pd or Ti). Moreover, effects of both offstoichiometry and 3dtransition elemental additions on martensitic transformation start temperature (MS) in PdTi alloys are evaluated as a function of electron-atom ratio (e/a). It has been found that M, is strongly related to the e/a value when 3d electrons of ternary additions are taken into account, although the effect of e/a on M, differs if their substitution behavior is different. Changes in M, per e/a are estimated to be 900 K when Pd-site substitution elements, such as Cr, Mn, Fe, Co, and Ni are added, while changes in M, per e/a are 790 K when Ti-site substitution elements such as V are added. This suggests that M, is influenced not only by the e/a value but also by atomic configurations.