Formation Of Modulated Structure Research Articles

Age-hardening and formation of modulated structures in austenitic Fe-Mn-Al-C alloys

Age-hardening and formation of modulated structures in austenitic Fe-Mn-Al-C alloys

On the theory of modulated structures

The reasons of the modulated structure formation through the second-order phase transition are briefly reviewed. Particular attention is paid to the relatively new case of the modulated structure formation due to the high-order invariants, linear in the order parameter gradients. Conditions of instability of the uniform state are discussed.

Laws of modulated structure formation in isomorphous decomposing alloys

Abstract Types and parameters of the space distribution of precipitating-phase particles in isomorphous decomposing alloys having a cubic crystalline lattice are determined. For this purpose, an analytical expression for the anisotropic elastic potential is derived, and the elastic energy of the various space distributions is calculated. Results of an electron-microscopy investigation of some isomorphous decomposing alloys are also reported.

Modulated structures and GP Zones in Al-Mg Alloys

The formation of modulated structures and GP zones in Al-Mg alloys aged at low temperatures was investigated using the mechanical properties, electrical resistivity measurements, and high resolution electron microscopy. It was shown in TEM investigations that a modulated structure was present in Al-10 wt pct Mg alloys at the beginning of low temperature aging which grew into GP zones having the Ll2 structure distributed periodically along the [100] direction. The miscibility gaps for GP zone formation and the spinodal temperature were determined from the results of resistivity measurements. It is suggested that in Al-Mg alloys spinodal decomposition can occur at the beginning of low temperature aging.

The formation of modulated structures in Ni-V alloys

The precipitation of the ordered Ni3V phase in binary Ni-V alloys has been studied by employing transmission electron microscopy, X-ray analysis, electrical resistivity and mechanical property measurements. The reaction consists of two stages, the first of which involves the formation of domain wave composition modulations manifested by a tweed microstructure and an accompanying strain field. The second stage is an amplification of the long-range order waves and is manifested by the formation of the ordered Ni3V phase. The strain fields produced by the domain waves interact with the strain field of the Ni3V nuclei leading to the formation of a single variant of tetragonal Ni3V in a given region of the matrix. The strength of the continuously ordered alloys is controlled by the strain in the lattice in agreement with the Mott and Nabarro theory of strengthening.

Structural changes in alnico alloys

The solid solution decomposition of Alnico 5 and Alnico 8 is studied by means of electron microscopy and X-rays. The following sequence of the decomposition process is found: Alnico 5: spherical zones-periodic modulated structure-rod-shaped particles of metastable tetragonal phases-final cubic phases; Alnico 8: the same sequence, but due to great importance of elastic energy at the decomposition, the three-dimensional modulated structure forms even during sharp quenching. The kinetics of modulated structure formation is studied. It is shown that the modulation wavelength increases according to a (time)nlaw. The activation energies of the decomposition process are determined. The spinodal mechanism of decomposition of the alloys in question is discussed.

Precipitation in Ni-12 at. % Ti Alloy

Precipitation behaviour in Ni-12 at. % Ti alloy was studied by means of thin foil transmission electron microscopy and electron diffraction technique. Hardness measurements showed two stage age-hardening at 500°C and 600°C. Thin foil observations revealed that the initial rapid increase in hardness was associated with the presence of superlattice. Slower second hardening was caused by the formation of modulated structure or periodic ordered structure. The increase in hardness by these periodic structures was found to be due to the internal strain hardening as originally suggested by Mott and Nabarro.

Formation of modulated structures in copper-nickel-iron alloys

By means of X-ray diffraction, the formation of modulated structures with periodic variations in composition due to a simple interchange of atoms has been studied in copper-nickel-iron alloys as a function of composition, temperature and time. The results are compared with two models suggested for this type of “exchange transformation”. The reaction characteristics seem to favor a kinetic model rather than the Guinier model. The activation energies for the transformation, evaluated at the first detectable stage and for a later stage, show that both these stages are diffusion-controlled inside as well as outside the spinodal. This indicates that the spinodal is of no real significance with regard to the nucleation process. The variation with temperature and composition of the first wavelength to form in the modulated structure is in agreement with the kinetic model.