Spinodal Alloys Research Articles

Applications of Spinodal Alloys

Applications of Spinodal Alloys

Heat treatments for improved mechanical properties of CuNiFe spinodal alloys

Nonconventional heat treatments were designed to produce spinodal microstructures in two CuNiFe alloys for the improvement of mechanical properties with a reduction in total aging time. The microstructures were characterized by measuring the Curie temperature to determine the composition of the NiFe rich phase, and by TEM to measure the spinodal wavelength. In most cases the material failed intergranularly. However, it was only after the longer aging times that any discontinuous grain boundary coarsening was observed. The measured yield strength of both alloys was found to be proportional to the difference in lattice parameter between the two phases and independent of wavelength and nearly independent of volume fraction.

Lattice Imaging of Spinodal Alloys

Spinodal decomposition in alloys has previously been studied1 by x-ray diffraction and conventional T.E.M., techniques which are used to reveal the wavelengths of the early stage composition modulations through diffracted intensities averaged over a large specimen area. The present paper describes recent progress on DIRECT detection by the lattice imaging technique of the localized variations in lattice parameter characteristic of spinodal decomposition. As in previous studies of ordered alloys this application of lattice imaging avoids the necessity of interpreting severe fringe distortions associated with lattice defects.

Spinodal decomposition in Cu-9wt% Ni-6wt% Sn—II. A critical examination of mechanical strength of spinodal alloys

The origin of mechanical strengthening due to the early stage spinodal decomposition process in a Cu-9 wt.% Ni-6 wt.% Sn alloy has been investigated. The wavelength (λ) and relative amplitude ( A) of the composition fluctuation were determined from the position and integrated intensity of X-ray sidebands for samples aged for varying times at 300°, 350° and 400°C. The incremental yield stress, Δσ (stress measured in tension at 0.01 per cent strain in excess of that measured for the undecomposed alloy) was found to increase substantially during aging (~ 80,000 psi at 400°C). As predicted by Cahn ( Acta Met. 11, 1275 (1963)) Δσ was proportional to A 2 λ: no other proposed hardening mechanism showed any correlation with the experimental data. It is further shown that the Cahn theory can equally well account for the early stage decomposition process in other, previously studied spinodal alloys. Measurements of resistivity change on isothermal aging at the three temperatures investigated were interpreted using the theory of Kolomets and Smirnov ( Fiz. Met. Metalloved. 14, 3 (1962)). The percentage decrease in resistivity was found to be proportional to A 2, as predicted. Accordingly, the product ¦ Δ gr ρ0 ¦ λ may also be used to show confirmation to Cahn's theory.

Spinodal decomposition in Cu-9wt PCT Ni-6wt PCT Sn-II: A critical examination of mechanical strength of spinodal alloys

Abstract The origin of mechanical strengthening due to the early stage spinodal decomposition process in a Cu-9 wt.% Ni-6 wt.% Sn alloy has been investigated. The wavelength (λ) and relative amplitude ( A ) of the composition fluctuation were determined from the position and integrated intensity of X-ray sidebands for samples aged for varying times at 300°, 350° and 400°C. The incremental yield stress, Δσ (stress measured in tension at 0.01 per cent strain in excess of that measured for the undecomposed alloy) was found to increase substantially during aging (~ 80,000 psi at 400°C). As predicted by Cahn ( Acta Met. 11 , 1275 (1963)) Δσ was proportional to A 2 λ : no other proposed hardening mechanism showed any correlation with the experimental data. It is further shown that the Cahn theory can equally well account for the early stage decomposition process in other, previously studied spinodal alloys. Measurements of resistivity change on isothermal aging at the three temperatures investigated were interpreted using the theory of Kolomets and Smirnov ( Fiz. Met. Metalloved. 14 , 3 (1962)). The percentage decrease in resistivity was found to be proportional to A 2 , as predicted. Accordingly, the product ¦ Δ gr ρ0 ¦ λ may also be used to show confirmation to Cahn's theory.

Enhanced strengthening of a spinodal fe-ni-cu alloy by martensitic transformation

A 61 at. Fe-20 at. Ni-19 at. Cu alloy has been designed which can be subsequently transformed to martensite following spinodal heat treatment (aging at 850°C). Morphological changes were followed by transmission electron microscopy.Ms temperatures were estimated by an electrical resistivity technique and microhardness measurements were done after each heat treatment. TheMs temperature increased rapidly, then were unchanged as particle coarsening started and wavelength λ increased. The hardness first increased rapidly, then decreased as λ increased. Although the specimens were brittle, the considerable increases in hardness achieved by martensitic transformation compared to the hardness of purely spinodal alloys indicates that this double phase transformation method may be promising for improving the strength of certain alloys.

Interphase interfaces in spinodal alloys

Detailed electron metallographic studies are being made of interfacial dislocations which are formed to relieve the elastic coherency strains developed upon long aging of spinodal alloys in the systems Cu-Ni-Fe and Cu-Mn-Al. The stresses generated by the coherency strains are sufficiently large to exceed the yield strength of the alloys so that the interface dislocations can be generated by plastic deformation. These slip dislocations accommodate the lattice mismatch at the interphase interfaces. The geometry of the arrangements is determined by the operative slip systems, by the degree of long range order (if any) and by the requirement that the Burgers vector or a large component thereof also lies in the interface plane. The model proposed for the formation of the interface dislocations suggests that during coarsening, the two coherent spinodal phases experience tensile and compressive stresses producing dislocations which glide on slip planes from opposite interfaces and are then incorporated into the interface. As a consequence, the interface planes change their orientation and {100} habit is lost. In Cu-Ni-Fe {110} faces develop and this shape change is expected from the slip model, since in this alloy the dislocations are generated on {111} slip planes and their 1 2 a 〈110〉 Burgers vectors lie in a (001) plane adjacent to the plane of the interface. Rearrangement of the dislocation structures at the interfaces may be accomplished by climb. In Cu-Mn-Al the dislocations lie in the {001} interface and have 〈100〉 Burgers vectors. These can only develop from slip dislocations after combination reactions and climb.

Electron Microscopy of Spinodal Alloys

Transmission microscopy, electron diffraction and scanning microscopy were used to study directly the microstructure of spinodal alloys and to correlate the fracture mode of aged tensile specimens with the microstruetural changes. The kinetics of spinodal decomposition for two copper-nickel-iron alloys were experimentally studied by heat treating specimens as described by Butler and Thomas. The aging times inside the spinodal ranged from one minute to 1000 hours. Thin foils for transmission electron microscopy were prepared by first using a chemical polish containing nitric, hydrochloric and acetic acids and then by electropolishing in a chromic oxide-acetic acid solution.

Magnetic Aging of Spinodal Alloys

The effect of a uniform magnetic field on the mechanism, kinetics, and resulting morphology of a spinodally decomposing alloy is investigated. It is shown that the magnetic field is most effective in forming uniformly spaced rods parallel to the magnetization if the aging is carried out just below the intersection (on the phase diagram) of the Curie temperature and the temperature where the spinodal instability sets in. This condition limits the effective magnetic aging alloys to the composition range near this intersection.