Plasticity of the spinodally decomposing Ni-11.8 at.% Ti alloy

Abstract

The spinodal decomposition of supersaturated Ni -11.8 at.% Ti single crystals (fcc structure) was studied during ageing at 853 and 1073 K. All the samples were homogenised at 1423 K and then waterquenched to room temperature prior to ageing. The quenched samples were found consisting of short-range ordered matrix and slightly L12 -ordered Ti-rich domains.During the first sixty hours of ageing at 853 K, a "metastable situation" was found, where the microstructure of the samples did not change very much. Between sixty and five hundred hours of ageing, concentration waves developed, which grew in amplitude and wavelength with increasing ageing time. Later on, coarsening started, leading to metastableY' particles (L12 -structure), which began to become incoherent after 4000 h of ageing. The kinetics of Ni -11.8 at.% Ti single crystals at 1073 K were investigated in order to study the phase transformation to the stable 9 -Ni3Ti phase (DO24 -structure). Not fully DO24 -ordered nuclei with ABAC stacking sequences were observed. Furthermore, defective stacking sequences, derived from the ABAC stacking sequence, were found to occur frequently. A mechanism based on Shockley partial dislocations is suggested for the transformation of the fcc structure to the ABAC stacking sequence of the DO24 -structure. The mechanical properties of quenched and aged Ni -11.8 at.% Ti single crystals were investigated by compressive, tensile and fatigue tests performed at room temperature. The quenched specimens showed planar glide in all types of tests, and even strain bursts, which are typical for short-range ordered single crystals, were recorded during fatigue tests. Superdislocations and dislocation pile-ups were observed by Transmission electron microscopy in quenched samples, which were deformed in compression. The samples aged at 853 K, which were consisting of a microstructure containing concentration waves, showed a very localised deformation behaviour. Instabilities were observed in compressive tests, and in tensile and fatigue tests a low ductility was noted. In all types of tests a very high yield strength was measured. It may be assumed, that these special mechanical properties are the result of the tendency to nucleate the stablev phase. The dislocation structure of samples consisting of a microstructure with concentration waves, was found to be dominated by regularly dissociated a/2<110> dislocations. This dissociation into Shockley partial dislocations takes place in the Ti-rich domains. The dislocations may be considered to act as microprobes which are very sensitive to the Ti concentration. A model based on planar slip is presented, which describes the formation of defective microtwins, i.e. ABAC stacking sequences containing a/2<110> dislocation segments, out of regularly split, paired dislocations under an external stress in a microstructure consisting of concentration waves. These microtwins are the basic elements of the ABAC stacking sequence of the stable? phase.Article published online by EDP Sciences and av...