Exploitable results The mechanisms of martensite formation and tempering in titanium alloys and their relationship to mechanical property development The aims of the research were to establish the mechanisms of formation of orthorhombic martensites which occur over certain composition ranges in titanium alloys, using X ray diffraction, low angle X ray scattering, transmission electron microscopy (at Imperial College) and calculation and modelling. The mechanical properties (tensile and fatigue) were to be determined (at the University of Hamburg-Harburg) as functions of the heat treatment condition of the martensite, both as quenched and tempered. Except where stated the work was carried out at the Institute of Metal Physics, Kiev. The formation of the orthorhombic structure has been explained as being due to the elastic distortions imposed on the h.c.p lattice as it decomposes into two constituents having different solute contents and coherently bonded in a two dimensionally modulated structure. Calculations of dull interatomic interactions has been employed and predicts bulk modulation in the basal plane different for isomorphous and eutectoid binary systems. It was also shown that when the interactions are strain dominated this results in beta isomorphous systems undergoing a spinodal mode of decomposition whereas a layered modulated structure along the c axis is induced in eutectoid forming systems. The electron microscopy observations of beta isomorphous Ti-Mo and Ti-Nb alloys are consistent with the theoretical predictions. However, modulated structures observed in the titanium-iron system (a eutectoid system) are not predicted theoretically and it is suggested that they result from modulations inherited from the parent beta phase: this proposal remains to be confirmed. In the beta isomorphous Ti-7%Mo alloy the as quenched yield stress is low (~530MPa) but increases very sharply on ageing at 450°C as the modulated structure of the as quenched condition develops further (>100MPa). As this occurs the tensile ductility falls to very low values (1%). S-N data show a 107 cycle fatigue strength of 300MPa, as quenched, rising to about 550MPa after 2 h ageing at 450°C. Further ageing decreases fatigue strength slightly due to development of intense slip bands and easier crack nucleation. È in corso la ricerca di dati su OpenAIRE... Si è verificato un errore durante la ricerca dei dati su OpenAIRE Nessun risultato disponibile
