The cellular reaction in dilute copper-titanium alloys
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NUMEROUSstudies of precipitation sequences in crystal phases, a variety of structures and compositions have been proposed) 5 17The present authors have dilute alloys of titanium in copper have been carried out in past years. In general, the emphasis has been placed shown that the fl unit cell which is applicable to the upon the early stages of the aging process, during which precipitate produced in dilute alloys is also orthoa high degree of mechanical strengthening (e.g. rhombic (a = 0.453, b = 0.434, c = 1.293 nm) ~sbut has oy -- 1540 MPa (220 ksi) in a 4.3 wt pet Ti alloy at peak a composition CuaTi. The prototype of the structure has hardness) may be achieved) been suggested to be Au4Zr ~5but the ordering scheme The first microstructural response to aging consists of observed in the latter alloy would not account for the the development of a modulated microstructure i.e. details of the diffraction evidence obtained by the periodic variations in composition and lattice parameter present authors. Modifications to the structure have lying principally along (100) directions.2-7 Associated therefore been proposed, and an attempt made to with such a response are 'sidebands' to the principal explain why the degree of order of the fi precipitates does not appear to be constant. 18,19 reflections in X-ray or electron diffraction patterns, The observations described in the present paper are leading to the suggestion that the initial decomposition occurs spinodally.2,5 the product of a continuing study of interfacial structures and their importance to phase transformations in Overaging leads to the development of discrete prea wide range of alloy systems. The experiments have cipitates of the metastable fl' phase, with the compobeen concentrated upon the production of the equisition Cu4Ti and a D 1a ordered structure (body centered librium phase, and the observations are of interest tetragonal, a = 0.584, c = 0.362 nm). The a(matrix)/fl' orientation relationship is (001)J/(001)~, [310L//[ 100]a,, particularly with regard to the cellular reaction. [T30],//[010]e, .4 Prolonged aging or high temperature transformation produces the equilibrium/3 phase. 2,3,6,8-13/3may appear as grain boundary precipitates, Widmanst~ttten plates or in the form of cellular colonies? In addition, an in-situ EXPERIMENTAL transformation from/3' has been reported: ~The overall A range of dilute alloys of titanium in copper was scheme of the phase transformations is common to made from the pure elements. The results quoted in this material both quenched and aged, and isothermally paper arise from alloys containing 1, 3 and 4.3 wt pct transformed though the kinetic details differ. titanium. These alloys were solution treated at 900 ~ The structure of the fl phase has been almost for half an hour, after homogenization, and were then exclusively reported in the recent phase transformation either isothermally transformed or quenched and aged literature 3,1~ as an orthorhombic unit cell (a = 0.453, b = 0.435, c = 0.517 nm) with composition Cu3Ti. 14How-
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