Flow and fracture of particle hardened copper-titanium alloys: Interplay of slip and twinning

The flow and fracture behavior of Cu-rich Cu-Ti alloys hardened by ordered, metastable, coherent {dollar}betaspprime{dollar} precipitates {dollar}rm (Cusb4Ti, D1sb{lcub}a{rcub}{dollar} superstructure) are particularly interesting because these alloys show an unusual propensity to twin profusely after small amounts of plastic flow by slip. The mechanical behavior was studied using standard tensile testing along with modern electron optical techniques including CTEM, SEM and HREM. The deformation substructure in the early stages of aging and in samples aged for longer times is characterized by the coupled motion of five dislocations of the same Burgers vector (superdislocations in the {dollar}rm D1sb{lcub}a{rcub}{dollar} structure) after small degrees of deformation. Characterization of the fine-scale structure of the mechanical twins which form in the particle hardened Cu-Ti alloys is also reported in this study. The growth of deformation twins occurs via the motion of so-called G-ledges at twin/matrix interfaces and via a coalescence of fine twins.; The atomic structure of the coherent twin/matrix interfaces and associated single and multiple layer ledges are also elucidated. Incoherent interfaces at the tip of deformation twins generated by {dollar}90spcirc{dollar} and {dollar}30spcirc{dollar} twinning dislocations have been investigated. The twin/twin and slip/twin interactions have been studied and analyzed as well as the effect of grain boundaries on deformation twinning. Deformation twins appear to nucleate at dislocation pile-ups serving as a stress relaxation mechanism. Shear transfer through obstacle twins is effected by slip and twining. The dominant strengthening mechanism is concluded to be coherency strain hardening and the interplay of particle strengthening and grain size hardening is analyzed. The particle dispersion was found to influence the Hall-Petch factor but a detailed understanding of these synergetic effects was not established. The fracture mode of the underaged Cu-Ti alloys was found to be dimpled ductile tearing.