| Nucleation and growth of new phase and two-phase coexistence are the key signatures of the first-order phase transition(PT)in materials and are widely used as the basic paradigms in the continuum modelling of deformation behavior of shape memory alloys.However,recent study shows that the PT is a multi-physical process which involves material nonlinearity,instability and multi-field coupling at different length and time scales.The current technology has been able to manufacture nanostructured materials by introducing defects such as dislocations and grain boundaries,which enhances the competition between interfacial and bulk energy and eventually brings fundamental changes of the PT behaviors(Ahadi and Sun 2013,2015).In addition to the size-dependent interfacial effect,the rate-dependent self-heating effect during the non-isothermal PT can also increase the energy barrier and suppress the nucleation events.In this paper,some relevant problems are investigated through theoretical analysis and FEM simulation on the multi-scale structure of materials and different physical processes.The researches are carried out as follows:(1)The roles of grain size(lg)and grain boundary thickness(lb)on the stress-induced PT behaviors of nanocrystalline shape memory alloys(SMAs)is investigated by using a Core-shell type "amorphous-crystallite composite" model.A non-dimensionalized length scale lg(= lg/lb)is identified as the governing parameter which is indicative of the energy competition between the crystallite and the grain boundary.Closed form analytical solutions of a reduced effective 1D model with embedded microstructure length scales of lg and lb are presented in this paper.It is shown that,with lg reduction,the energy of the elastic non-transformable grain boundary will gradually become dominant in the PT process,and eventually bring fundamental changes of the deformation behaviors:breakdown of two-phase coexistence and vanishing of superelastic hysteresis.The predictions are supported by experimental data of nanocrystalline NiTi SMAs.(2)The structure instability and self-organization phenomena are investigated by a non-local strain gradient model with non-convex free energy function and by 2D FEM simulations.It is demonstrated that,with the increase of strain rate,the way of phase transition changes from the sequential nucleation and growth mode in the low rates range(≤ 3.0 x 10-2/s)to the emergence of periodic patterns in the medium rates range(3.0 x 10-2/s～3.0 × 100/s)and eventually to stable homogeneous mode in the high rates range(≥ 3.0 ×100/s).In addition,the spacing between neighboring domains is governed by a power-law scaling to the loading rate,where the exponent changes from-1/2(low rates range)to-1/6(medium rates range).Competing length scales of bulk and interfacial energy and competing time scales of heat release and transfer were the core idea to interpret the rate effects on spatiotemporal pattern.The numerical results agree qualitatively well with the experimental observations in thin strips of nano-gained polycrystalline NiTi shape memory alloy.(3)The dynamics of pattern formation and evolution during non-isothermal martensitic transformation is investigated by perturbation analysis.Two key governing parameters are identified:a non-dimensional parameter A0 representing the ratio of intrinsic isothermal softening and thermal hardening,and a non-dimensional external driving parameter k representing the ratio of time scales between heat release and heat conduction.It is found that the nucleation-growth paradigm breaks down due to fast self-heating once the adiabatic elastic modulus becomes positive(A0>1).With the decrease ofk,the mode of PT gradually changes from the nucleation-growth of scattered domains to emergence of periodic domain patterns and eventually to stable and homogenous deformation.Moreover,the scaling law of the spatiotemporal patterns and its experiment verification using Digital Image Correlation method on nano-gained polycrystalline NiTi thin strips are presented. |
