Strain Glass Transition In Ferroelastic Systems: Experimental And Simulation Study

Strain glass transition was recently discovered in ferroelastic systems and show typical glasstransiotn characteristic. With the discovery of strain glass, abnormal properties (theappearance of nano-sized strain domains, frequency dispersion, small hysteresis and freezingprocess…) from that of normal martensitic transition were described. Special applicationswere found based on the abnormal"strain glass"state, such as shape memory, superelasticityand good internal friction. The discovery of strain glass transition not only rich the field ofglass materials or disorder materials, but also open new application potential and promte thefinding and study of abnormal strain states in ferroelastic systems.Recent experiments have revealed that the strain glass transition is a general phenomena indoped ferroelastic systems. Point defects were considered as the key factor for theappearance of strain glass transition. However, the study about the origin of two abnormalstrain states; precursory tweed and strain glass, and their relationship with the well-knownaustenite and martensite (the para- and ferroelastic states) is still unclear. Especially, there isno complete phase diagram which can describe all the abnomal strain state and theirrelationship with the point defects. Therefore, the present thesis focus on the study ofabnormal strain states in ferroelastic systems through experimental and simulation method.Firstly, the strain glass transition was found in a new ferroelastic systems - Ti50Ni50-xFex(atypical shape memory alloys), a complete experimental phase diagram including abnormalprecursory strain state and strain glass state was established. According to our experimentalresults and complete phase diagram, the abnormal strain states in ferroelastic systems can beunderstood through the freezing process of strain glass. Precursory strain states also existsabove the strain glass state, it is actually a partially frozen strain glass state. Theexperimental discovery provide a chance to improve the properties of smart materialsthrough controling the doped point defects. The above experimental understanding and theestablishement of phase diagram supply the fundamental for the seeking of the physical origin of the abnormal strain states in doped ferroelastic systems. A complete theoriticalmodel is needed to explain and predict the abnormal strain states in ferroelastic systems. Insecond part, a Landau free energy model is proposed, which assumes that point defects alterthe global thermodynamic stability of martensite and create local lattice distortions thatinteract with the strain order parameters and break the symmetry of the Landau potential.Phase field simulations based on the model have predicted all the important signatures of astrain glass found in experiment. Moreover, the generic''phase diagram''constructed fromthe simulation results shows clearly the relationships among all the strain states, whichagrees well with experimental measurements.Furthermore, our model can predict that high-density coherent nano-precipitates anddislocations (as strain centers) can also generate strain glass, suggesting that local latticedistortions may play an important role in the formation of strain glasses. In addition, with thediscovery of strain glass state in ferroelastic systems, a general view for the study of glassstates in ferroic materials was possible established. It gives us a chance to supply a possibleway to understand the similar glass states in ferroelectric systems.