Ferroelastic Nanostructures and Nanoscale Transitions: Ferroics with Point Defects

For decades, a kind of nanoscale microstructure, known as the premartensitic “tweed structure” or “mottled structure,” has been widely observed in various martensitic or ferroelastic materials prior to their martensitic transformation, but its origin has remained obscure. Recently, a similar nanoscale microstructure also has been reported in highly doped ferroelastic systems, but it does not change into martensite; instead, it undergoes a nanoscale freezing transition—“strain glass” transition—and is frozen into a nanodomained strain glass state. This article provides a concise review of the recent experimental and modeling/simulation effort that is leading to a unified understanding of both premartensitic tweed and strain glass. The discussion shows that the premartensitic tweed or strain glass is characterized by nano-sized quasistatic ferroelastic domains caused by the existence of random point defects or dopants in ferroelastic systems. The mechanisms behind the point-defect-induced nanostructures and glass phenomena will be reviewed, and their significance in ferroic functional materials will be discussed.

Effects of Aging and Thermal Cycling Treatments on the Damping Capacity of Cu-Mn Alloys

ABSTRACTThe effects of aging and thermal cycling treatments on the damping capacity and microstructure of Cu-(47, 55, 65)%Mn alloys and Cu-65%Mn-4.5%Ni alloy have been studied using an optical microscope, a transmission electron microscope (TEM), and a cantilever type damping measuring apparatus. The maximum damping capacity appeared after aging at 400°C for 18 hours in Cu-47%Mn, 8 hours in Cu-55%Mn, 4 hours in Cu-65%Mn, and 36 hours in Cu-65%Mn-4.5%Ni alloy, respectively. Storage at 100°C slightly reduced damping capacity of these alloys, and the higher the Mn content, the smaller the amount of the degradation. This result might be ascribed to α-Mn precipitation inside microtwins in Cu-65%Mn alloy, and to the microstructural change from tweed structure to mottled structure in the other alloys. The thermal cycling treatment between room temperature (20°C) and 250°C led to an increase in damping capacity, which is probably due to the refinement of microstructure(tweed or twin).

Simulation and interpretation of image contrast and diffuse scattering of tweed structure

The so-called tweed structure exhibits a roughly periodic lenticular-domain image (Fig.l) associated with the streaking of diffuse scattering around the fundamental reflections in an electron diffraction pattern (Fig.2(c)). Tweed was observed in high Tc oxides YBa2(Cu1-xMx)3O7-δ (M = Fe, Co, Al, x≥0.03; M = Cu, δ∼0.6-0.8), as well as in some binary alloys, which usually show statistical fluctuation in their composition or in an order parameter.Although tweed has long been studied, crystallographic aspects of tweed modulation is not well understood. To shed light on the details of the structure, we performed simulations both on the tweed image and diffuse scattering, based on our TEM observations of YBa2(Cu1-xMx)3O7-δ

Direct observation of the flux line lattice in Al-doped YBa2Cu3O7-δ

There has been much interest in the effect of partial substitution of Cu atoms in YBa2Cu3O7-δ (YBCO) by dopant atoms such as Fe, Al and Co. It has been found that an increase in the doping level alters the structure of YBCO - the twin domain size drops to 30-100Å for x>0.025 and the domains form the so-called 'tweed' structure. HREM has confirmed that within the domains the structure remains orthorhombic but on a macroscopic scale is quasi-tetragonal. This, in turn, leads to a change in superconducting properties: a rapid decrease in the transition temperature Tc, critical current, etc.In the present contribution the first results are reported on the direct observation of the magnetic flux structure in YBa2(Cu1-xAlx)3O7-δ single crystals (x=0.04) which was also expected to be influenced by Al doping. The flux line (vortex) distribution was observed using the high resolution Bitter pattern technique which involves evaporation of fine ferromagnetic particles (both Fe and Co were used in our experiments) to ‘decorate’ vortices at the surface of a superconductor.