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Accelerating materials discovery crucially relies on strategies that efficiently sample the search space to label a pool of unlabeled data. This is important if the available labeled data sets are relatively small compared to the unlabeled data pool. Active learning with efficient sampling methods provides the means to guide the decision making to minimize the number of experiments or iterations required to find targeted properties. We review here different sampling strategies and show how they are utilized within an active learning loop in materials science.

The alloy Ti50(Pd40Cr10) undergoes a strain glass transition around room temperature evidenced by frequency dispersion of dynamic mechanical properties and lack of average structure change from that of the high symmetry austenite phase. However, since the strain glass transition is not a thermodynamic phase transition but a dynamic freezing process governed by the kinetics, a quantitative characterization of the slowing down of dynamics during the strain glass transition is still lacking. In the present study, the probability distribution function (PDF) of the relaxation time of the strain glass alloy is investigated spanning the whole transition temperature range (253 K-313 K). The slowing down of dynamics of the strain glass is indicated by the rapid increase of the characteristic relaxation time ([Formula: see text]) upon cooling. The [Formula: see text], as a function of temperature, shows a transition from Vogel-Fulcher relationship to an Arrhenius relationship. Such a change suggests two fundamentally different states: unfrozen strain glass state and frozen strain glass state. Furthermore, the spread of the PDF is connected to the fraction of quasi-static nanodomains, which helps the understanding of the dynamic freezing process in the strain glass.

We study the plastic deformation of the ω phase which is obtained when Titanium undergoes a phase transformation under pressure. We perform molecular dynamics simulations under uniaxial loading and find that the ω phase not only shows brittle fracture upon loading in the [0001] direction, but also exhibits "superplastic" deformation features along the [100] direction. The brittle fracture is analogous to that which occurs in metallic glass by means of shear banding whereas the ductility is mediated by the α (hcp) to ω (hexagonal) phase transformation. We further show that the elastic deformation of the m phase is anisotropic; it can be non-uniform upon [0001] uniaxial compression. Our results provide insight into the mechanical behaviour of the m phase and imply that the transformation mediated ductility can lead to improvement of the plasticity of co-containing Titanium alloys.

The time dependence of physical properties in the paraelectric phase was probed recently in a Mn(3+) doped (Ba0.8Sr0.2)TiO3 ceramic, providing a simple situation (without spontaneous polarization or domain walls) to quantify the role of the oxygen vacancy during aging. In the present study, we propose a quantitative model for paraelectric aging to understand how the distribution of the oxygen vacancy evolves with time and consequently influences the dielectric response of the paraelectric phase. First, by comparing dielectric behavior of paraelectric aging in a Mn(3+) doped (Ba0.75Sr0.25)TiO3 ceramic and the dielectric tunable effect, an internal bias field E(in) related to the oxygen vacancy is shown to exist in the paraelectric phase. Second, by introducing such a time dependent E(in) in a Landau-type model, we reproduce the dielectric response of Mn(3+) doped (Ba0.8Sr0.2)TiO3 ceramic during paraelectric aging. It is suggested that the increase of dielectric permittivity can be ascribed to the decrease of E(in) with time. The investigation of paraelectric aging is helpful for understanding the role of the oxygen vacancy in influencing the physical properties of ferroelectric materials.

The time evolution of the physical properties of martensite during martensite ageing is traditionally explained by the symmetry-conforming short-range order (SC-SRO) principle, which requires the spatial configuration of crystal defects to follow the symmetry change of the host lattice. In the present study, we show that the volume change of the host lattice also contributes to the ageing effects in Cu-Al-Ni shape memory alloy besides the symmetry change. To substantiate this statement the gradual increase of the storage modulus with time at constant temperature was measured by dynamic mechanical analysis (DMA) and the experimental results were quantitatively described in the framework of the symmetry-conforming Landau theory of martensitic transformations in a crystal with defects. The comparison of experimental and theoretical results confirmed that the time dependence of the storage modulus is caused by two different physical mechanisms. Evaluations showing that the first mechanism is driven by the spontaneous symmetry change and the second mechanism is caused by the volume change after the martensitic transformation was carried out.

We made a detailed comparison of physical properties between the tetragonal-rhombohedral (T-R) phase boundary (in the Ba(SnTi)O3-x(BaCa)TiO3 system) and the tetragonal-orthorhombic (T-O) phase boundary (in the Ba(SnxTi1−x)O3 system). The contrasting results suggest that the T-R phase boundary enhances the piezoelectricity more significantly than the T-O phase boundary. Such difference is considered to stem from the dissimilar anisotropy for polarization rotation, the different elastic softening and the different domain wall contribution between T-R and T-O phase boundaries.

Multiferroicity in CoCr2O4 is generally regarded to emerge from the onset of noncollinear spiral magnetic order (T s ≈ 28 K) due to the inverse Dzyaloshinskii–Moriya interaction, far below the temperature of collinear ferrimagnetic transition (T c ≈ 96 K). So far, the possibility of multiferroicity appearing in collinear magnetic order has been rarely explored. Here, we report a magnetodielectric effect emerging from the onset of collinear ferrimagnetic transition T c in CoCr2O4. Based on the postulation of non-centric crystal symmetry and Landau theory, we deduce that such a magnetodielectric effect is attributed to the coexistence of electric and magnetic ordering by the magnetoelectric coupling below T c. Our results suggest that CoCr2O4 is a natural multiferroic in both noncollinear spiral and collinear magnetic ordering.

We report high pyroelectricity in lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–50BCT) ceramics which were previously reported to exhibit high piezoelectricity. The pyroelectric coefficient measured by the Byer–Roundy method upon heating for dc-poled samples shows a high value of 5.84 × 10−4 C m−2 K−1 at 300 K, comparable to those of PZT-based ceramics and significantly higher than many other lead-free pyroelectric materials. The pyroelectric coefficient calculated from the remanent polarization versus temperature relationship obtained during a cooling process is 17.17 × 10−4 C m−2 K−1 at 300 K, which is considerably larger than the value obtained by the Byer–Roundy method. This difference is attributed to the thermal hysteresis of the rhombohedral–tetragonal phase transition between the heating and cooling processes.

Perovskite ferroelectrics naturally exhibit multiple energetically equivalent domain states; thus, a certain domain state or a certain domain pattern is not expected to reappear once it is disturbed by a phase transition; i.e., there should exist no "domain memory". This letter, however, provides direct evidence with in situ optical microscopy for a domain memory effect in an aged K+-doped BaTiO3 single crystal during a ferroelectric-ferroelectric transition cycle between tetragonal and orthorhombic phase. By contrast, the domain memory effect is absent in an unaged sample. These results suggest that the domain memory effect is associated with point defect migration during aging. Our findings can be explained by a symmetry-conforming tendency of point defects.

Different ferroelectric domains are degenerate states of the same ferroelectric phase; thus they are energetically equivalent and, in principle there exists no preference for a particular domain pattern. However, the existence of point defects is considered to stabilize certain preferential domain states. In order to study the temperature violation on such stabilized domains, we performed in?situ observation on an aged K + -doped (Ba, Sr)TiO3 single crystal and found that both the domain configuration and polarization state can be memorized after experiencing a thermally activated ferro?para?ferro transition cycle, as manifested by a reappearance of the same domain pattern and double P?E hysteresis loop. In contrast, after the sample was aged in the paraelectric state (>10?min), these memory effects disappeared. The above memory effects are considered to originate from the interaction between point defects and the crystal symmetry driven by a symmetry-conforming tendency of point defects. Such a mechanism suggests that the memory effects are relevant to the existence of acceptor dopant and associated mobile oxygen vacancies, and they are not restricted to a particular dopant. Thus similar memory effects are expected to exist in a wide range of ferroelectric materials with acceptor doping.

Very recently, a high-performance Pb-free piezoelectric system Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (abbreviated as BZT–xBCT) has been reported, showing a very high d 33 of 620 pC N−1 at x = 50. However, the Curie temperature T C (for x = 50) is relatively low (93 °C), thus restricting its applications. We report a method to increase the T C of the lead-free system BZT–xBCT, that is, increasing the T C of both tetragonal and rhombohedral terminals. The modified system Ba(Zr0.15Ti0.85)O3–x(Ba0.8Ca0.2)TiO3 showed a higher T C of 114 °C at x = 53. We also found that both d 33 and ε along the morphotropic phase boundary (MPB) decreased with a deviation from the cubic–tetragonal–rhombohedral triple point. This result is explained by considering an increase in polarization rotation barrier with a deviation in composition from the triple point. Our results implied that to achieve a high d 33 with a high T C, a nearly vertical MPB is required. Our work may give a new insight into how to design ‘ideal’ Pb-free piezoelectrics with both high piezoelectricity and high T C.