Cluster-spin glass and ferroelectric relaxors have been observed in defect-containing ferromagnetic systems and ferroelectric systems, respectively. However, it is unclear whether or not an analogous glass state exists in the physically parallel ferroelastic (or martensitic) systems. In the 1990s, theoretical studies suggested that premartensitic tweed could be viewed as a strain glass. However, there has been no experimental verification of this hypothesis. In this paper, we provide an experimental test of this hypothesis by measuring the possible glass signatures in two well-known premartensitic tweed systems prior to their martensitic transformation: one Ni₆₃Al₃₇ and the other Ti₅₀Ni₄₇Fe₃ martensitic alloy. Our experiments show that no glass signatures exist for the premartensitic tweed in both systems. There is no mechanical susceptibility/modulus anomaly in the tweed temperature regime, suggesting no glass transition exists. The tweed remains ergodic, inconsistent with a frozen glass. These two critical experiments show that premartensitic tweed is not a frozen glass state. We demonstrate that strain glass exists in ferroelastic/martensitic systems but only in defect-containing ferroelastic/martensitic systems with defect concentration exceeding a critical value. This strain glass is a mechanical analogue of cluster-spin glass or ferroelectric relaxors, and possesses all the features of a glass. We further show that the tweed is equivalent to an ‘unfrozen state’ of a strain glass. Finally, we demonstrate that the microscopic origin of the strain glass can be easily understood in analogy with the behavior of a ‘defect-containing domino array’.

A frozen short-range, strain-ordered state has been observed in several doped ferroelastic/martensitic alloys. The reported strain glass behavior has been attributed to atomic-scale point defects such as dopant atoms. We report here how nanoscale precipitates can also lead to such glassy behavior. Nanosized, randomly distributed Ti${}_{3}$Ni${}_{4}$-like precipitates, produced by aging/annealing at 473 K for 3 h, prohibit the B2 $\rightarrow${} B19$'${} martensitic transition that occurs in a precipitate-free state. The strain glass transition is characterized by a mechanical susceptibility/modulus anomaly with Vogel-Fulcher type frequency-dependence, ergodicity-breaking, invariance in average structure and nanosized strain domains. Our work emphasizes that heterogeneities or in general disordering effects in ferroelastics will also give rise to signatures characteristic of strain glass behavior.

In ferroelectric and relaxor-ferroelectric materials, piezoelectric and dielectric properties are significantly enhanced at the morphotropic phase boundary (MPB), a boundary between different ferroelectric phases with different macroscopic symmetries. By contrast, in relaxor systems, such an MPB does not exist because relaxors of different compositions possess the same macroscopic symmetry. Here, we report the existence of a morphotropic relaxor boundary (MRB) in the single phase relaxor region of a ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3\text{−}x{\mathrm{BaTiO}}_3$ system, which is a composition-induced boundary between two relaxors with different local polar symmetries (tetragonal versus rhombohedral) but with the same macroscopic cubic symmetry. At the MRB the electrostrain increases by $\sim 3$ times and the permittivity increases by $\sim 1.5$ times over a wide temperature range of more than 100 K, as compared with off-MRB compositions. Our Letter demonstrates that the MRB may become an effective mechanism to enhance the dielectric and electrostrictive properties of relaxors.

Futuristic technologies such as morphing aircrafts and super-strong artificial muscles depend on metal alloys being as strong as ultrahigh-strength steel yet as flexible as a polymer^1,2,3. However, achieving such ‘strong yet flexible’ alloys has proven challenging^4,5,6,7,8,9 because of the inevitable trade-off between strength and flexibility^5,8,10. Here we report a Ti–50.8 at.% Ni strain glass alloy showing a combination of ultrahigh yield strength of σ_y ≈ 1.8 GPa and polymer-like ultralow elastic modulus of E ≈ 10.5 GPa, together with super-large rubber-like elastic strain of approximately 8%. As a result, it possesses a high flexibility figure of merit of σ_y/E ≈ 0.17 compared with existing structural materials. In addition, it can maintain such properties over a wide temperature range of −80 °C to +80 °C and demonstrates excellent fatigue resistance at high strain. The alloy was fabricated by a simple three-step thermomechanical treatment that is scalable to industrial lines, which leads not only to ultrahigh strength because of deformation strengthening, but also to ultralow modulus by the formation of a unique ‘dual-seed strain glass’ microstructure, composed of a strain glass matrix embedded with a small number of aligned R and B19′ martensite ‘seeds’. In situ X-ray diffractometry shows that the polymer-like deformation behaviour of the alloy originates from a nucleation-free reversible transition between strain glass and R and B19′ martensite during loading and unloading. This exotic alloy with the potential for mass producibility may open a new horizon for many futuristic technologies, such as morphing aerospace vehicles, superman-type artificial muscles and artificial organs.

We report that $R$ martensite isothermally forms with time in a solution-treated ${\mathrm{Ti}}_{48.7}{\mathrm{Ni}}_{51.3}$ single crystal. This abnormal formation originates from the growth of a short-range ordered $R$ phase with time, i.e., the “crystallization” of strain glass. The established time-composition-temperature Ti-Ni diagram shows a time evolution of the $R$ phase and composition-temperature phase diagram. The presence or absence of the $R$ phase in this new diagram, as well as in other conditions (like doping Fe or aging), is explained in a unified framework of free-energy landscape. Our finding suggests a new mechanism for the isothermal martensite formation, which could be applied to other metal and ceramic martensitic systems to find new phases and novel properties.