Toughening Ceramics down to Cryogenic Temperatures by Reentrant Strain-Glass Transition

Ceramics, often exhibiting important functional properties like piezoelectricity, superconductivity, and magnetism, are usually mechanically brittle at room temperature and even more brittle at low temperature due to their ionic or covalent bonding nature. The brittleness in their working temperature range (mostly from room down to cryogenic temperatures) has been a limiting factor for the usefulness of these ceramics. In this Letter, we report a surprising “low-temperature toughening” phenomenon in a La-doped ${\mathrm{CaTiO}}_3$ perovskite ceramic, where a $2.5\times$ increase of fracture toughness $K_{\mathrm{IC}}$ from 1.9 to $4.8\mathrm{MPa}{\mathrm{m}}^{1/2}$ occurs when cooling from above room temperature (323 K) down to a cryogenic temperature of 123 K, the lowest temperature our experiment can reach. In situ microscopic observations in combination with macroscopic characterizations show that this desired but counterintuitive phenomenon stems from a reentrant strain-glass transition, during which nanosized orthorhombic ferroelastic domains gradually emerge from the existing tetragonal ferroelastic matrix. The temperature stability of this unique microstructure and its stress-induced transition into the macroscopic orthorhombic phase provide a low-temperature toughening mechanism over a wide temperature range and explain the observed phenomenon. Our finding may open a way to design tough ceramics with a wide temperature range and shed light on the nature of reentrant transitions in other ferroic systems.

Figure 1

(a) Phase diagram of ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_{2x/3}{\mathrm{TiO}}_3$ (CT-100xLa) system, where $T_{C\text{−}T}$, $T_{T\text{−}O}$, and $T_{\mathrm{RSTG}}$ represent the ferroelastic transition temperatures from cubic ($C$) to tetragonal ($T$), $T$ to orthorhombic ($O$), and the reentrant strain-glass transition temperature, respectively. (b) Temperature-dependent fracture toughness ($K_{\mathrm{IC}}$), flexural strength (${\sigma }_f$), and fracture strain (${ϵ}_f$) at fracture point (Fig. S4 [28]) of RSTG sample CT-70La. (c) Temperature-dependent $K_{\mathrm{IC}}$, ${\sigma }_f$, and ${ϵ}_f$ of ferroelastic sample CT-60La.

Figure 2

Macroscopic evidence for RSTG transition in CT-70La. (a) An invariance of the average structure through $T_{\mathrm{RSTG}}$: tetragonal at 473 K (above $T_{\mathrm{RSTG}}$) and 173 K (below $T_{\mathrm{RSTG}}$), while cubic at 1545 K. (b1) Frequency dispersion of both storage modulus dip and internal friction peak. (b2) The enlarged modulus curve around $T_{\mathrm{RSTG}}$. (b3) $T_{\mathrm{RSTG}}$ follows the Vogel-Fulcher relation.

Figure 3

Microscopic evidence of the RSTG in CT-70La. (a),(b) In situ microstructure evolution and diffraction pattern along ${[001]}_C$ zone axis at 573 K (above $T_{\mathrm{RSTG}}$) and 298 K (below $T_{\mathrm{RSTG}}$). Ferroelastic $T$ domain pattern will not change (blue arrows mark the domain walls) while nanodomains appear inside the large ferroelastic domains after RSTG transition, accompanied by the formation of $1/2(110)$ superlattice spots indicated by yellow circles in (b2). (c) High resolution TEM images from 573 K (c1) to 298 K (c2). (c3) Inverse fast Fourier transform (FFT) image shows the nanodomains (yellow dashed lines) are around several to tens nanometers. (c4),(c5) FFT and local symmetry (mm, O phase) of the nanodomains marked by the red square in (c3). (c6),(c7) FFT and local symmetry (4mm, $T$ phase) of the ferroelastic matrix marked by the green square in (c3).

Figure 4

Signatures of the ferroelastic $O$ phase in CT-45La. (a) XRD profiles indicating a structure change from $C$ at 1545 K to T at 773 K and then to $O$ at 298 K. (b) Frequency independent storage modulus dip and internal friction peak around $T_{T\text{−}O}$. (c) In situ microstructure evolution from $T$ phase at 773 K to $O$ phase at 298 K, while the insets are the corresponding selected area diffraction patterns.

Figure 5

Mechanism of low-temperature toughening effect associated with stress-induced RSTG-$O$ transition. (a) Stress-induced formation of $O$ phase in RSTG CT-70La as indicated by the appearance of ${(102)}_O$ peak in the XRD pattern of the fractured surface. (b) Hysteretic-plastic deformation behavior of the RSTG sample and its temperature dependence obtained by compression loading-unloading test. Remnant strain (${ϵ}_r$) increases with lowering temperature below $T_{\mathrm{RSTG}}$. Insets are several representative strain-stress curves of Fig. S7. (c) Schematic illustration of the microscopic origin of the low-temperature toughening effect.