Granular shape memory ceramic packings

Abstract

Although bulk shape memory ceramics (SMCs) are brittle, in particulate form they exhibit large recoverable strains in both shape memory and superelastic modes. Here, we investigate the fundamentals of mechanically-and thermally-triggered martensitic transformation of granular SMC packings. Specifically, (ZrO2)1-x-(CeO2)x is studied in three different composition regimes. In the shape memory regime (below the martensite finish temperature), confined uniaxial compression leads to martensite reorientation in the granular SMC packing, with the peak intensity of preferred crystallographic orientation increasing with external loading. In the intermediate regime (between austenite start and martensite start temperatures), confined uniaxial compression leads to irreversible martensitic transformation with the transformed volume increasing with external loading. This provides direct evidence of stress-induced martensitic transformation in granular SMCs. In the superelastic regime (above the austenite finish temperature), confined uniaxial compression leads to forward (during loading) and reverse (during unloading) martensitic transformation, manifesting in a large hysteresis loop in each load-unload cycle with remarkably high energy dissipation density. Based on finite element modeling of SMC particles in contact, we explore the martensitic transformation under non-uniform Hertzian stresses, which in turn provides insight on the experimental results.

In situ investigation of stress-induced martensitic transformation in granular shape memory ceramic packings

“…Increasing the macroscopic load gradually increases the number of particles that experience critical stress, resulting in the observed continuous increase of the martensite phase fraction. Such a continuous mode of transformation has indeed been observed in previous work using ex situ phase characterization [28]. However, without in situ phase characterization, it was unclear how much of the transformed phase reversed during unloading.…”

“…In the granular packing form, fracture reduces the particle size and is much less deleterious because the fractured particles may still be able to undergo SIMT during further mechanical loading as long as the new particle size is above a critical value. Recently, Yu and coworkers have confirmed that significant SIMT can occur in ZrO 2 -CeO 2 shape memory ceramic packings by examining the metastable austenite phase via ex situ characterization [28]. This prior work specifically supports the feasibility of using the granular packing form to scale shape memory ceramics for bulk-level energy dissipation applications.…”

“…Single crystal shape memory ceramic particles were synthesized with two compositions, ZrO 2 -12 at%CeO 2 [24,28]. The specific transformation behavior depends on the relationship between room temperature T 0 and the transformation temperatures, M F (martensite finish), M S (martensite start), A S (austenite start), and A F (austenite finish).…”

“…The powders were fabricated by co-precipitation followed by calcination, high energy ball milling, and annealing, with the synthesis procedures detailed in Ref. [28]. The synthesized particle size ranged from 1 to 5 mm.…”

“…However, the exact transformation characteristics and mechanisms in granular packings are still unclear. First, ex situ characterization in previous work does not offer transformation information during unloading [28], so the questions remain regarding how much of the transformed phase reverses during unloading and whether reversible SIMT is possible in granular packings. Second, because of the complicated dynamic microstructure evolution with load [29], many transformation characteristics cannot be derived by simply examining the loaddisplacement curves.…”