Altmetric: 23Views: 185More detail Article | OPEN Superior room-temperature ductility of typically brittle quasicrystals at small sizes

Abstract

The discovery of quasicrystals three decades ago unveiled a class of matter that exhibits long-range order but lacks translational periodicity. Owing to their unique structures, quasicrystals possess many unusual properties. However, a well-known bottleneck that impedes their widespread application is their intrinsic brittleness: plastic deformation has been found to only be possible at high temperatures or under hydrostatic pressures, and their deformation mechanism at low temperatures is still unclear. Here, we report that typically brittle quasicrystals can exhibit remarkable ductility of over 50% strains and high strengths of ∼4.5 GPa at room temperature and sub-micrometer scales. In contrast to the generally accepted dominant deformation mechanism in quasicrystals—dislocation climb, our observation suggests that dislocation glide may govern plasticity under high-stress and low-temperature conditions. The ability to plastically deform quasicrystals at room temperature should lead to an improved understanding of their deformation mechanism and application in small-scale devices.

Introduction

In materials science, plasticity describes the non-reversible deformation of a solid in response to applied forces and determines the ability of a material to change its shape permanently without breaking. Regular crystalline materials, including most metals and ceramics, are generally plastically deformed through dislocation motion1 or twinning2. The plasticity of amorphous solids, such as metallic glasses, is based on the formation and propagation of shear bands3. In quasicrystals4, despite their lack of periodicity, plastic deformation can also be achieved by dislocation activities5. In contrast to the situation in periodic crystals, every movement of a dislocation in a quasicrystal creates a cloud behind, which is called phason fault6. As a consequence, the dislocation motion gets hindered and the material appears brittle. Although a great variety of quasicrystals have been synthesized7,8, and some have even been discovered in nature9, and found to be technologically interesting10,11,12,13 and useful14, only few of them can be found in applications so far, mainly limited by their poor ductility and formability at room temperature. Hence, improving the room-temperature ductility of quasicrystals is not only of academic interest but also essential for technological applications.

Early studies of the plastic deformation of quasicrystals focused on an easily grown icosahedral quasicrystal, i-Al–Pd–Mn, in the high-temperature regime above ∼600 °C (∼70% of its melting temperature). These studies demonstrated that the plastic deformation of i-Al–Pd–Mn was dominated by dislocation climb—with the Burgers vector out of the plane of dislocation motion, rather than dislocation glide—with the Burgers vector restricted in the plane of dislocation motion15. It is generally believed that dislocation climb is a much easier deformation mode in quasicrystals than dislocation glide16. Although there are some hints that the glide motion may be possible in low-temperature conditions as suggested by numerical simulations17 or under high hydrostatic pressures18, the required stress to activate glide is extremely high, on the order of 1/10 of its shear modulus—a stress level generally leading to fracture without showing any ductility. It has been a long-standing question concerning the deformation mechanism in quasicrystals at room temperature. Despite several investigators have sought to explore the plastic deformation of quasicrystals at or near room temperature using indentation or by confining gas or solid pressures19,20,21,22, so far there has been no common conclusion: the explanations include shear banding similar to metallic glasses23, phase transformation24,25, grain-boundary glide21, pure dislocation climb22, dislocation climb dominant26 and crystallization27. Therefore, one has to conclude that the plastic deformation of quasicrystals under a wide range of temperatures and pressures has been poorly understood—much in contrast to crystalline and amorphous solids. Two fundamental questions are still open: can steady-state plastic deformation be achieved at room temperature? If so, what is the underlying deformation mechanism?

Unveiling room-temperature plasticity in quasicrystals hence relies on a new method to suppress fracture before plastic yielding in a simple loading experiment. Our strategy is to increase the fracture strength over the yield strength in a quasicrystal by reducing the sample size. Although similar methods have been explored for other brittle materials such as ceramics28 and metallic glasses29,30, it has, to our knowledge, not previously been reported for quasicrystals—a large family of unusual solids. In this study, we demonstrate a brittle-to-ductile transition in quasicrystals at room temperature due to a sample size reduction—a submicron-sized quasicrystal pillar exhibits superior ductility at room temperature. Furthermore, we suggest that dislocation glide may control the plastic deformation of quasicrystals at room temperature and attempt to shed light on the underlying deformation mechanism in the low-temperature regime.

Results

A model to predict brittle-to-ductile transition

To estimate at what size range a typically brittle quasicrystal may become ductile, we compared the different deformation mechanisms as a function of the sample size: dislocation activities, crack propagation31and mass transport by diffusion32. We identified three deformation regimes: cracking-controlled, displacive-deformation-controlled (dislocations or shear bands) and diffusion-controlled, as illustrated inFig. 1. We estimated the critical size, rp, for the brittle-to-ductile transition to be ∼500 nm, and the size of the diffusion-controlled zone,rd, to be around 10 nm (see the detailed analysis in Methods section at the end of the article). Our targeted sample size to attain steady-state plasticity thus falls in a range from ∼100 to ∼500 nm.

Figure 1: Deformation map for small-scale i-Al–Pd–Mn quasicrystals.

Semi-quantitative predictions for room temperature deformation. If D>rp, defined as the intersection of the fracture strength, σf (the blue dashed lines), and the yield strength, σy(the black solid line), the material fails by cracking without notable plasticity, following the Griffith’s criterion37, σf=KIc/[α(πa)−1/2] with KIc the fracture toughness of the material, α a geometrical parameter on the order of unity and a the size of pre-existing cracks or flaws. The σf shows a smaller-is-stronger trend. If D<rd, defined as the intersection of σy and σd, the diffusion governs the strength, following  with K, surface diffusivity, , strain rate, and T, temperature. The σd shows a smaller-is-weaker phenomenon. In betweenrp and rd, the curves of σf, σd and σy are crossed and define a zone controlled by displacive deformation. The size range of this zone may vary by flaw sizes and strain rates, as illustrated.