Amorphization by dislocation accumulation in shear bands

Abstract

Microcrystalline γ-Y₂Si₂O₇ was indented at room temperature and the deformation microstructure was investigated by transmission electron microscopy in the vicinity of the indent. The volume directly beneath the indent comprises nanometer-sized grains delimited by an amorphous phase while dislocations dominate in the periphery either as dense slip bands in the border of the indent or, further away, as individual dislocations. The amorphous layers and the slip bands are a few nanometers thick. They lie along well-defined crystallographic planes. The microstructural organization is consistent with a stress-induced amorphization process whereby, under severe mechanical conditions, the crystal to amorphous transformation is mediated by slip bands containing a high density of dislocations. It is suggested that the damage tolerance of γ-Y₂Si₂O₇, which is exceptional for a ceramic material, benefits from this transformation.

Introduction

Amorphization is a solid-state transformation that can be achieved in almost every category of crystalline materials including metal alloys [1], [2], [3], intermetallics [1], [4], [5], semiconductors [6], [7], [8], [9], [10], ceramics [11], [12], minerals [13] and very few organic compounds [14]. The crystalline to amorphous (c → a) transformation proceeds from the massive displacement of atoms into metastable positions and takes place when the crystal has reached a non-equilibrium state whose free energy is higher than that of the amorphous phase [5], [15], [16], [17]. It is sometimes associated with a shear instability [18]. It can be achieved along several processing routes such as irradiation [12], [19], interdiffusion [3], [20], [21] or dehydration [14]. It can be also activated under an indenter [7], [22], [23] or upon shock-wave loading [11] and other severe mechanical conditions such as large deformation [24], [25], mechanical alloying (MA) and mechanical milling (MM) of crystalline powders [1], [2], [5], [6], [8], [17], [26], [27]. The mechanism involved in the c → a transformation depends on the processing route. In MA, crystal destabilization is not purely a mechanical process but involves a solid-state reaction, whereas in MM, amorphization is thought to result in part from the accumulation of structural defects [17].

Whereas the nucleation of amorphization is not necessarily mediated by crystal defects as shown by Si [7], [9], [10] and B₄C [11], there is repeated evidence of deformation-induced amorphous phases being nucleated at highly distorted lattice sites such as grain boundaries, twins, and dislocations [8], [22]. Grain boundaries have a substantial and even overwhelming influence as this was shown by varying the size of nanoscale grains (for reviews, see Refs. [24], [25]). The part taken by dislocations in deformation-induced amorphization is, on the other hand, rather ill-defined. In addition to being nucleation sites, dislocations are thought to favor the solid-state transformation in enhancing diffusion in the case of metal couples [28]. It has been shown that the dislocation density is dramatically increased under severe deformation conditions such as MA, where dislocations self-organize in shear bands forming cells, thus taking part in grain refinement [29], but there is no indication that the dynamical properties of dislocations such as self-organization as cells, storage or shear localization contribute directly to amorphization. The development of a dense dislocation structure during MM is sometimes reported [30] though not interpreted. In fact, transmission electron microscopy (TEM) observations have almost never been conducted at a scale adequate to pinpoint a direct relation between this and the nucleation of an amorphous phase. In the few documented instances of a c → a transformation forming amorphous layers oriented in the host crystal such as in B₄C [11], dislocations were not observed and it was thus concluded that they played no role. Quartz, on the other hand, transforms differently, depending upon whether it is compressed to high pressure in which case amorphization occurs along with shear, or else shocked, in which case it does not [31]. However, amorphization seems to precede shear band formation. In effect, it is by annealing amorphous lamellae, the dominant deformation structure in naturally shocked α-quartz, that planar deformation structures including dislocation bands, Brazil-twin lamellae and transformation lamellae are generated [13], [32]. On the other hand, depending on load orientation, α-berlinite AlPO₄, which is isostructural to α-quartz, deforms either by individual dislocations or by amorphous shear layers [31]. Under certain load orientations, pervasive amorphous shear lamellae are found in the weak planes of berlinite though no trace of dislocation activity is detected. In brief, while severe deformation normally involves dislocations in great numbers, no direct evidence on the role played collectively by dislocations in deformation-induced amorphization has been given and it is unclear whether there is a relation between mobile dislocation and the c → a transformation.

In this work, the defect distribution in the vicinity of indented γ-Y₂Si₂O₇ samples was investigated by transmission electron microscopy under diffraction contrast and high-resolution imaging. γ-Y₂Si₂O₇ is an interesting ceramic material in several respects. It is a grain boundary phase in Si₃N₄ ceramics doped with Y₂O₃ and/or SiO₂ as sintering aids and, as such, it plays a beneficial role in enhancing the high-temperature mechanical properties of Si₃N₄ [33], [34], [35]. Y₂Si₂O₇, which can be prepared as a single-phase bulk material [36], may be used in harsh environments under corrosive media or fast cooling/heating rates due to its good erosion resistance and low coefficient of thermal expansion [37]. With a melting point of 1775 °C, it is one of the most refractory silicates. Y₂Si₂O₇ is therefore an excellent candidate as a high-temperature structural ceramic as well as an environmental/thermal barrier coating and oxidation protective coating on Si₃N₄ and SiC-based composites. The mechanical properties of Y₂Si₂O₇ are incompletely characterized, in particular the origin of its excellent damage tolerance and good machinability is not understood [38]. In the observations reported below, we investigate the deformation microstructure under indentation and we show that amorphous layers are formed along specific crystallographic planes as a result of considerable strains highly localized in dislocation slip bands.