||The deformation behavior of γ phase LiAlO2 (γ-LAO) single crystal under various loading conditions has been investigated in the present study. The loading conditions used in this study included uniaxial compression, Vickers microhardness, and nanoindentation. In addition, the deformation microstructure was characterized by using various techniques, including optical microscopy (OM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD).|
In the high temperature compression test, it was found that the LiAlO2 single crystal deformed via a twinning process while compressed along  direction at temperatures from 773 K to 1173 K. The twinning elements for the observed deformation twin are summarized as K1: (1-12), K2: (-112), η1: [-111], η2: [1-11], and s = 0.305. No dislocation slips were observed associated with the twinning. The necessary stress for the initiation of twinning increased with decreasing temperature. At 673K, twinning was not found even after loading to a stress of 550 MPa. When the crystal was compressed along  direction, no twin but cracks was observed after loaded to 660 MPa.
By using the nanoindentation test with Berkovich indenter at room temperature, the hardness of the (100) surface is determined to be ~10 GPa, and that of (001) surface is ~12.5 GPa. The values of elastic modulus are determined as 145 GPa and 168 GPa for (100) and (001) surface, respectively. The deformed microstructure induced by the nanoindentation was characterized by examining the cross-section from the volume underneath the indent with transmission electron microscope. The phase transformation from γ-LAO to β-LAO was found in the deformed volume after indentation on the (100) surface. The crystallographic orientation relationship between γ-LAO and β-LAO exhibits a nearly parallel relation of aγ//cβ, bγ//aβ, and cγ//bβ. Two sets of habit plane between these two phases are found as (10-1)γ // (01-1)β and (101)γ // (011)β. After indentation on the (001) surface, a deformed structure consisting of dislocation lines was observed.
The anisotropy of hardness and cracking behavior of γ-LAO were studied by using Vickers indentation test at room temperature. The hardness of the (001) plane is about 10.5 GPa, when the diagonal directions of the indent are along  and , however, it decreases to about 9 GPa when the diagonal directions are along  and [1-10]. The hardness of the (100) plane is about 8.5 GPa when the diagonal directions of the indent are along  and , but it decreases to 6.5 GPa as the diagonal directions change to  and [0-11]. Both radial cracks and lateral cracks were found, which depended on the indenting plane and the orientation of indent. Based on the observation, the crack planes were determined. For indentation loading on the (100) surface, the crack planes are the (001) plane when the indent diagonal directions are along  and [0-11] directions, while (001), (011), and (01-1) planes are the potential cracking planes when the diagonal directions are along the  and  directions. For indentation loading on the (001) surface with diagonal directions along  and [-110], the favorable cracking planes are (110), (-110), (010) and (100), and when the diagonal directions of the indent change to  and  directions, the potential cracking planes are (010) and (100) planes.