**2. Materials and methods**

### **2.1 Production of single and bi-crystals**

The Al crystals of several different crystallographic orientations were obtained using a standard Bridgman method, while the Al bi-crystals of crystallite orientations {100}<011>/{110}<001> (Goss/shear) were produced applying a modified Bridgman technique of horizontal crystallization.

The samples of Al single- and bi-crystals, of dimensions 10x10x10mm of various orientations were cut out and subjected to tests of channel-die compression at room and liquid nitrogen temperature (77K) using an INSTRON testing machine equipped with channel-die (Fig. 1), which ensured plastic flow merely in the normal direction (ND) and in the elongation direction (ED), parallel to the channel axis, since the deformation in the transverse direction (TD) was held back by the channel walls.

The samples were deformed in a multi-stage way in order to obtain appropriate values of intermediate and final degrees of deformation. After each deformation stage they were trimmed to satisfy the slenderness ratio. In each case the traverse speed of the testing machine was 0.05 mm/min.

An apparatus recording the AE descriptors was coupled with the testing machine. Both systems are unique, fully computerized set for a simultaneous measurement of external compressive force and AE descriptors. A broad-band piezoelectric sensor recorded AE signals in the range from 100 kHz up to 1 MHz. The contact of the sensor with the sample was maintained with the aid of steel rail which served as a washer in the channel-die, as well as an acoustic wave-guide. In order to eliminate unwanted effects of friction against the wall of the channel, each sample was covered with a Teflon foil. The more detailed description of the AE method is presented in the next section 2.2.

Fig. 1. Scheme of the device for the channel-die compression test
