**3. Experimental Verification of Dislocation Emission**

We briefly mentioned before the use of TEM in thin films for detecting evidence of dislocation emission from GBs. Additionally, a brief description is given of the experiments, which were conducted to observe in-situ dynamics of dislocation emission from GBs for bulk specimens at elevated temperatures. An uncommonly known technique of photo-emission was used. The instrument used for this purpose, is known as the photo emission electron microscope (PEEM). A brief description of the instrument and an electron micrograph are presented below. Details of the experiment is reported by Li et al. [25]. A schematic diagram of PEEM is shown in Figure 2.

**Figure 2.** Schematic diagram of photoelectron emission microscopy.

A cylindrical sample of a titanium alloy was used as the target in the electron optical system. The sample was heated to a predetermined temperature. It served as a cathode in the electron accelerating field. A three-stage lens system of the microscope, as shown in Figure 2, was used to image the emitting photoelectrons on to a fluorescent screen, and recorded by a video camera, along with a television monitor. Details of the instrument and its applications are described elsewhere [26–35]. This system allows continuous monitoring of changes in the microstructure, in in bulk samples, up to a temperature of 1500 ◦C. Prior to the photo-electron emission experiment, the polished surface of the sample was cleaned by bombarding argon at a glancing angle, conducted in an adjoining chamber. The intensity of emitted electrons is a function of the crystallographic orientations, chemical composition of the sample, and surface topography, during heating or cooling, at a temperature above the transformation of the titanium alloy. A bulge emerging from the boundary appeared, as shown in Figure 3.

**Figure 3.** A bulge formed at a grain boundary of β-Ti6A1-2V-6Zr alloy during rapid heating (photoemission electron microscope PEEM).

The contrast seen in Figure 3 is interpreted as emission of defects from grain boundary ledges. Details of these observations are presented in [25].

Autruffe et al. [36], using TEM, with samples subjected to uniaxial tensile straining showed that dislocation profiles extending from the GBs, and associated with ledges on the boundary plane, increased in frequency with increasing strain. They conclude that GBs are the principal sources for dislocations. In these observations, dislocation profiles resembling dislocation pile-ups were directly observed to form at GB ledges.

Similar and very extensive direct experimental evidence of dislocation emission from GBs have been provided by Murr and his coworkers [7], showing dislocation emission from GB ledges in 304 stainless steel. (see Figure 4).

**Figure 4.** (**a**–**d**) schematics of grain boundary ledge formation, and (**e**,**f**) showing dislocation emission from grain boundary ledges in 304 stainless steel. From Murr [7].
