*4.1. The Determination of Coarse Particles in Cu-Cr-Zr System Alloys*

Many investigations on the coarse particle types and their roles in Cu-Cr-Zr system alloys have been carried out by means of phase diagram theory, SEM, TEM and EDS, and great progress has been made. Batra et al. [19] investigated coarse particles with a size of about 0.2 μm × 0.4 μm in the Cu-Cr-Zr alloy by electron diffraction and concluded that the particles were b.c.c structure Cr particles. Club-shaped, hexagonal-shaped and sphericalshaped coarse particles were observed in Cu-0.43Cr-0.17Zr- 0.05Mg-0.05RE alloy by Mu et al. [31], and EDS analysis showed that the coarse particles were Cr phase and (Cu, Zr) compound. Though the coarse Cr phase has been confirmed by most researchers, the type and structure of the (Cu, Zr) compounds is still uncertain. Kawakatsu et al. [32] studied the phase equilibrium at the copper corner of the isothermal diagram in Cu-Cr-Zr alloy and suggested that Zr existed in the form of Cu3Zr in the alloy. Tang et al. [26] considered the coarse particles as a Cu4Zr intermetallic phase in Cu-Cr-Zr-Mg alloy, but other's work on Cu-Cr-Zr alloy implied that Cu51Zr14 should exist [27]. Theoretical studies [33] and SEM experimental studies [34] of Cu-Cr-Zr alloys showed that Cr phase and Cu5Zr intermetallic phase should be present. These results accord well with those of Holzwarth [23], Correia [35] and Sun [36], who investigated the coarse particles in Cu-Cr-Zr alloy with SEM and EDS analysis.

Due to the spatial resolution limitation of electron microscopes in the mode of EDS (e.g., X-ray beam effectively analyzed by SEM-EDS can only be focused to a few hundred nanometers), the previous EDS studies on the coarse phase with a size of a few microns or a few hundred nanometers in Cu-Cr-Zr alloys could only show a qualitative result, and these failed to precise determine the coarse particle types, structure and orientation relationship with the matrix. However, electron beams can be focused to a few nanometers in TEM electron diffraction mode, which greatly improves the accuracy and resolution of phase analysis.

The present work studied the presence of coarse particles in Cu-Cr-Zr-Ni-Si alloy as well as its chemical composition using SEM, TEM and corresponding EDS analysis, and established relations between the types and morphology of coarse particles. Three types of coarse particles were found in the Cu-Cr-Zr-Ni-Si alloy: needle-like (Cu, Zr) particles with a large aspect ratio, nearly spherical (Cu, Cr, Si) particles and lath (Cu, Cr, Zr, Ni, Si)-rich complex particles with a small aspect ratio. It can be inferred that the morphology of coarse particles varies with the composition.

Furthermore, the types, structure and crystallographic orientation relationships of the coarse particles were precisely examined using TEM electron diffraction and central dark field imaging techniques. Needle-like (Cu, Zr) particles were identified to be an f.c.c

structure Cu5Zr intermetallic phase, which belongs to space group F − 43 m (216) with a

lattice parameter of 0.687 nm, having [011]*Cu*//[011]*Cu*5*Zr*, ( − 31 − 1)*Cu*//( − 11 − 1)*Cu*5*Zr* orientation relationship with matrix. The Cu5Zr phase grows approximately along the <111> direction of the copper matrix. As far as the authors know, this is the first time to show the selected area electron diffraction of Cu5Zr intermetallic phase with a zone axis of [011] and determine the orientation relationship with the matrix. The nearly spherical (Cu, Cr, Si) particles were identified as Cr9.1Si0.9 intermetallic phases with a b.c.c structure, which had a typical N–W orientation relationship with the matrix. The relationship is the same as the results reported by Luo [37], Dahmen [38] and Hall [39] in Cu-Cr alloys. From the bright field images and dark field images in Figures 4 and 5, it can also be found that a distinct interface exists between the Cu5Zr phase and Cr9.1Si0.9 phase. In addition, SEM and TEM results showed that the lath-like complex particles were composed of Si, Cr, Ni, Zr and Cu elements; however, these structure types and the orientation relationship need to be further studied.
