**1. Introduction**

High-strength and high-electrical-conductivity copper alloys have been extensively used in railway contact wires [1,2], integrated circuit lead frame [3], heat exchangers [4] and other electrical and electronic engineering contexts [5]. These kinds of alloys have mainly included Cu-Fe systems, Cu-Ni systems, Cu-Cr systems and Cu-Mg systems [1–7], and Cu-Cr system alloys are potentially the most representative of copper alloys with the best balance of mechanical and electrical properties, especially Cu-Cr-Zr alloys [8–12]. In the past years, many investigations have focused on Cu-Cr-Zr alloys to improve physical and mechanical properties such as strength, conductivity, ductility and thermal stability with alloying, plastic deformation, rotary forging, friction stir processing and equal channel angular pressing [8–12], and the authors of this paper also carried out some studies in this field [13–16]. These previous studies have greatly promoted the developments and applications of Cu-Cr-Zr alloys in various fields.

It has been established unambiguously that Cu-Cr-Zr alloys are mainly strengthened by nanosized precipitates, and the characteristics of the nanosized precipitates such as structure, morphology, crystallographic orientation relationships and precipitation sequence were widely studied in the previous works [17–22]. It was reported that the microstructure had a bimodal particle distribution in Cu-Cr-Zr system alloys, while it has not been reported for the binary Cu-Cr alloy [23,24]. Therefore, some researchers studied the coarse particles in Cu-Cr-Zr system alloys [25–29]. Suzuki et al. [25] considered that the dispersed precipitates located on the {111} plane of Cu were Cu3Zr phase. Tang et al. [26] reported that intermetallic precipitates on the grain boundary in the Cu-Cr-Zr-Mg alloy were Cu4Zr. Huang et al. [27] concluded that the coarse intermetallic particles were Cu51Zr14 with an hcp structure using transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) analysis. Theoretical studies conducted by Ge [28] and Cui [29] showed that the Cu51Zr14, Cu10Zr7, CuZr2, CuZr phase should exist in Cu-Zr metallic glass. Although the previous researchers confirmed that Cr particles and (Cu, Zr) particles were present in Cu-Cr-Zr system alloys by means of scanning electron microscope (SEM) and TEM investigations, there has been no accurate experimental evidence and unanimous agreement on

**Citation:** Xia, C.; Ni, C.; Pang, Y.; Jia, Y.; Deng, S.; Zheng, W. Study of Coarse Particle Types, Structure and Crystallographic Orientation Relationships with Matrix in Cu-Cr-Zr-Ni-Si Alloy. *Crystals* **2023**, *13*, 518. https://doi.org/10.3390/ cryst13030518

Academic Editors: Andrea Di Schino and Claudio Testani

Received: 27 February 2023 Revised: 14 March 2023 Accepted: 15 March 2023 Published: 17 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the types and structure of coarse particles or their crystallographic orientation relationships with the matrix so far.

The present study focuses on the coarse particle types, structure and crystallographic orientation relationship with matrix by means of SEM and TEM combined with EDS analysis, electron diffraction and central dark field imaging technique. The purpose is to clarify the controversies about coarse particles in Cu-Cr-Zr alloys and propose some suggestions for designing and processing of the system alloys.

### **2. Materials and Methods**

Material with a composition of Cu-0.39Cr-0.24Zr-0.12Ni-0.027Si (wt.%) was melted using electrolytic copper, pure chromium, magnesium, silicon and copper-13 wt.% zirconium master alloy in a vacuum-induction melting furnace, and then cast in an iron mold with a size of 35 × 120 × 180 mm. The ingot was planed on both sides to remove surface defects and homogenized at 920 ◦C for 5 h and then hot rolled from 30 mm to 5 mm in thickness, followed by quickly quenching into cold water. Samples were cut and then solution-treated at 960 ◦C for 1 h in an air atmosphere muffle furnace.

Microstructure was characterized using an FEI Sirion 200 scanning electron microscope equipped with EDS. SEM image observations and EDS analyses of coarse particles were operated at a voltage of 15 kV. TEM specimens were mechanically thinned to 0.1 mm and punched into discs of 3 mm in diameter, and then thinned by jet polishing in a 30 vol.% nitric acid and 70 vol.% methanol solution at about −30 ◦C. Microstructure observations, electron diffraction and energy dispersive analyses were carried out using a JEM 2100F transmission electron microscope with an accelerating voltage of 200 kV. Jade software was used to determine the crystal structure and lattice parameters, and CaRIne software was applied to construct the cells of the matrix and precipitated phases and simulate the crystal diffraction pattern under the specific zone axes.
