Microstructural and mechanical properties of Cu-7vol.%ZrB₂ alloy produced by powder metallurgy techniques

1 Introduction

Copper-zirconium alloys are widely used in automotive, electrical, and aerospace industry [1, 2]. In past decades, the interest in copper matrix composites has increased for their potential engineering applications resulting from high values of electrical and thermal conductivity, excellent properties of wear resistance, anticorrosion, and good mechanical properties [3]. Copper alloys with low content of alloying elements offer the highest possible thermal conductivity and very good mechanical properties sufficient to withstand the static and cyclic loads [4, 5]. Today’s developments of alloys and composites with copper matrix are based on intensively researching the three groups of systems [6, 7]: materials in which the precipitates are formed from solid solutions of copper; those in which the precipitates or dispersoids are formed from the stable intermetallic compounds without copper, and materials where the copper matrix is being reinforced by metal borides.

Synthesis of copper-based composites reinforced with nano- and micro-zirconium diboride (ZrB₂) particles is achieved by using the technique of PM. Combining mechanical alloying with hot pressing process can contribute to the development of a potential process for production of advanced materials such as metal borides in copper matrix. The base of this technique is that the mechanically activated particles formed ZrB₂ particles in situ during hot pressing process. Adding ZrB₂ as a hardening phase to the copper matrix can significantly improve mechanical properties, wear and spark resistance, as well as maintain high electrical and thermal conductivity of Cu-based composites [8–11].

The aim of this work was to establish the better microstructural homogeneity in copper matrix and to produce the material with better mechanical properties. The present study examines the influence of synthesis parameters on structural and mechanical properties of Cu-7vol.%ZrB₂ alloy obtained by PM techniques.

2 Materials and methods

The powders used for study as starting materials were copper (99.5% purity, 30 μm, Pometon TIR Ltd, Bor, Serbia), zirconium (99.5% purity, 3 μm, Merck, Darmstadt, Germany), and amorphous boron (97% purity, 12 μm, Heraeus, Hanau, Germany). The particle size of powders was measured in Mastersizer 2000 (Malvern Instruments Ltd, Malvern, UK). Starting powders were weighed to give stoichiometric Cu-7vol.%ZrB₂ and mixture was homogenized for 1 h. The homogenized mixtures were mechanically alloyed in Netzsch (NETZSCH-Feinmahltechnik GmbH, Selb, Germany) attritor mill with ball-to-powder weigh ratio of 5:1. Steel balls (diameter 6 mm) were used in the mill. Attrition milling was carried out in protective atmosphere (argon) for 5, and up to 30 h, with stirring speed of 330 rpm. Mechanically alloyed powders were hot pressed in a graphite mold (10-mm diameter) under a pressure of 35 MPa, with retention time of 2.5 h, at temperature of 950°C. Hot pressing was carried out in ASTRO (Astro Industries, Santa Barbara, California) furnace with the heating rate 15°C/min. Density of the compacts was determined by Archimedes method in water. The powder mixtures and corresponding compacts were characterized by X-ray powder diffraction (XRD) analysis which was performed using a Bruker system 3 SAXS, Ultima IV type 2 (Rigaku, Tokyo, Japan) with CuKα Ni-filtered radiation. Vickers macro hardness of compacts was determined under the load of 1 kg using Buehler Hardness Tester (Buehler, An ITW Company, Lake Bluff, Illinois). Presented values were obtained from an average of 12 indents. Also, the microstructure of powders and compacts was examined with a JEOL-JSM 5800LV (JEOL Ltd, Tokyo, Japan) scanning electron microscope at an accelerating voltage of 20 kV equipped with an EDS. Room-temperature compressive test of the studied alloy was carried out using 1185 Instron^® (High Wycombe, UK)-type testing machine at a strain rate of 1 mm per minute. Two end surfaces of the cylindrical samples were polished to produce parallel surfaces, which are perpendicular to the longitudinal axis with the length/diameter ratio of 2.

3 Results and discussion

Particles of mechanically alloyed powders for two different times of the mechanical treatment are shown on scanning electron (SEM) photomicrographs in Figure 1A–D. After 1 h of homogenization the powders are mechanically alloyed for at least 5 h up to maximum 30 h. The changes in particle sizes are shown in Figure 2. The average particle size of mechanically alloyed powders has increased at longer milling time of 30 h as a consequence of the forming relatively coarse agglomerated particles. SEM analysis showed that during mechanical treatment the particles change their morphology and microstructure as a consequence of repeated deformation, fracturing, and welding. According to photomicrographs shown in Figure 1A,B the shape has become more rounded and the size more uniform as the time of mechanical alloying increased, due to the welding predominance in the attriting process. The particles are soft and they are deformed due to high energy collisions of balls to particles, leading to microforging of powder particles. Forming of characteristic layers during mechanical alloying, as a result of cold welding and deformation processes caused by high pressure on the interface between the particles and balls, is shown in photomicrograph Figure 1C,D. As mechanical alloying time increases the effect of work hardening on brittleness and fracturing of the particles is increased. Also, a great influence on these processes has the presence of Zr and B particles by the amount of dissolved Zr atoms in Cu matrix or entrapment of Zr atoms between Cu particles during ball-particle collisions [12, 13]. After collisions between balls and particles of Cu, Zr, and B during milling, Zr particles with lower ductility were broken faster than Cu particles, and as such caused the breaking of Cu particles.

Figure 1

The shape and size of mechanically alloyed powders for different time periods, SEM photomicrographs: (A) 5 h, (B) 30 h, and SEM photomicrographs of particles with characteristic layers, (C) 5 h, (D) 30 h.

Figure 2

The average particle size of mechanically alloyed powders for different milling time periods.

X-ray analysis of mechanically alloyed powders and compacts displayed that reinforcing ZrB₂ particles are always formed after hot pressing process irrespective of mechanical alloying time (Figure 3). Furthermore, XRD analysis of the compacts showed that lattice constants of copper has been slightly changed compared to powders for 0.083%.

Figure 3

XRD pattern of the Cu-7vol.%ZrB₂ alloy compact.

Microstructure of the hot-pressed samples mechanically alloyed 5 h and 25 h is shown in Figure 4A,B. SEM analysis showed that the distribution and size of ZrB₂ particles in Cu matrix are changing with the increased milling time. The presence of ZrB₂ particles which can be seen in the structure of the copper matrix as an individual or as agglomerates was confirmed by XRD analysis (Figure 3) and EDS (Figure 5). It is assumed that at assigned temperature during hot pressing, entire amount of Zr and B reacts and form in situ particles of ZrB₂. Volume fraction of 7vol.%ZrB₂, which is confirmed by XRD analysis, is calculated by using the rules of mixture applied on starting powder mixture.

Figure 4

Microstructure of compacts made by PM process from powders mechanically alloyed for: (A) 5 h, (B) 25 h.

Figure 5

SEM-EDS image of Cu-7vol.%ZrB₂ alloy compact after 30 h of milling with spectrum diagram.

The basic characteristics of the microstructure of hot-pressed samples are: the presence of individual and agglomerated particles in the Cu matrix, the distribution of reinforcements in the Cu matrix, a specified percentage of residual porosity and clearly defined areas of recrystallized grains. The presence of reinforcing ZrB₂ particles in situ formed during the hot pressing process has already been noted (Figure 3). As these particles can occur in the Cu matrix as an individual or in the form of agglomerates, the expectation that the agglomerations of ZrB₂ particles would disappear during consolidation was not realized. Thus, the residual porosity appears to be a result of the presence of these ZrB₂ agglomerations. The bonding between the particles within agglomerations is weak and the rearrangement that occurred during hot pressing resulted in the pores formation. Comparing the photomicrographs shown in Figure 4A,B it can be seen that with extension of mechanical alloying time homogeneous particle distribution of reinforcements in the structure of metal matrix can be obtained. The fracturing of particles is a dominant mechanism in the process of mechanical alloying at longer time duration. In addition to the more uniform particle size, longer time duration has an important influence on better distribution of reinforcements in the metal matrix, as well as reducing the amount of agglomerated particles. For compacts with shorter time of mechanical alloying some changes in the microstructure may be distinguished, i.e., the dark areas (Figure 5A) indicate recrystallization which occurred during hot pressing process. Also, it must be noted that recrystallization was mostly initiated at the boundaries of the powder particles, but in a smaller extent and it is also visible at the corners of particles where the concentration of stresses imposed during compaction was highest.

The densities of all alloy compacts were above 92% up to 96% of theoretical density of Cu-7vol.%ZrB₂ alloy after hot pressing process. High compact density can be achieved through optimization of the hot pressing parameters: temperatures, pressure, or retention time. However, compared to pure copper compact, the full densification of Cu-7vol.%ZrB₂ alloy was not achieved because of hardening effects due to presence of ZrB₂ particles.

Generally, the mechanical behavior of Cu-7vol.%ZrB₂ alloy was affected by the presence of reinforcement particles dispersed in matrix material which decreased the grain size and increased the density of dislocations. The grain size is reduced by the formation of subgrains when dislocations are rearranged into boundaries within the grain [14]. Subgrains are formed in those grains with high dislocation density surrounding a ZrB₂ particle. Also, ZrB₂ particles act as obstacles to dislocation motion. Figure 6 shows dependence of the macro hardness value on the mechanical alloying time.

An increase in macro hardness occurs as a result of more equal distribution of formed ZrB₂ particles. Also, the curve on Figure 6 shows slight decrease of macro hardness value after 25 h of milling. This decrease is a consequence of an increasing number of relatively coarse agglomerated ZrB₂ particles (Spectrum 1, Figure 5), in Cu matrix for extended milling time. These agglomerates have smaller diameter than agglomerates which are formed in microstructure of compacts with shorter time of mechanical alloying. Increased number of smaller agglomerates enables recrystallization and grain growth in the structure areas without dispersed ZrB₂ particles. The number of areas with smaller recrystallized grains (Spectrum 3, Figure 5), is less than in microstructures of compacts with shorter time of mechanical alloying (Figure 4A) which indicates higher values of macro hardness after 30 h of mechanical alloying than until 20 h.

Figure 6

Influence of milling time on macro hardness values of Cu-7vol.%ZrB₂ alloy compacts.

In accordance with the above mentioned macro hardness results, the influence of milling time on compressive properties remains the same, as it can be seen in the curves on Figure 7. More uniform particle size of mechanically alloyed powders and alloying elements, as well as the better distribution of reinforcements in copper matrix are the result of extended mechanical alloying time. All of these factors have a more dominating influence on compression properties of Cu-7vol.%ZrB₂ alloy than the present recrystallized grains formed during the hot pressing process. Only after 25 h of milling there is a change in the composite microstructure (Figure 5) which led to a decrease of compression strength value. The presence of ZrB₂ particles can be detrimental to the compressive strength of Cu-7vol.%ZrB₂ alloy, especially if any of the following mechanisms has occurred: matrix cracking, particle/matrix debonding, or ZrB₂ particle agglomerate decohesion. Crack path, which can originate from particle/matrix decohesion, fracture of ZrB₂ agglomerate or Cu matrix depends on the homogeneity of alloy microstructure.

Figure 7

Compressive properties of Cu-7vol.%ZrB₂ alloy at room temperature.

Fractographic morphologies induced by compressive fracture are shown in Figure 8A–D. It can be concluded from the images that the fracture process of Cu-7vol.%ZrB₂ alloys depends on the milling time of starting powders. Uniform crack path (Figure 8A) is typical for less homogenous alloys, while zigzag path (Figure 8C) characterizes homogenous alloys. This latter type of crack propagation is a consequence of homogenous distribution of ZrB₂ particles, which block the propagating crack and deflect it. The energy level at the crack tip, which is necessary for its propagation, decreases after deflection and further crack propagation is disabled. Therefore, more energy (larger applied force) has to be introduced to the system for continuation of crack movement. This fact explains why more homogenous alloys have better mechanical properties. Otherwise, when the alloying time is not sufficient for achieving good homogenization of ZrB₂ particles, a crack can propagate easily through the matrix. Agglomerates of ZrB₂ particles have influenced the values of compression strength and their presence was more pronounced in the structure of mechanically treated alloys with shorter milling times (Figure 8B). This figure indicates that an agglomeration of ZrB₂ particles along with the formed micro voids, introduce stress concentration sites. Due to very weak bond between the particles of ZrB₂ in the agglomerates, a particle decohesion will easily occur under applied stress and the crack can easily appear.

Figure 8

Fracture surface of Cu-7vol.%ZrB₂ alloy compacts depending on the milling time: (A), (B) 5 h, (C), (D) 25 h.

4 Conclusions

1. Copper alloy reinforced with in situ formed ZrB₂ particles has been produced by powder metallurgy techniques up to near theoretical density.

2. Morphology and microstructure of mechanically alloyed powders change during milling time as a consequence of deformation and cold welding of the particles.

3. During the hot pressing process of mechanically alloyed powders the micro and nano particles of ZrB₂ reinforcements have occurred.

4. Distribution of ZrB₂ particles and agglomerates in the copper matrix depends on the time of mechanical alloying and displays strong influence on mechanical properties of Cu-7vol.%ZrB₂ alloy.

5. Increasing the time of mechanical alloying up to 25 h leads to an enhancement in macro hardness, compressive and fracture properties.