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Woven carbon fibers reinforced Al-matrix composites were successfully prepared through a liquid metallurgy process in pulsed magnet field (PMF). Various wetting conditions and PMF application methods were respectively adopted to reveal the infiltration mechanism, based on which a model and hypothesis of the electromagnetic infiltration were developed. There was an exact match between the metallization depth in electroless plating process and the infiltrating depth in spontaneous infiltrating process, and an obvious improvement in the infiltration result was obtained with the introduction of PMF. The finite-element simulation result indicated that the molten matrix in PMF could offer periodic squeeze and impact effects on the fibers. With the application of PMF, the bending strength of the obtained composite improved from 112 to 203 MPa. Furthermore, the electromagnetic infiltration morphology in PMF possessed the gradient and synchronization characteristics, which was different from the spontaneous infiltration morphology with a step-by-step characteristic.

The trace Hf element (0.2 wt%) was added into the Cu-0.4Cr-0.2Zr (wt.%) alloy to achieve synchronous improvements in strength, ductility and electrical conductivity. The roles of the Hf element in the microstructural evolution and the mechanical and electrical properties were investigated via transmission electron microscopy (TEM), X-ray line broadening analysis and tensile tests. The results showed that the trace Hf element effectively decreased the stacking fault energy of copper alloys. The synergetic effect of the Hf element and the intermediate aging treatment contributed to the formation of high-density dislocations, profuse deformation twins and refined deformation bands in the Cu-0.4Cr-0.2Zr-0.2Hf alloy, thereby enhancing its tensile strength by ∼51 MPa. The addition of Hf also significantly improved the ductility of the copper alloys due to the presence of deformation twins. High strength (628 MPa), high ductility (∼5%) and high electrical conductivity (80.35% IACS) have been achieved simultaneously in the Cu-0.4Cr-0.2Zr-0.2Hf alloy subjected to the intermediate aging treatment.

A two-step cryorolling and aging process was developed to fabricate high strength and high electrical conductivity CuCrZr alloys. The microstructural evolution, mechanical and electrical properties of Cu-1Cr-0.1Zr (wt.%) alloys subjected to the two-step cryorolling and aging process were investigated. The results showed that this two-step process reduced the width of the deformation bands from 113 nm to 80 nm. The introduction of precipitates in the matrix during intermediate aging was responsible for the narrowing of the deformation bands. Even though the cryorolled sample underwent detwinning, its ultimate tensile strength improved by ∼35 MPa. Ordered face-centered cubic (fcc) Cr precipitates ∼5 nm in size were observed in the Cu matrix, and these precipitates had a pinning effect on the motion of boundaries. A desired balance between the ultimate tensile strength (648 MPa) and electrical conductivity (79.80% IACS) was achieved by the coexistence of refined deformation bands, nanoscale deformation twins and nanoprecipitates.

Due to their high strength and high electrical conductivity, CuNiSi system alloys are widely used as lead frames and connectors. In this work, CuNiSi alloys alloyed with Cr and Zr were rolled at liquid nitrogen temperature and subjected to various aging treatments. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis showed that in addition to forming Cr3Si and Ni2SiZr particles, elemental Cr and Zr distributed evenly in the matrix. Ni and Si segregated at the interface between the particles and matrix to form Ni2Si precipitates. During the aging process, Cr alloying increased the hardness while slightly reducing the electrical conductivity of the alloy. This was ascribed to the refined precipitates and hindered precipitation kinetics. Zr alloying promoted both the precipitate growth and the precipitation kinetics of the CuNiSi alloys, while decreasing their hardness and enhancing their electrical conductivity. According to the results of the synchrotron X-ray diffraction (XRD) analysis, Cr alloying increased the dislocation density of CuNiSi alloys and suppressed the annihilation of dislocations during aging. This promoted strength through work hardening. Compared to CuNiSi alloys aged at 723 K for 2 h, the ultimate tensile strength of the alloys obtained after Cr alloying using a two-step rolling aging process was increased by 154 MPa and their electrical conductivity was increased by 4.8% International Annealed Copper Standard (IACS).

The effect of cryorolling on the precipitation process of deformed Cu–Ni–Si alloys was investigated through in situ synchrotron X-ray diffraction technique. The results demonstrate that the precipitation process is significantly accelerated by cryorolling. Cryorolling produces higher dislocation density, which provides more heterogeneous nucleation sites for Ni2Si precipitates, hence promotes precipitation. In the early stage of aging, the enhanced nucleation of precipitates accelerates the depletion of supersaturation, and finer precipitates are obtained. In addition, recrystallization is promoted as a result of high stored energy in the cryorolled Cu–Ni–Si alloys, which facilitates the formation of discontinuous precipitation in the late stage of aging.

The effects of Nb addition on the microstructures and mechanical properties of the casting Cu-9Ni-6Sn alloy were investigated in the present work. Microstructures of the as-cast and heat treated samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that, with the addition of Nb from 0.0 wt% to 0.35 wt%, the average grain size of Cu-9Ni-6Sn alloys in the as-cast condition is reduced from 570 µm to 156 µm due to the formation of Nb-bearing intermetallic compounds. The occurrence of NbNi3 particles after solution treatment is considered as an important factor that leads to the postponed hardening response to age treatment in the early stage. For the peak-aged samples with 0.35 wt% Nb addition, the tensile strength is increased from 768.9 to 913.3 MPa when compared to that of the Nb-free samples. In addition, for the peak-aged samples, the hardness increment is mainly attributed to the coherent stress generated by the ordered γ' phases, which results in practically identical hardness (~350 HV) regardless of the Nb additions. Furthermore, an optimized combination of UTS (719.5 MPa), elongation (13.76%) and electrical conductivity (11.37% IACS) of the Cu-9Ni-6Sn alloy can be achieved by the 0.35 wt% Nb addition accompanied with aging treatment at 375 ℃ for 1 h.

In this work, TiB 2 particulate reinforced copper matrix composites were fabricated by casting method, based on in situ precipitation reaction between B and Ti elements to form TiB 2 particles in molten copper. The microstructures of the Cu-TiB 2 composites were characterized using SEM and TEM. The results show that TiB 2 particles are successfully formed in the copper matrix and the interfaces between these particles and the matrix are clean and well bonded. The friction and wear characteristics of the Cu-TiB 2 composites were determined by carrying out dry sliding tests on pin-on-disk machine under varying loads, sliding speeds and sliding distances. A comparison between the volume wear losses of the composites under different conditions shows a significant improvement in wear property of the composites with respect to the pure copper. Furthermore, the mechanical properties of the composites with different TiB 2 levels were also investigated. Both tensile strength and hardness are significantly improved with the increasing amount of TiB 2 in copper matrix, while compromises of the elongation and electrical conductivity nevertheless occur in all cases.

The microstructure and texture evolution of the room temperature and cryogenically rolled Cu-0.3%Zr alloy were investigated at a range of true strains (0.36, 0.69, 1.20, 2.30) by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The high angle boundary spacing of lamellar grains along the normal direction is mainly dependent on the geometrical effect, and cryorolling has a minor refining effect on the width of the lamellae. However, the deformation twins and shear bands within the lamellar structure of the cryorolled CuZr alloy contribute to the grain refinement and improve the mechanical properties. The cryorolling enhances the inhomogeneous deformation and forms brass-type shear bands. In contrast to the copper-type texture in the room temperature rolled CuZr alloy, the cryorolled CuZr alloy exhibits a typical brass-type texture. This texture discrepancy is mainly attributed to the closely spaced twin/matrix lamellae and brass-type shear bands induced by cryorolling. The twin/matrix lamellae stimulate the initial formation of brass-type texture, and the shear bands promote its development. Unlike the room temperature rolled CuZr alloy, the Goss component is observed in the remnant grains in the cryorolled CuZr alloy. The occurrence of the Goss component may be attributed to the suppression of cross slip during cryorolling.

Cu-1Cr-0.1Zr (wt%) alloy strips were fabricated by a two-stage cryorolling and an intermediate aging treatment. The microstructures, mechanical/electrical properties and precipitation behaviors of the alloy were investigated. The results show that a desired combination of the tensile strength (690.13 MPa) and electrical conductivity (67% IACS) were obtained after the primary 30% thickness reduction and intermediate aging at 450 °C for 2 h followed by a secondary 60% thickness reduction at the cryogenic temperature. The improved strength can be attributed to the interactions of twin boundaries strengthening, grain boundaries strengthening, precipitation strengthening and strain hardening. For the alloy manufactured by cryorolling and intermediate aging treatment, the twin/matrix lamellar thickness decreases 38% and 55% compared with the alloy manufactured by cryorolling and room temperature rolling, respectively. In particular, results from energy-dispersive spectroscopy (EDS) mapping show that a core-shell structure with the shell of Zr and the core of Cr appeared after the intermediately aging treatment.

In situ Cu-TiB2 composites can be synthesized via metallurgical process by mixing Cu-B and Cu-Ti master alloys before casting. In this paper, La, as a rare earth element was added to the composites to improve the comprehensive properties and the effects of its addition on the microstructures, mechanical and electrical characteristics of Cu-TiB2 composites were investigated. Results show that addition of La significantly diminishes the average size and facilitates a homogeneous distribution of TiB2 particles in the copper matrix. As a result, an improvement in mechanical properties was achieved. In particular, a remarkable change in the conductivity of the composites appears with the variation of La content. The mechanisms of particle refinement and improvements in properties by La alloying were analyzed.

Cu-Ni-Si alloys are widely used for electrical applications owing to high strength and high electrical conductivity. In this work, the effects of Cr and Zr additions on microstructure and properties of Cu-Ni-Si alloys were investigated. The addition of Cr and Zr results in formation of Cr 3 Si and Ni 2 SiZr intermetallic compounds, respectively, thus increases the electrical conductivity of Cu-Ni-Si alloy and refines the microstructure. Microstructure analysis confirms the presence of δ-Ni 2 Si precipitates, which strengthens the alloy through Orowan mechanism. Alloying with Zr element deteriorates the mechanical property of Cu-Ni-Si alloy, whereas in the presence of Cr, to the contrary, the ultimate tensile strength is increased, whether Zr is incorporated or not. Moreover, the addition of Zr can decrease the stacking fault energy (SFE) and promote formation of deformation twins. The best integrated performance is obtained through co-addition of Cr and Zr elements. The ultimate strength, elongation and electrical conductivity are 706 MPa, 9.5% and 48.2% IACS, respectively. The precipitation kinetics was discussed in terms of Avrami equation and the activation energy of Ni 2 Si precipitation was obtained. The calculated activation energies of precipitation are 105, 89, 115 and 111 kJ/mol for Cu-Ni-Si, Cu-Ni-Si-Zr, Cu-Ni-Si-Cr and Cu-Ni-Si-Cr-Zr alloy, respectively.

Synchrotron X-ray radiography was used to in situ study the solute distribution and the dendritic growth during the bottom-up solidification of Sn–50 wt%Pb alloy under a traveling magnetic field (TMF) for the first time. The buoyance driven evolution and motion of the plumes containing Sn-rich melt are directly observed in the solidification front before the application of TMF. A forced melt flow from left to right is induced with the application of TMF, which results in the redistribution of the solute concentration (facilitate the solute transportation and reduce the local fluctuations considerably) and the change of the dendrite morphologies (promote/suppress the growth of the secondary arms, remelting and fragmentation of dendrites). Meanwhile, the concentration variations of Sn around the solidification front are quantitatively analyzed through the extraction of gray level from sequenced X-ray images.

In this work, the response of Cu–Cr–Zr alloys to aging under direct current (DC) pulses was investigated. The results demonstrated that well-balanced mechanical properties and electrical conductivity can be achieved in case the alloys were properly treated. The aging behavior of Cu–Cr–Zr alloys is sensitive to DC pulses in the temperature range between 400 and 450 °C, whereas at temperatures out of this range, DC pulses have minor influences. The hardness of aged Cu–Cr–Zr alloy is increased by DC pulses while the electrical conductivity is slightly decreased. The peak hardness is increased by 10 HV after aging at 400 °C for 1 h. With the imposition of DC pulses, the precipitate size of Cr rich precipitates in aged Cu–Cr–Zr alloys is reduced from 238 nm to 178 nm, exhibiting stronger Orowan strengthening. After aging at 450 °C for 2 h with DC pulses, the Cu–Cr–Zr alloy has an average tensile strength of 609 MPa, elongation of 12.8%, and electrical conductivity of 77.6% IACS. A retarding effect of DC pulses on precipitation process is confirmed by synchrotron X-ray diffraction analysis, which is responsible for the slight decrease in electrical conductivity. Keywords: Direct current pulses, Mechanical properties, Electrical conductivity, Precipitation, Synchrotron X-ray radiation, Cu–Cr–Zr alloys

Due to the contradiction between mechanical properties and electrical conductivity, it is not easy to fabricate materials with both high strength and good wear resistance with favourable electrical conductivity for the application of electrical materials. In addition, strength and wear resistance do not always present a uniform growth trend at the same time. Herein, a novel copper matrix composite reinforced by in situ synthesized ZrB2 microparticles and nano Cu5Zr precipitates is successfully prepared by a casting method and sequential heat treatments. The Cu/dual-scale particulate composite possesses a desired trade-off of strength, electrical conductivity and wear resistance. ZrB2 microparticles form from Zr and B elements in copper melts, and nanoscale Cu5Zr precipitates form in the matrix after solid solution and aging treatments. The ZrB2 microparticles, nano Cu5Zr precipitates, and well-bonded interfaces contribute to a high tensile strength of 591 MPa and superior wear resistance, with a relative electrical conductivity of 83.7% International Annealed Copper Standard.

In situ TiB2 particulate reinforced copper matrix composites were prepared using Cu–Ti and Cu–B master alloys. Rotating magnetic field (RMF) was employed to improve the dispersion of in situ TiB2 particles during casting. The influences of RMF on the microstructures, TiB2 distribution and mechanical properties of thus prepared composites were investigated. The results show an improved distribution of TiB2 particles in the copper matrix, which resulted in the improvement of mechanical properties of Cu/TiB2 composites. The ultimate tensile strength and Vickers hardness of Cu–1 wt.% TiB2 with a 30% reduction hot rolling and a 90% reduction cool rolling increased from 405 MPa, 125 HV to 470 MPa, 171 HV by the variation of field currents, respectively. Moreover, the action mechanism of RMF on TiB2 distribution in the copper matrix was explained and analyzed.

The effects of trace La additions on the microstructures and mechanical properties of Cu–0.2 wt% Zr alloy have been investigated. Traditional thermo-mechanical processing was applied to the Cu–Zr alloys. Isochronal aging and isothermal aging were used to optimize the aging conditions. According to the hardness and conductivity tests, the optimized aging condition should be 150 min isothermal holding at 400 °C following 85% cold-rolling. If the as-cast sample is subjected to aging, the addition of La obviously refines the microstructures. After cold-rolling, however, the La addition has no significant effects on the grain size of the aged sample. SEM and TEM are combined to observe the distribution and morphology of the precipitates, which are identified to be Cu5Zr. The addition of La can improve the hardness and conductivity simultaneously in the as-cast samples after aging, while it improves the hardness and strength and decreases the conductivity slightly in the cold-rolled samples after aging. XRD and TEM results show that the lattice parameter and the dislocation density are increased by the addition of La.

Cu-0.3 wt% Zr alloys incorporated with varying ZrB 2 levels were prepared by adjusting Zr/B addition rates via in-situ synthesis. Micro-scale ZrB 2 particles formed through in-situ reactions between Zr and B in copper melt and nano-scale Cu 5 Zr precipitates formed upon aging treatment. The composites thus produced exhibited desired combination of mechanical properties and electrical conductivity. This paper investigates the effects of cryorolling and aging treatment on the microstructures and properties of the composites. Compared with traditional rolling process, the ultimate tensile strength of cryorolled Cu-0.3Zr–1ZrB 2 composites increased from 541.9 MPa to 599.6 MPa without sacrificing too much electrical conductivity. The contributions of different strengthening mechanisms due to the dual-scale particles, twins, and dislocations were quantitatively calculated and the results showed good agreement with the experimentally measured data. Also revealed in this work is that the mechanical performance of Cu-0.3Zr- x ZrB 2 composites is relatively superior with respect to Cu-0.3Zr at 573 K, indicating that ZrB 2 is in favor of enhancing the resistance to thermo-softening.

To address the trade-off between strength and electrical conductivity, we propose a strategy: introducing precipitated particles into a structure composed of deformation twins. A Cu-0.3%Zr alloy was designed to verify our strategy. Zirconium was dissolved into a copper matrix by solution treatment prior to cryorolling and precipitated in the form of Cu5Zr from copper matrix via a subsequent aging treatment. The microstructure evolutions of the processed samples were investigated by transmission electron microscopy and X-ray diffraction analysis, and the mechanical and physical behaviours were evaluated through tensile and electrical conductivity tests. The results demonstrated that superior tensile strength (602.04 MPa) and electrical conductivity (81.4% IACS) was achieved. This strategy provides a new route for balancing the strength and electrical conductivity of copper alloys, which can be developed for large-scale industrial application.

Particulate reinforced metal matrix composites are confronted with the problem of particle aggregation emerging in the process of solidification. It sharply deteriorates the mechanical properties of the composites. In order to improve the particle distribution, in situ TiB2/Cu composites were prepared using Ti and Cu-B master alloys in a vacuum medium frequency induction furnace equipped with a rotating magnetic field (RMF). The effect of RMF magnetic field intensity employed on the particles distribution, mechanical properties and electrical conductivity of the TiB2/Cu composites were investigated. The results show that with the applied RMF, TiB2 particles are homogeneously distributed in the copper matrix, which significantly improves the mechanical properties and electrical conductivity of TiB2/Cu composites. The mechanism of RMF may be ascribed to the following two aspects. On one hand, the electromagnetic body force generated by appropriate RMF drives forced convection in the equatorial plane of composite melt during solidification. On the other hand, a secondary flow in the meridional plane is engendered by a radial pressure gradient, thus making a strong agitation in the melt. These two effects result in a homogenous dispersion of TiB2 particles in the copper matrix, and hence excellent properties of TiB2/Cu composites were obtained.