Synthesis, microstructure, and hardness of rapidly solidified Cu-Cr alloys
Introduction
The material system Cu-Cr is of high relevance for applications requiring a combination of high electrical conductivity [1], corrosion resistance [2] and strength [3] to ensure long service times. Cu with its face-centered cubic (fcc) crystal structure and Cr with its body-centered cubic (bcc) structure form a eutectic system with the eutectic invariant at a temperature of ∼1077 °C at a Cr concentration of 1.56 at.%. The mutual solubility limits are extremely low (≪ 1 at.%) [4]. In the composition range of ∼45–∼75 at.% Cr the liquidus is rather flat indicating a metastable miscibility gap below the liquidus. As a consequence, synthesis of such alloy compositions with conventional casting techniques leads to strong elemental partitioning during the comparatively slow solidification, and microstructures consisting of virtually pure Cr and Cu grains, respectively. In contrast, bulk and thin film Cu-Cr alloys can be fabricated as single phase materials with compositions significantly exceeding the solubility limits of the phase diagram (i.e. as supersaturated single phase materials) giving rise to nanocrystalline microstructures with enhanced mechanical strength [5]. Mechanical alloying of powders [6] and severe plastic deformation [7] are routes to obtain nanocrystalline bulk Cu-Cr alloys. Sauvage et al. [8] and Bachmaier et al. [9] fabricated supersaturated solid solutions by starting with 50 μm sized Cr particles in a Cu matrix exposed to high pressure torsion under high pressure of 6 GPa and very high total strains of up to 45,000%. The severe plastic deformation creates high dislocation densities which are assumed to support the intermixing of Cu and Cr. Grain sizes in the range of 10–20 nm were achieved with supersaturated solid solutions of Cr containing up to 15 at.% Cu for a starting composition of ∼48 at.% Cr and ∼52 at.% Cu. In contrast to the bcc Cr(Cu) phase, the fcc Cu phase remained nearly Cr-free. The differences in chemo-mechanical intermixing of the bcc and fcc phases is not yet fully understood but differences in plasticity, capillary pressures and diffusion coefficients [8] are discussed as the main reasons.
Physical vapor deposition is another synthesis route reported in literature to create supersaturated Cu-Cr alloy compositions which are not expected from the equilibrium phase diagram [10,11]. In that case, a supersaturated solid solution forms due to the rapid condensation of the vapor on the substrate preventing to reach thermodynamic equilibrium. The condensation rate is mainly controlled by the adsorption of the vapor atoms which can occur within picoseconds on the surface and depends on the vibrational frequency, temperature, desorption energy, and surface diffusion [12]. If surface diffusion is largely suppressed by a low substrate temperature, typically below 0.3 of the absolute melting temperature, a supersaturated nanocrystalline microstructure can evolve. Recently, Harzer et al. [13] fabricated supersaturated Cu-Cr films with compositions far from equilibrium. Single phase bcc films evolve for Cu contents as high as 67 at.%, while fcc single phase films grow with Cr contents of up to ∼4 at.%. Cu-Cr alloy films containing 15 at.% Cr were two-phase fcc and bcc with the Cu rich fcc phase containing ∼5 at.% Cr and the bcc phase containing up to ∼60 at.% Cu [13]. The compositions of the fcc and bcc phases for the supersaturated thin films exceed by far the phase compositions realized by severe plastic deformation [8].
Both, the supersaturated thin films and the severe plastically deformed Cu-Cr alloys reveal excellent strength due to the small grain sizes of less than 100 nm. This raises the question of a simple and cheap upscaling route to bulk alloys with supersaturated compositions and small grain sizes. Possible approaches to achieve non-equilibrium microstructures and chemical compositions are rapid solidification techniques such as melt spinning and splat quenching. Laser melting is another approach, which would also indicate if additive manufacturing could be a possible future process route. While several studies in literature analyze a possible miscibility gap in the liquid regime [14,15] or extending the Cr contents in Cu alloys to several at.% to improve precipitation strengthening of Cu [16,17,18], this aspect has not been addressed in the literature. In our study we compare the microstructure formation of two different rapid solidification techniques, splat quenching and laser surface treatment of a Cu-(32 ± 2) at.% Cr master alloy. We selected this composition as it matches well with previously investigated Cu-Cr thin films [13] and is also similar to the composition used for severe plastically deformed bulk samples reported in literature [9]. Laser induced surface melting of bulk samples provides a simple and fast route to locally modify the microstructure of a bulk alloy and serves as a model experiment for 3D printing. Additionally, the convection in the confined melt pool and the rapid heat conduction by the surrounding bulk material may suppress elemental partitioning towards equilibrium phases. The microstructure analyses are performed using X-ray diffraction (XRD), optical and scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) techniques. The mechanical properties were studied using micro- and nanoindentation and related to the corresponding microstructure.
Section snippets
Material synthesis
The base alloy with the nominal composition Cu-(32 ± 2) at.% Cr was produced by melting technically pure Cu and Cr in an induction furnace (16 kW, Al2O3 crucible, Ar atmosphere of 600 mbar) and cast into a Cu mold of 20 mm internal diameter. Several wet chemical analyses of the cast material revealed Cr concentrations between 30.6 at.% (26.5 wt%) and 33.0 at.% (28.7 wt%), indicating inhomogeneous Cr distribution stemming from elemental partitioning during solidification due to the lack of
X-ray diffraction
XRD was used to identify the crystal structures, to determine the lattice parameters and the phase fractions for the two different synthesis routes, i.e. splat quenching and laser melting. The XRD patterns of the Cu-Cr alloys (Fig. 1) reveal that for both synthesis routes fcc and bcc solid solutions coexist. Both samples are composed of two-phases identified as Cu Fm-3m and Cr Im-3m. The lattice parameters for the fcc phase are determined [21] to (3.618 ± 10 – 4) Å (laser molten) and (3.619 ± 10
Discussion
Cu-Cr alloys are well known for forming stable binary compounds [22]. According to the Cu-Cr binary phase diagram [23], the liquid phase is expected to exhibit a metastable miscibility gap [24] leading to the separation into two liquids, one rich in Cu and the other rich in Cr. Our observations indirectly support this by the lack of orientation relationship between the bcc phase and the fcc matrix. Typically, a precipitation process would cause the formation of a low energy orientation
Conclusions
The microstructure of a splat quenched and a laser molten Cu-Cr alloy made from a master alloy with (68 ± 2) at.% Cu and with (32 ± 2) at.% Cr was investigated by XRD, SEM and TEM. Both synthesis routes result in a metastable two-phase microstructure during rapid solidification. The Cu matrix contains ca. 1 at.% Cr in both cases. For splat quenched samples the bcc particles contain 20 at.% Cu while laser melting yield 14 at.% Cu. Splat quenching leads to a fine and homogenous bcc particle size
Acknowledgments
Alena Folger and Stephan Gleich are recognized for their contribution to the EDS measurements, Jürgen Wichert for the laser treatment, Xufei Fang for his measurement of the roughness and Tanja Sondermann for her information technology support.
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