Effect of continuous extrusion on the microstructure and mechanical properties of a CuCrZr alloy
Introduction
Taking the excellent mechanical properties, high electrical conductivity, and high thermal conductivity, CuCrZr alloys are being considered as the primary candidate material for applications, such as the International Thermonuclear Experimental Reactor first wall, electrical contact, and the trolley contact wire for high speed railway, etc. [1], [2], [3]. Due to the work hardening in cold deformation process and precipitation hardening in aging treatment, CuCrZr alloy could gain high tensile strength. So, higher contents of Cr and Zr are required to reach better mechanical properties. However, this method will bring about metallurgical problems and inevitability decrease the electrical conductivity. Thus, it is required to improve the mechanical properties of the CuCrZr without an increase of alloying element contents [2], [4], [5].
Continuous extrusion forming process [6] is a severe plastic deformation technology, and is considered as an alternative way to improve the strength without losing electrical conductivity significantly. In addition, the advantage of producing significant long continuous products, which is hard for the conventional forming processes, makes the CEF process competitive in industrial production. The CEF technology has been adapted to improve the strength of the CuMg alloys, which are used for high speed railway contact wires, and the grain size can be refined to about 2 μm through recrystallization [7]. Besides, the electrical conductivity of Cu–Mg alloys is not decreased significantly after the CEF process. However, relevant researches on the application of continuous extrusion on CuCrZr alloys almost have not been conducted up to now because the CEF process is complicated for the precipitation of hardened alloy, which not only involves the grain refinement, but also influences the precipitation process.
In the present study, the CuCrZr rods were continually extruded on the TLJ400 equipment. The microstructures, mechanical properties and electrical conductivities of the alloys produced by CEF process are compared with that by the conventional process.
Section snippets
Experiments procedure
The CuCrZr alloy was produced in a vacuum induction furnace by adding electrolytic copper, CuCr intermediate alloy and pure Zr. The billets were hot extruded into a diameter of 20 mm. The compositions of experimental alloys are analyzed and listed in Table 1.
After solution annealing (1233 K for 2 h), the as-hot-extruded rods were subjected to the corresponding processing, respectively:
- (A)
Cold drawing (75%) and aging (723 K for 2 h);
- (B)
CEF process, cold drawing (75%) and aging (723 K for 2 h).
For the CEF
Microstructure
Fig. 1 shows the microstructure of Cu–0.16Cr–0.12Zr alloy after solution treated at 1233 K for 2 h. A homogeneously distributed and coarse grained microstructure can be found in the alloy with a grain size of about 400 μm. Within the grains, annealing twins can be observed clearly.
The microstructure of Cu–0.16Cr–0.12Zr alloy after the CEF process is shown in Fig. 2. Large equiaxed grains obtained from solution treatment disappeared, instead, a deformed morphology with intense shear bands (SB)
Grain refinement of continuous extrusion
The EBSD result of continuous extruded rod is shown in Fig. 4. The microstructure is elongated along the extrusion direction. Most of the grains are very fine and in a sub-micron scale. According to the EBSD data, grains are divided into recrystallized, sub-structured and deformed ones depending upon the grain-to-grain misorientation angles and marked by different colors. Thus, it can be seen in Fig. 4 that the sub-structured grains (marked by yellow regions) are dominating here, with a
Conclusions
- 1.
Cu–0.16Cr–0.12Zr alloy with continuous extrusion forming process reaches an high strength of 590 MPa and an elongation of 17.2%, which are 40 MPa and 1.5% higher than that of the alloy with the traditional hot working process, respectively. Meanwhile, the electrical conductivity has a negligible drop from 78.7% to 77.6% IACS.
- 2.
Ultrafine (sub)grains of sub-micron scale are obtained by continuous dynamic recrystallization after continuous extrusion process, which has great contribution to the
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