Analysis of microstructural effects on mechanical properties of copper alloys 2017 Journal of Science Advanced Materials...
Journal of Science: Advanced Materials and Devices (2017) 128e139 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Analysis of microstructural effects on mechanical properties of copper alloys Mitsuhiro Okayasu a, *, Takuya Muranaga a, Ayana Endo b a b Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama, 700-8530, Japan Department of Materials Science and Engineering, Ehime University, Bunkyo-cho, Matsuyama, Ehime, 790-8577, Japan a r t i c l e i n f o a b s t r a c t Article history: Received 16 November 2016 Received in revised form 17 December 2016 Accepted 22 December 2016 Available online 30 December 2016 With the aim of obtaining copper alloys with favorable mechanical properties (high strength and high ductility) for various engineering applications, the microstructural characteristics of two conventional copper alloys d an aluminum bronze (AlBC; CueAl9.3eFe3.8eNi2eMn0.8) and a brass (HB: CueAl4eZn25 eFe3eMn3.8) d and a recently developed aluminum bronze (CADZ: CueAl10.5eFe3.1eNi3.5eMn1.1eSn3.7), were controlled by subjecting the alloys to two different processes (rolling and casting) under various conditions For the rolling process, the rolling rate and temperature were varied, whereas for the casting process, the solidification rate was varied Microstructural characteristics, as examined by electron backscatter diffraction analysis, were found to differ among the alloys Complicated microstructures formed in CADZ led to high hardness and high tensile strength (sUTS), but low ductility (εf) For CADZ, casting at a high solidification rate allowed an increase in ductility to be obtained as a result of finegrained structure and low internal stress In contrast, high ductility (with a fracture strain of more than 30%) was found for both cast AlBC and cast HB; moreover, both of these alloys possessed high tensile strength when produced by warm rolling at 473 K For CADZ, on the other hand, no clear effect of rolling on tensile strength could be found, owing to the many microcracks caused by its brittleness The results of this study indicate that copper alloys with excellent mechanical properties can be produced This is especially the case for the conventional alloys, with a high tensile strength sUTS ¼ 900 MPa and a high fracture strain εf ¼ 10% being obtained for warm-rolled brass © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Bronze Brass Strength Ductility Microstructural characteristics Introduction Copper alloys, including bronzes, are currently employed in a wide range of engineering applications because of their high ductility, high corrosion resistance, non-magnetic properties, excellent machinability, and high hardness [1] Copper is used for electric wiring and in heat exchangers, pumps, tubing, and several other products, while aluminum bronze and high-strength brass are found in marine applications, for example in propellers and propeller shafts [2] Furthermore, shiny brass is widely employed for coins and for musical instruments However, in spite of their excellent material characteristics, there is still scope for technical improvements to increase the strength and ductility of these alloys To achieve improvements in mechanical strength, several copper * Corresponding author Fax: ỵ81 86 251 8025 E-mail address: mitsuhiro.okayasu@utoronto.ca (M Okayasu) Peer review under responsibility of Vietnam National University, Hanoi alloys with high dislocation density and fine microstructure, containing solid solutions, have been proposed The mechanical strength of ultrafine-grained or nanocrystalline CueAl alloys, prepared by equal-channel angular pressing (ECAP), has been investigated, and the strength and uniform elongation of these alloys have been simultaneously improved by lowering the stacking fault energy [3] The hardness of even nanocrystalline copper with grain size as small as 10 nm still follows the HallePetch relation [4] A variety of methods have been used to make high-strength copper alloys Maki et al [5] attempted to create a higher-strength CueMg alloy through a solid-solution hardening effect, in which supersaturation with Mg increases the strength compared with that of a representative solid-solution CueSn alloy [5] A high tensile strength of 600 MPa was reported by Sarma et al [6], who produced a CueAl alloy with ultrafine-grained microstructure and very fine annealing twins by cryorolling and annealing at 523 K for 15 The higher strength of this CueAl alloy was interpreted in terms of the enhanced solid-solution strengthening effect of Al, which is about 1.7 times higher than the corresponding effect in CueZn alloys http://dx.doi.org/10.1016/j.jsamd.2016.12.003 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig Maximum rolling rates for copper alloys at different temperatures [6] In recent years, CueZn30eAl0.8 alloys exhibiting nanostructure have been fabricated by cryomilling of brass powders and subsequent spark plasma sintering [7] Such alloys have a high compressive yield strength of 950 MPa, which is much higher than 129 the values of 200e400 MPa found in commercially available alloys This increase in mechanical strength has been attributed to precipitation hardening and grain boundary strengthening [7] The effect of grain size on yield stress was examined in polycrystalline copper and CueAl alloys at 77 and 293 K, and the yield stress was found to satisfy the HallePetch relation in both materials [8] The influence of hydrogen on the mechanical properties of aluminum bronze was investigated, and it was found that neither tensile nor fatigue properties were affected [9] After low-temperature thermal treatment, strained CueAl alloys exhibited high mechanical strength, which is caused by increases both in the degree of order and in the electron-to-atom (e/a) ratio [10] The effects of microstructural characteristics on the mechanical strength of CueNi26eZn17 alloy were investigated, and it was found that solidsolution strengthening of the alloy was affected by the interaction of Ni and Zn atoms with screw dislocations and by the effective interaction caused by the modulus mismatch [11] In order to understand the material properties of copper alloys, it is important to investigate their microstructural characteristics, including texture The textures of copper alloys after rolling and recrystallization were analyzed by electron backscatter diffraction analysis (EBSD) [12] The evaluation of grain boundaries in copper bicrystals during one- Fig Optical micrographs of copper alloys made by casting and by rolling: (a) CADZ; (b) AlBC; (c) HB 130 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig (continued) pass ECAP was systematically investigated by several methods, including EBSD [13] The above literature survey shows that there are various approaches that can be adopted to improve the mechanical properties of copper alloys, including grain refinement, solid solutions, and high dislocation density In many practical applications, it is desirable to reduce the weight of components and structures made from such alloys by enhancing their mechanical properties Thus, in the present work, an attempt is made to create copper alloys with favorable tensile properties (high strength and ductility) via microstructural modification using forging and casting processes under various conditions To analyze the mechanical strength and ductility of these alloys, their microstructural characteristics are investigated by EBSD Experimental 2.1 Sample preparations Two commercial copper alloys, namely, an aluminum bronze (AlBC: CueAl9.3eFe3.8eNi2eMn0.8) and a brass (HB: CueAl4eZn25eFe3eMn3.8), were studied, as well as a newly developed aluminum bronze (CADZ: CueAl10.5eFe3.1eNi3.2e Mn1.1eSn3.7) It should be pointed out that CADZ was developed on the basis of a CueAl10.5 alloy in Dozen-Kogyo Co Ltd The material characteristics of CADZ were originally developed by described in detail elsewhere [14] The test samples of the alloys were produced by casting and forging (rolling) In the casting process, two different cooling rates, and thus solidification speeds, were adopted At the low cooling rate (slow cooling, SC: 20 K/s), the melts were solidified slowly in a furnace In this case, the solidification process was carried out under an argon gas atmosphere to prevent oxidation At the high cooling rate (rapid cooling, RC: 150 K/s), the melts were solidified rapidly in a copper mold The solidification speeds for both the rapid and slow cooling processes were measured directly using a thermocouple In the rolling process, the alloys were forged at different deformation rates, using a 10-ton twinrolling machine (Yoshida Kinen Co., Ltd.) with high-strength rollers made of hot rolled steel (SKD11: 150 mm diameter  200 mm) Samples of thickness 10 mm were forged under severe deformation at different temperatures: 293 K (cold rolling, CF), 493 K (warm rolling, WF), and 1073 K (hot rolling, HF) M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 131 Fig (continued) 2.2 Material properties Results and discussion Tensile tests were conducted at room temperature using a hydraulic servo-controlled testing machine with 50 kN capacity Rectangular dumbbell-shaped specimens were employed with dimensions mm  20 mm  mm The loading speed was set at mm/min until final failure The tensile properties (ultimate tensile strength sUTS and fracture strain εf) were evaluated via tensile stress versus tensile strain curves, which were monitored by a data acquisition system in conjunction with a computer through a standard load cell and strain gauge Hardness measurements were made using a micro-Vickers tester at 2.94 N for 15 s In this test, a diamond indenter was loaded manually at about 0.3 N/s to the sample surface, which had been polished to a mirror finish The microstructural and lattice characteristics of the alloys were investigated by EBSD using a field emission scanning electron microscope (SEM; JEOL JSM-7000F), with an acceleration voltage of 15 kV, a beam current of nA, and a step size of 20 mm The samples were sectioned to less than 10 mm thick, and the sample faces for the observation were polished to a mirror finish in a vibropolisher, using colloidal silica for no longer than h 3.1 Microstructural characteristics Fig shows the maximum possible rolling rates for the alloys With the cold-rolling process, the maximum rolling rate of CADZ is about 9%, which is about 50% and 33% lower than those for HB and AlBC, respectively With the warm-rolling process, the rolling rate is still as low as 12% for CADZ, although severe deformation of more than 75% is obtained for HB and AlBC after warm rolling With the hot-rolling process, a high rolling rate of more than 75% is obtained for the three alloys Fig shows optical micrographs of the three alloys, made using both the casting and the rolling processes Essentially, the three alloys consist of matrix and eutectic structures The main eutectic phases of the CADZ sample are found to be (Fe, Ni)3Cu, CueNieSn and CueAl [14], as indicated by the arrows in cast CADZ The AlBC sample is essentially formed from eutectic Fe-, CueAleNi-, and CueAl-based phases, while for the HB sample, eutectic Fe-, CueZneAl-, and CueZn-based phases are observed The grain size clearly varies for all the cast alloys, where the higher the cooling 132 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig (aec) Inverse pole figure (IPF) and misorientation angle maps of (a) CADZ, (b) AlBC, and (c) HB (d) IPF map for CADZ with and without Fe element rate, the smaller the grain size For the rolled samples, no clear changes in grain size can be detected, especially for CADZ This could be due to the low rolling rates for CF- and WF-CADZ On the other hand, grain growth (or recrystallization) occurs for HF-CADZ and for HF-HB For CF- and WF-AlBC, slightly strained microstructural formations can be seen To understand these microstructural characteristics in detail, an EBSD analysis was carried out Fig displays the inverse pole figure (IPF) and misorientation (MO) angle maps of the cast alloys, obtained by EBSD As can be seen, complicated microstructures with high MO angles are formed almost throughout both the cast and rolled CADZ samples In contrast, high MO angles are found mainly in the eutectic phases of AlBC, whereas high MO angles are widely distributed in RC- and CF- AlBC Similar trends are observed in the corresponding HB samples The MO angles for the CADZ samples are overall higher than those for AlBC and HB The higher MO angles for the CF samples are considered to be due to increased dislocation density, while the low MO angles for the HF samples result from a reduction in internal stress due to the high-temperature processing In addition, it is notable that deformation twins can be clearly detected in the rolled AIBC samples, but not in the others This can be attributed to the different extents of stacking fault energy (SFE): the lower the SFE, the weaker the deformation twins In previous work, it has been reported that the SFE decreases with an increasing proportion of Al in the alloy composition: for example, the SFE of CueAl2.3 alloy is about times higher than that of CueAl11.6 alloy [15] Since it has M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig (continued) 133 134 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig (continued) M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 135 Fig (continued) Table Grain sizes of the copper alloys CADZ, AlBC, and HB (SD: standard deviation) Cu alloys Casting process Rapid cooling (mm) As-cast (mm) Slow cooling (mm) CADZ AlBC HB 38 (SD: 8.6 mm) 132 (SD: 16.3 mm) 220 (SD: 34.6 mm) 15 (SD: 10.9 mm) 45 (SD: 14.0 mm) 60 (SD: 52.1 mm) 38 (SD: 4.6 mm) 81 (SD: 19.7 mm) 680 (SD: 176.1 mm) been reported that the SFE of CueZn24 alloy is about times higher than that of CueAl8 alloy [16], our AIBC should have a much lower SFE compared with HB, leading to deformation twinning in the former but not the latter The grain size of the cast alloys was measured directly, and the results are summarized in Table It should be pointed out first that for the Cu-based phases, measurements were made of straight diagonal lines on each grain, and the grain size was determined as the mean value of more than 50 measurement data Since grain formation is not clearly seen for CADZ, image analysis was conducted on the cast CADZ, with the Fe element being removed from the IPF maps; see Fig 3(d) From Table 1, it can be seen that the grain size varies, depending on the sample and the casting speed The differences in microstructural characteristics lead to differences in mechanical properties The average grain size of the alloys made by rapid cooling is less than 38 mm The grain size increases with Fig (a) Vickers hardness of copper alloys made by casting (b) Relationship between Vickers hardness and grain size 136 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig Vickers hardness of copper alloys made by rolling: (a) CADZ; (b) AlBC; (c) HB decreasing cooling rate: for example, for HB, a large grain size of 680 mm is obtained, which is more than 10 times greater than that for AIBC 3.2 Mechanical properties Figs and show Vickers hardness data for the three alloys made by rolling and casting processes under different conditions For the cast samples shown in Fig 4(a), a high hardness is obtained overall for CADZ: for example, the value of about 2.5 GPa for AS-CADZ is about 15% and 70% higher than those for AS-HB and AS-AlBC, respectively An improvement in hardness is obvious for all the alloys with a higher solidification rate (the RC samples): for example, for RC-CADZ, the hardness is as high as 3.3 GPa, which is more than 1.4 times that for SC-CADZ On the other hand, the lowest hardness of about 1.4 GPa is obtained for the SC-AlBC samples These differences in hardness are due to a number of reasons, including the fact that different grain sizes lead to different grain boundary strengths Fig 4(b) shows the relationship between grain size and hardness for the alloys Although there are only a few data points, clear correlations can be seen, and the HallePetch relation appears to be satisfied A similar HallePetch relation is also obtained for nanocrystalline copper (10 nm) [4] For the rolled samples shown in Fig 5, the hardness value increases with increasing rolling rate and decreasing rolling temperature These trends are presumably due to the differences in dislocation density, deformation twinning, and internal stress arising during the rolling process, as indicated by the distributions of MO angles seen in Fig In particular, a high hardness is obtained for the cold-rolling process, owing to dislocation tangling, despite the low rolling rate On the other hand, the low hardness of the samples made by hot rolling is a consequence of their recrystallization and grain growth, as described previously It should also be pointed out that the deformation characteristics of AIBC and HB can vary depending on the SFE, as mentioned above In general, it appears that deformation twining occurs for the alloys with lower SFE, namely, the AIBC samples This deformation occurs when dislocation is dominated by the rolling process, i.e., work hardening occurs [17] Fig shows representative tensile stress versus tensile strain curves for the three alloys made by rolling and by casting, while Fig summarizes their tensile properties in terms of ultimate tensile strength versus fracture strain It should be pointed out that more than three specimens were employed here to obtain the tensile properties From the stressestrain curves, it can be seen that high ductility is obtained for the cast samples, with the fracture strain for AIBC being higher than that for HB and CADZ The reason for this is the presence of deformation twinning in AIBC, as mentioned above Huang et al [18] reported that the deformation twins in coarse-grained Cu occurred mainly in shear bands and at their intersections, as a result of the very high local stress caused by M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 severe plastic deformation On the other hand, a high tensile strength is obtained overall for the rolled samples compared with the cast ones In particular, higher tensile strengths sUTS are obtained overall for AlBC and HB made at a high rolling rate and a rapid cooling rate The highest sUTS values (>900 MPa) are obtained for WF-AlBC, WF-HB, and RC-CADZ On the other hand, low sUTS values are found for HF-HB and HF-AlBC, even when high rolling rates were applied The data plots of tensile properties are relatively scattered for CADZ, which may be due to the low sample quality 137 Fig shows an SEM image of the HF-CADZ sample after rolling but before the tensile test As can be seen, several microcracks have been generated along the grain boundaries, as indicated by the dashed lines Such microcracks could lead to a deterioration in mechanical properties It should be noted that no clear microcracks were detected in the other rolled alloys, because of their high ductility For the cast samples in Fig 7, higher tensile strengths are obtained for the alloys made at a high solidification rate (the RC Fig Stressestrain curves for copper alloys made by rolling and by casting: (a) CADZ; (b) AlBC; (c) HB 138 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 alloys) For cast CADZ, the highest sUTS value is obtained for RCCADZ, and is higher than the value for the corresponding rolled alloy This may be a consequence of the fine-grained structure as well as the high sample quality (with no microcracks) The tensile strength of the cast samples decreases with decreasing solidification rate Unlike the tensile strength of CADZ, high tensile strengths of both HB and AlBC result from cold and warm rolling at a high rolling rate In addition, the rolled AlBC and HB alloys (e.g., the CF and WF samples) show a raised ductility εf of more than 15%, although this strain value is lower than those for the RCAlBC and RC-HB samples From this result, it can be considered that the cast and rolled samples are overall located on the rightand left-hand sides, respectively On the other hand, no clear trend in tensile properties is seen for CADZ This may be due to its low Fig SEM image of the hot-rolled CADZ sample, showing some microcracks deformability and the microcracks generated by the rolling process, as mentioned above Conclusion The mechanical properties of copper alloys made by different processes have been investigated The results can be summarized as follows: The mechanical properties of the alloys depend on the production process: rolling or casting For the CADZ alloy, high mechanical strength was obtained for the rapidly cooled cast sample, although low ductility was found High ductility (> 30% in some cases) was obtained for cast AlBC and HB alloys High tensile strength with high ductility was obtained by warm rolling at a high rolling rate, especially for HB and AlBC The high hardness of the CADZ alloy was attributed to severe lattice strains almost throughout the material Vickers hardness was clearly related to grain size for all three alloys, with larger grains leading to lower hardness, i.e., the HallePetch relationship The CADZ alloy could not be subjected to intense rolling owing to its brittleness, arising from its complicated microstructure A large number of microcracks were created in rolled CADZ, resulting in reduced tensile strength On the other hand, intense rolling was possible for the HB and AlBC alloys, allowing samples to be produced with high strength and high ductility Acknowledgements The authors appreciate financial support from the Japan Copper and Brass Association, and the Cu alloys used in the present work were provided by Dozen-Kogyo Co Ltd References Fig Relationship between ultimate tensile strength and tensile strain for copper alloys made by rolling and by casting: (a) CADZ; (b) AlBC; (c) HB [1] H Imai, Y Kosaka, A Kojima, S Li, K Kondoh, J Umeda, H Atsumi, 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(continued) 133 134 M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 Fig (continued) M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) ... Zhang, Microstructural evolution and mechanical properties of Cu-Al M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 [4] [5] [6] [7] [8] [9] [10] [11] alloys. .. intersections, as a result of the very high local stress caused by M Okayasu et al / Journal of Science: Advanced Materials and Devices (2017) 128e139 severe plastic deformation On the other hand,