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Nanostructures and nanoprecipitates induce high strength and high electrical conductivity in a CuCrZr alloy
Journal of Materials Science & Technology 48 (2020) 18–22
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Research Article
Nanostructures and nanoprecipitates induce high strength and high electrical conductivity in a CuCrZr alloy Z.Y. Zhang a,b , L.X. Sun a,c,∗ , N.R. Tao a a b c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 October 2019 Received in revised form 9 December 2019 Accepted 12 December 2019 Available online 22 February 2020 Keywords: CuCrZr alloy Nanotwins Precipitation hardening Aging Electrical conductivity
a b s t r a c t The mixed nanostructure mainly consisting of nanotwins and nanograins was obtained in a solid solution CuCrZr alloy by means of dynamic plastic deformation at cryogenic temperature. After subsequent aging treatments, the precipitation of Cr at nanometer scale provided further strengthening and brought substantial recovery of electrical conductivity. The aged nanostructured CuCrZr alloy exhibited a high tensile strength of 832 MPa and a high electrical conductivity of 71.2 % IACS. The details of precipitation tuned by nanotwin boundaries were demonstrated in this work. The combined strengthening of nanostructures and nanoprecipitates was discussed. © 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction CuCrZr alloys have attracted great attentions in the past decades, due to their excellent balance between high electrical conductivity and tailored mechanical properties. They have shown great potential in engineering applications, such as lead frames for integrated circuit in microelectronics industry, contact wires of high-speed railway, etc [1–3]. Elevating the strength furtherly while keeping high electrical conductivity is the key issue for CuCrZr alloys. However, high strength and high electrical conductivity are mutually exclusive in metals and alloys because the methods used to strengthen materials rise the defect density in the matrix which increase electrical resistance at the same time [4]. Attempts have been made to pursue this goal while methods and technologies have been invented in the past decades. Ultrafinegrained (UFG) CuCrZr alloys, with the grain size from 100 nm to about 500 nm, were prepared by severe plastic deformation (SPD) with or without aging treatments, which showed the strength of 500−700 MPa and the electrical conductivity of 60 %–80 % International Annealed Copper Standard (IACS) [5–8]. However, it is still a challenge to further raise the strength of the bulk CuCrZr alloys. On the one hand, it is difficult to refine coarse grains (CGs)
∗ Corresponding author. E-mail address: [email protected] (L.X. Sun).
into nanograins (NGs) by using conventional plastic deformation techniques. On the other hand, the numerous boundaries in the nanostructures (NSs) would increase electrical resistance seriously [4]. Strengthening with the high density of nanotwins (NTs) has been proven an effective way in many metals and alloys [9–11]. Twin boundaries (TBs) strengthen the metals and alloys like conventional high angle grain boundaries (HAGBs) [12,13], while the electrical resistance induced by coherent TBs is one order of magnitude lower than conventional HAGBs [14,15]. It is the chosen microstructure for high strength and high conductivity alloys. The ultrahigh strength around 1 GPa was obtained in pure Cu thin foils with a high density of NTs prepared by electrical deposition, while the electrical conductivity only dropped to 97 % IACS slightly [14]. The strength of bulk nanostructured pure Cu reached 610 MPa with high electrical conductivity of 95 % IACS, when a high density of NTs was introduced by means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT) [16]. Our previous work has reported the mixed nanostructures mainly consisting of NTs and NGs achieved by DPD in a pre-annealed CuCrZr alloy (hereafter referred to as Annl-DPD samples), which exhibited the strength of 700 MPa and the electrical conductivity of 78.5 % IACS [9]. In this work, an alternative processing approach was applied to achieving higher strength of the CuCrZr alloy. Firstly, a solid solution CuCrZr alloy was subjected to DPD to obtain a mixed nanostructure of NGs and NTs that strengthen the alloy. Then, sub-
https://doi.org/10.1016/j.jmst.2019.12.022 1005-0302/© 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
Z.Y. Zhang et al. / Journal of Materials Science & Technology 48 (2020) 18–22
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Fig. 1. Typical cross-sectional bright-field TEM images of solid solution treated DPD samples (SST-DPD): (a) nanotwins (NTs); (b) nanograins (NGs); (c) dislocation structures (DSs).
sequent aging treatments induced numerous nanoprecipitates in the nanostructured matrix. It is therefore reasonable to expect that the strengthening from precipitates would further raise the strength of the alloy. The detailed microstructures and strengthening mechanism are introduced as follows, based on the comparison between DPD pure Cu and Annl-DPD CuCrZr alloy without precipitation hardening as mentioned above. 2. Materials and experimental procedures A commercial C18150 alloy (i.e. CuCrZr alloy) with 1.0 wt% Cr and 0.1 wt% Zr was used in this work. The samples were homogenized at 1000 ◦ C followed by water quenching. The solubility of Cr in Cu is 0.4 wt% at 1000 ◦ C [17]. There were dispersed Cr particles with the size ranging from a few hundred nanometers to several micrometers in the samples, similar to the Annl-DPD samples. Cylindrical samples with a diameter of 16 mm and a height of 22 mm were deformed to a strain of 2.0 by using DPD at liquid nitrogen temperature (SST-DPD samples). The strain rate of DPD is in the range of 102 –103 s−1 . The DPD strain was defined as ε= ln(L0 /Lf ), where L0 and Lf are the initial and final thicknesses of the treated samples, respectively. The DPD setup and processing parameters have been described in detail in previous works [18–20]. The SSTDPD samples were then isothermally aged at 400 ◦ C for different durations from 10 min to 49 h. Microstructural characterizations were carried out on a FEI Verios 460 field emission gun scanning electron microscope (SEM), a FEI Talos F200x field emission gun transmission electron microscope (TEM) operated at 200 kV and an aberration-corrected FEI Titan G2 microscope (TEM) operated at 300 kV. Microhardness measurements were performed on a Qness Q10A + microhardness tester. Tensile tests were conducted under a strain rate of 5 × 10−3 s-1 on an Instron 5848 micro-tester with a laser extensometer. Dog-bone shaped tensile samples with the gauge dimension of 5 mm × 1 mm × 0.5 mm were cut perpendicularly to the DPD loading direction. The electrical conductivity was measured using a Sigmatest 2.069 electrical conductimeter with the eddy current method. 3. Results and discussion The microstructure of the solid solution treated CuCrZr alloy after DPD (SST-DPD samples) is mainly composed of three types of structures: nanotwins (NTs, Fig. 1(a)), nanograins in lamellae (NGs, Fig. 1(b)) and not well-developed dislocation structures (DSs, Fig. 1(c)), similar to that observed in the DPD Cu samples [21]. The NTs are in the form of bundles, with the volume fraction of 23 %. The twin lamellae are too thin to be distinguished from the
matrix lamellae. So the average twin/matrix (T/M) thickness is used here, which is 20 nm after DPD. The NGs mainly exhibited as elongated ‘bamboo-like’ laminates, with the volume fraction of 62 % and the average transverse grain size of 38 nm. Large proportion of NGs in lamellae originated from the fragmentation of NT lamellae and inherited the T/M interfaces [21]. Some of NGs transformed from the breaking up of lamellae with low angle grain boundaries (LAGBs) [21,22]. The remained dislocation structures present elongated cells with the average transverse size of 138 nm. It is noticeable that the primary microstructures of the SST-DPD and Annl-DPD samples [9] are similar. They are mainly composed of NT bundles, NGs in lamellae and DSs. There are dispersed Cr particles in the nanostructured matrix. The obvious difference between the two mixed microstructures is that there are about 0.4 wt% Cr atoms dissolved in the SST-DPD samples while negligible Cr atoms (<0.07 wt%) dissolved in the Annl-DPD samples [17]. With the addition of Cr and Zr to Cu, the SFE declines from 74 mJ/m2 for Cu to 37 mJ/m2 for Cu-1.0 wt% Cr-0.1 wt% Zr alloy [23]. As demonstrated in previous investigations [24], the thickness of deformation twins decreases monotonically with a reduction in SFE and the grain size of the NGs correlates with the T/M lamellar thickness. Therefore, the T/M spacing in the SST-DPD samples decreases to 20 nm and the grain size of NGs drops to 38 nm, while the corresponding sizes are 25 nm and 75 nm in the Annl-DPD samples. A typical bright-field TEM image (Fig. 2) shows microstructures of SST-DPD samples after aging treatment at 400 ◦ C for 3 h (aged DPD samples). The average thickness of T/M lamellae in twin bundles increases slightly from 20 nm to 28 nm while the average grain size of NGs rises from 38 nm to 68 nm. The thickness T/M lamellae and the grain size of NGs are still in the nanometer scales. There is no obvious change in volume fraction and the transverse size of DSs after aging treatment. It is notable that static recrystallized (SRX) grains could be observed in the bottom of Fig. 2. The average size and the volume fraction of SRX grains measured via SEM are 1.14 m and 8 vol.%, respectively. A high magnification TEM image of aged DPD samples is shown in Fig. 3(a). Insets of SAED patterns indicate that the bottom-left of Fig. 3(a) is one bundle of NTs and the top-right of Fig. 3(a) is NGs in lamellae. The high density of dislocations and the feature of deformation structures, could be found kept in the T/M lamellae and pinning on TBs after the aging treatment. Similarly, the high density of dislocations and lamellar grain boundaries in the aged lamellar NGs keep the same as those in the as-deformed ones in Fig. 1(b). The primary microstructural difference between the SST-DPD and aged DPD samples is the nanoprecipitates formed during aging treatment. Fig. 3(b) shows the EDS element mapping of Cr of the square region in Fig. 3(a). Numerous nanoprecipitates form after
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Z.Y. Zhang et al. / Journal of Materials Science & Technology 48 (2020) 18–22
Fig. 2. Bright-field TEM morphology of SST-DPD samples after aging at 400 ◦ C for 3 h (aged DPD).
aging treatment in both NT bundles and NGs. In NT regions, the nanoprecipitates locates in parallel lines, with regular or periodic interval and uniform size. Carefully comparison with the morphology of nanotwins in Fig. 3(a) indicates that the nanoprecipitates precipitated just on the twin boundaries (TBs). Seldom nanoprecipitates are found inside T/M lamellae. The atomic resolution HAADF image (Fig. 3(c)) and corresponding EDS element mapping (Fig. 3(d)) of a typical FCC Cr-rich nanoprecipitate on twin boundary using aberration-corrected TEM show that the FCC Cr-rich nanoprecipitate is highly coherent with the Cu matrix. The preferred nucleation site is the TB steps formed by interactions between TBs and dislocations or secondary twins. The size of these nanoprecipitates ranges from 1 nm to 3 nm with a typical size of 2 nm. In NG regions, nanoprecipitates locate on grain boundaries (GBs) and inside some wider. There are some coarse nanoprecipitates on GBs with the size near 5 nm, which have been pointed with white arrows in Fig. 3(b). Obviously, the nanoprecipitates prefer to nucleate on boundaries of nanostructures. The TB steps and the GBs provide higher energy for nucleation of precipitates than the interiors of the grains
[25,26]. For the present nanostructured CuCrZr alloy, a high density of dislocations still exists in NTs and NGs, leaving the boundaries at the state of high stored energy. The high boundary density and nanoscale spacing between the adjacent boundaries enable Cr atoms segregating and precipitating in neighborhood of several nanometers. Cr atoms are exhausted in the grain interiors of the nano-lamellae of T/M and NGs. Therefore, the coarsening of the nanoprecipitates are suppressed effectively. According to the Smith–Zener pinning effect [27], the nanoprecipitates dispersed uniformly in nanometer scale could strongly pin the boundaries of nanostructures, which inhibits the coarsening of the nanostructures and enhances the thermal stability. That is the reason why the mixed nanostructures could survive at 400 ◦ C for such long duration. A long duration aging treatment has been conducted at 400 ◦ C for 49 h. Fig. 4 shows the variations of microhardness and electrical conductivity of the SST-DPD samples with aging time. The microhardness rises rapidly at the beginning of aging treatment and reaches the peak at 3 h, demonstrating the substantial precipitation of Cr. The microhardness increases from 1.96 GPa to 2.25 GPa at the peak-aged state. Then the microhardness decreases slightly with the aging duration but still maintained 2.04 GPa after 49 h, which is at the same level as the SST-DPD samples. Considering about the slight recrystallization taking place in the nanostructures (Fig. 2), the maintained high hardness means most of the nanostructures could be kept even after such long time treatment at 400 ◦ C, which shows significantly improved thermal stability of the nanostructures. The pronounced recovery of electrical conductivity occurs accompanied by the reduced microhardness at the early state of aging treatment. That is also mainly determined by the desolvation of Cr atoms from the supersaturated matrix. It is well known that the solid solution of alloying atoms significantly increases the electrical resistance. For example, when 3 wt% Ti is added in pure Cu, the electrical conductivity drops from 100 % IACS to 5% IACS sharply [28]. The formation of precipitates during aging treatment can effectively decompose the supersaturated matrix and thereby enable the electrical conductivity to recovery. Hence, the aging treatment is a sound method that enhances the electrical conductive alloys and improve the electrical conductivity simultaneously. Besides, the previous investigation indicates that the electrical resistivity of TBs is one order magnitude less than that of GBs [14]. It means the introduction of NTs and NGs (mainly coming from the break-up of NT bundles or lamellae with LAGBs) in this work only causes leads to slight decrement of the electrical con-
Fig. 3. (a) High magnification TEM image of SST-DPD samples after aging at 400 ◦ C for 3 h (aged DPD). Insets are SAED patterns of the areas marked with circles; (b) EDS element mapping of Cr taken from the square region in (a). Coarsened precipitates have been pointed out by white arrows; (c, d) atomic resolution HAADF-STEM image and EDS element mapping of Cr on the twin boundary, respectively.
Z.Y. Zhang et al. / Journal of Materials Science & Technology 48 (2020) 18–22
Fig. 4. Variations of microhardness and electrical conductivity of SST-DPD samples as a function of the aging time at 400 ◦ C.
Fig. 5. Tensile engineering stress-strain curves of the solid solution treated coarse grained samples (SST-CG), pre-annealed DPD samples (Annl-DPD), solid solution treated samples after DPD (SST-DPD) and aging at 400 ◦ C for 3 h (aged DPD), respectively.
ductivity, in contrast to conventional nanostructured alloys mostly with mostly the common HAGBs [4]. The corresponding tensile curves of the solid solution treated coarse grained samples (SST-CG), solid solution treated samples after DPD (SST-DPD) and aging at 400 ◦ C for 3 h (aged DPD) are listed in Fig. 5. A curve of pre-annealed nanostructured CuCrZr (Annl-DPD) from the previous work [9] is also shown for reference. The DPD deformation brings obvious increment in the strength of the solid solution treated CuCrZr samples. It is mainly attributed to the introduction of NTs and NGs in the matrix (Fig. 1), which is consistent with the pre-annealed ones (Annl-DPD) [9].The tensile strength of SST-DPD samples (746 MPa) is higher than that of Annl-DPD samples (700 MPa). This is attributed to the difference in the sizes of nanostructures, which had been analyzed based on Fig. 1 and the size statistics. After aging at 400 ◦ C for 3 h, i.e. at the peak aging state, the tensile strength further rises to 832 MPa. The microstructures had been characterized and statistically measured before and after aging. Most of the NT bundles and NG regions remained in nanometer scales with only slight coarsening. It is the foundation why there is no softening after aging at 400 ◦ C for such a long time. Due to the few SRX region after aging (Fig. 2), the contribution of the nanostructured matrix on strengthening must be reduced compared with that in the SST-DPD samples. Therefore, it is reasonable that the contribution of nanoprecipitation on strength is substantial, with an increment even larger than 86 MPa after aging treatment. By introducing nanostructures and nanoprecipitates in the CuCrZr alloys, a good combination of strength (832 MPa) and elec-
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Fig. 6. Combinations of the electrical conductivity and tensile strength of SST-DPD and aged DPD CuCrZr alloy in comparison with that of the reported CuCr(Zr/Ag) alloys.
trical conductivity (71.2 % IACS) is obtained. Higher electrical conductivity about 79.4 % IACS could be obtained with increasing the aging duration time, with the strength of 704 MPa. Fig. 6 summaries the electrical conductivities vs. tensile strengths synergy of CuCrZr alloys with the solid solution treated nanostructures and aged nanostructures, including the pre-annealed nanostructure, solution state ultrafine-grained ones and aged UFGs from literatures for comparison [5–9,29–31]. The five structures are marked in the Fig. 6 as NS (SST), NS + P (aged), NS (Pre-Annl), UFG (SST) and UFG + P (aged), respectively. The electrical conductivities recover remarkably after aging treatments, in both of the UFGs and NSs. However, the tensile strength of the aged UFGs is usually less than 700 MPa. The finer nanostructures provide stronger matrix. The combination of nanostructure strengthening and precipitation hardening is the key point to break through the strength limit of 800 MPa. The TBs with better thermal stability and lower electrical resistivity are the essential microstructures here, comparing with conventional HAGBs in the UFGs or conventional NGs. The NTs and NGs provide high strength and survive after long time aging treatment when precipitation hardening and recovery of conductivity take place. They also tune the finely dispersed precipitates precipitation and suppress coarsening of the precipitates. The combined strengthening from nanotwins and nanoprecipitates is the chosen way for electrical conductive alloys. 4. Conclusion In this work, a combined strengthening method was reported to strengthen the alloys with nanostructured matrix and a high density of nanoprecipitations. It provides an alternative way to further strengthening the high strength alloys with nanostructures by a high density of nanoprecipitates tuned by nanostructured matrix. In the CuCrZr alloy, the high strength of 832 MPa was obtained with the matrix of nanotwin/nanograin and high density of the nanoprecipitates locating at the high density of boundaries. The electrical conductivity of 71.2 % IACS and substantially improved thermal stability were achieved simultaneously. Acknowledgments This work was financially supported by the Ministry of Science & Technology of China (No. 2017YFA0204401), the Chinese Academy of Sciences (No. zdyz201701), the Liaoning Revitalization Talents Program (No. XLYC1808008), the National Natural Science Foundation of China (Nos. 51501192 and 51771196), the Fundamental Research Funds for the Central Universities (No. 3072019CF1017) and the Key Research Program of Frontier Science, Chinese Academy of Sciences.
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Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Research Article
Nanostructures and nanoprecipitates induce high strength and high electrical conductivity in a CuCrZr alloy Z.Y. Zhang a,b , L.X. Sun a,c,∗ , N.R. Tao a a b c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 October 2019 Received in revised form 9 December 2019 Accepted 12 December 2019 Available online 22 February 2020 Keywords: CuCrZr alloy Nanotwins Precipitation hardening Aging Electrical conductivity
a b s t r a c t The mixed nanostructure mainly consisting of nanotwins and nanograins was obtained in a solid solution CuCrZr alloy by means of dynamic plastic deformation at cryogenic temperature. After subsequent aging treatments, the precipitation of Cr at nanometer scale provided further strengthening and brought substantial recovery of electrical conductivity. The aged nanostructured CuCrZr alloy exhibited a high tensile strength of 832 MPa and a high electrical conductivity of 71.2 % IACS. The details of precipitation tuned by nanotwin boundaries were demonstrated in this work. The combined strengthening of nanostructures and nanoprecipitates was discussed. © 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction CuCrZr alloys have attracted great attentions in the past decades, due to their excellent balance between high electrical conductivity and tailored mechanical properties. They have shown great potential in engineering applications, such as lead frames for integrated circuit in microelectronics industry, contact wires of high-speed railway, etc [1–3]. Elevating the strength furtherly while keeping high electrical conductivity is the key issue for CuCrZr alloys. However, high strength and high electrical conductivity are mutually exclusive in metals and alloys because the methods used to strengthen materials rise the defect density in the matrix which increase electrical resistance at the same time [4]. Attempts have been made to pursue this goal while methods and technologies have been invented in the past decades. Ultrafinegrained (UFG) CuCrZr alloys, with the grain size from 100 nm to about 500 nm, were prepared by severe plastic deformation (SPD) with or without aging treatments, which showed the strength of 500−700 MPa and the electrical conductivity of 60 %–80 % International Annealed Copper Standard (IACS) [5–8]. However, it is still a challenge to further raise the strength of the bulk CuCrZr alloys. On the one hand, it is difficult to refine coarse grains (CGs)
∗ Corresponding author. E-mail address: [email protected] (L.X. Sun).
into nanograins (NGs) by using conventional plastic deformation techniques. On the other hand, the numerous boundaries in the nanostructures (NSs) would increase electrical resistance seriously [4]. Strengthening with the high density of nanotwins (NTs) has been proven an effective way in many metals and alloys [9–11]. Twin boundaries (TBs) strengthen the metals and alloys like conventional high angle grain boundaries (HAGBs) [12,13], while the electrical resistance induced by coherent TBs is one order of magnitude lower than conventional HAGBs [14,15]. It is the chosen microstructure for high strength and high conductivity alloys. The ultrahigh strength around 1 GPa was obtained in pure Cu thin foils with a high density of NTs prepared by electrical deposition, while the electrical conductivity only dropped to 97 % IACS slightly [14]. The strength of bulk nanostructured pure Cu reached 610 MPa with high electrical conductivity of 95 % IACS, when a high density of NTs was introduced by means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT) [16]. Our previous work has reported the mixed nanostructures mainly consisting of NTs and NGs achieved by DPD in a pre-annealed CuCrZr alloy (hereafter referred to as Annl-DPD samples), which exhibited the strength of 700 MPa and the electrical conductivity of 78.5 % IACS [9]. In this work, an alternative processing approach was applied to achieving higher strength of the CuCrZr alloy. Firstly, a solid solution CuCrZr alloy was subjected to DPD to obtain a mixed nanostructure of NGs and NTs that strengthen the alloy. Then, sub-
https://doi.org/10.1016/j.jmst.2019.12.022 1005-0302/© 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
Z.Y. Zhang et al. / Journal of Materials Science & Technology 48 (2020) 18–22
19
Fig. 1. Typical cross-sectional bright-field TEM images of solid solution treated DPD samples (SST-DPD): (a) nanotwins (NTs); (b) nanograins (NGs); (c) dislocation structures (DSs).
sequent aging treatments induced numerous nanoprecipitates in the nanostructured matrix. It is therefore reasonable to expect that the strengthening from precipitates would further raise the strength of the alloy. The detailed microstructures and strengthening mechanism are introduced as follows, based on the comparison between DPD pure Cu and Annl-DPD CuCrZr alloy without precipitation hardening as mentioned above. 2. Materials and experimental procedures A commercial C18150 alloy (i.e. CuCrZr alloy) with 1.0 wt% Cr and 0.1 wt% Zr was used in this work. The samples were homogenized at 1000 ◦ C followed by water quenching. The solubility of Cr in Cu is 0.4 wt% at 1000 ◦ C [17]. There were dispersed Cr particles with the size ranging from a few hundred nanometers to several micrometers in the samples, similar to the Annl-DPD samples. Cylindrical samples with a diameter of 16 mm and a height of 22 mm were deformed to a strain of 2.0 by using DPD at liquid nitrogen temperature (SST-DPD samples). The strain rate of DPD is in the range of 102 –103 s−1 . The DPD strain was defined as ε= ln(L0 /Lf ), where L0 and Lf are the initial and final thicknesses of the treated samples, respectively. The DPD setup and processing parameters have been described in detail in previous works [18–20]. The SSTDPD samples were then isothermally aged at 400 ◦ C for different durations from 10 min to 49 h. Microstructural characterizations were carried out on a FEI Verios 460 field emission gun scanning electron microscope (SEM), a FEI Talos F200x field emission gun transmission electron microscope (TEM) operated at 200 kV and an aberration-corrected FEI Titan G2 microscope (TEM) operated at 300 kV. Microhardness measurements were performed on a Qness Q10A + microhardness tester. Tensile tests were conducted under a strain rate of 5 × 10−3 s-1 on an Instron 5848 micro-tester with a laser extensometer. Dog-bone shaped tensile samples with the gauge dimension of 5 mm × 1 mm × 0.5 mm were cut perpendicularly to the DPD loading direction. The electrical conductivity was measured using a Sigmatest 2.069 electrical conductimeter with the eddy current method. 3. Results and discussion The microstructure of the solid solution treated CuCrZr alloy after DPD (SST-DPD samples) is mainly composed of three types of structures: nanotwins (NTs, Fig. 1(a)), nanograins in lamellae (NGs, Fig. 1(b)) and not well-developed dislocation structures (DSs, Fig. 1(c)), similar to that observed in the DPD Cu samples [21]. The NTs are in the form of bundles, with the volume fraction of 23 %. The twin lamellae are too thin to be distinguished from the
matrix lamellae. So the average twin/matrix (T/M) thickness is used here, which is 20 nm after DPD. The NGs mainly exhibited as elongated ‘bamboo-like’ laminates, with the volume fraction of 62 % and the average transverse grain size of 38 nm. Large proportion of NGs in lamellae originated from the fragmentation of NT lamellae and inherited the T/M interfaces [21]. Some of NGs transformed from the breaking up of lamellae with low angle grain boundaries (LAGBs) [21,22]. The remained dislocation structures present elongated cells with the average transverse size of 138 nm. It is noticeable that the primary microstructures of the SST-DPD and Annl-DPD samples [9] are similar. They are mainly composed of NT bundles, NGs in lamellae and DSs. There are dispersed Cr particles in the nanostructured matrix. The obvious difference between the two mixed microstructures is that there are about 0.4 wt% Cr atoms dissolved in the SST-DPD samples while negligible Cr atoms (<0.07 wt%) dissolved in the Annl-DPD samples [17]. With the addition of Cr and Zr to Cu, the SFE declines from 74 mJ/m2 for Cu to 37 mJ/m2 for Cu-1.0 wt% Cr-0.1 wt% Zr alloy [23]. As demonstrated in previous investigations [24], the thickness of deformation twins decreases monotonically with a reduction in SFE and the grain size of the NGs correlates with the T/M lamellar thickness. Therefore, the T/M spacing in the SST-DPD samples decreases to 20 nm and the grain size of NGs drops to 38 nm, while the corresponding sizes are 25 nm and 75 nm in the Annl-DPD samples. A typical bright-field TEM image (Fig. 2) shows microstructures of SST-DPD samples after aging treatment at 400 ◦ C for 3 h (aged DPD samples). The average thickness of T/M lamellae in twin bundles increases slightly from 20 nm to 28 nm while the average grain size of NGs rises from 38 nm to 68 nm. The thickness T/M lamellae and the grain size of NGs are still in the nanometer scales. There is no obvious change in volume fraction and the transverse size of DSs after aging treatment. It is notable that static recrystallized (SRX) grains could be observed in the bottom of Fig. 2. The average size and the volume fraction of SRX grains measured via SEM are 1.14 m and 8 vol.%, respectively. A high magnification TEM image of aged DPD samples is shown in Fig. 3(a). Insets of SAED patterns indicate that the bottom-left of Fig. 3(a) is one bundle of NTs and the top-right of Fig. 3(a) is NGs in lamellae. The high density of dislocations and the feature of deformation structures, could be found kept in the T/M lamellae and pinning on TBs after the aging treatment. Similarly, the high density of dislocations and lamellar grain boundaries in the aged lamellar NGs keep the same as those in the as-deformed ones in Fig. 1(b). The primary microstructural difference between the SST-DPD and aged DPD samples is the nanoprecipitates formed during aging treatment. Fig. 3(b) shows the EDS element mapping of Cr of the square region in Fig. 3(a). Numerous nanoprecipitates form after
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Fig. 2. Bright-field TEM morphology of SST-DPD samples after aging at 400 ◦ C for 3 h (aged DPD).
aging treatment in both NT bundles and NGs. In NT regions, the nanoprecipitates locates in parallel lines, with regular or periodic interval and uniform size. Carefully comparison with the morphology of nanotwins in Fig. 3(a) indicates that the nanoprecipitates precipitated just on the twin boundaries (TBs). Seldom nanoprecipitates are found inside T/M lamellae. The atomic resolution HAADF image (Fig. 3(c)) and corresponding EDS element mapping (Fig. 3(d)) of a typical FCC Cr-rich nanoprecipitate on twin boundary using aberration-corrected TEM show that the FCC Cr-rich nanoprecipitate is highly coherent with the Cu matrix. The preferred nucleation site is the TB steps formed by interactions between TBs and dislocations or secondary twins. The size of these nanoprecipitates ranges from 1 nm to 3 nm with a typical size of 2 nm. In NG regions, nanoprecipitates locate on grain boundaries (GBs) and inside some wider. There are some coarse nanoprecipitates on GBs with the size near 5 nm, which have been pointed with white arrows in Fig. 3(b). Obviously, the nanoprecipitates prefer to nucleate on boundaries of nanostructures. The TB steps and the GBs provide higher energy for nucleation of precipitates than the interiors of the grains
[25,26]. For the present nanostructured CuCrZr alloy, a high density of dislocations still exists in NTs and NGs, leaving the boundaries at the state of high stored energy. The high boundary density and nanoscale spacing between the adjacent boundaries enable Cr atoms segregating and precipitating in neighborhood of several nanometers. Cr atoms are exhausted in the grain interiors of the nano-lamellae of T/M and NGs. Therefore, the coarsening of the nanoprecipitates are suppressed effectively. According to the Smith–Zener pinning effect [27], the nanoprecipitates dispersed uniformly in nanometer scale could strongly pin the boundaries of nanostructures, which inhibits the coarsening of the nanostructures and enhances the thermal stability. That is the reason why the mixed nanostructures could survive at 400 ◦ C for such long duration. A long duration aging treatment has been conducted at 400 ◦ C for 49 h. Fig. 4 shows the variations of microhardness and electrical conductivity of the SST-DPD samples with aging time. The microhardness rises rapidly at the beginning of aging treatment and reaches the peak at 3 h, demonstrating the substantial precipitation of Cr. The microhardness increases from 1.96 GPa to 2.25 GPa at the peak-aged state. Then the microhardness decreases slightly with the aging duration but still maintained 2.04 GPa after 49 h, which is at the same level as the SST-DPD samples. Considering about the slight recrystallization taking place in the nanostructures (Fig. 2), the maintained high hardness means most of the nanostructures could be kept even after such long time treatment at 400 ◦ C, which shows significantly improved thermal stability of the nanostructures. The pronounced recovery of electrical conductivity occurs accompanied by the reduced microhardness at the early state of aging treatment. That is also mainly determined by the desolvation of Cr atoms from the supersaturated matrix. It is well known that the solid solution of alloying atoms significantly increases the electrical resistance. For example, when 3 wt% Ti is added in pure Cu, the electrical conductivity drops from 100 % IACS to 5% IACS sharply [28]. The formation of precipitates during aging treatment can effectively decompose the supersaturated matrix and thereby enable the electrical conductivity to recovery. Hence, the aging treatment is a sound method that enhances the electrical conductive alloys and improve the electrical conductivity simultaneously. Besides, the previous investigation indicates that the electrical resistivity of TBs is one order magnitude less than that of GBs [14]. It means the introduction of NTs and NGs (mainly coming from the break-up of NT bundles or lamellae with LAGBs) in this work only causes leads to slight decrement of the electrical con-
Fig. 3. (a) High magnification TEM image of SST-DPD samples after aging at 400 ◦ C for 3 h (aged DPD). Insets are SAED patterns of the areas marked with circles; (b) EDS element mapping of Cr taken from the square region in (a). Coarsened precipitates have been pointed out by white arrows; (c, d) atomic resolution HAADF-STEM image and EDS element mapping of Cr on the twin boundary, respectively.
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Fig. 4. Variations of microhardness and electrical conductivity of SST-DPD samples as a function of the aging time at 400 ◦ C.
Fig. 5. Tensile engineering stress-strain curves of the solid solution treated coarse grained samples (SST-CG), pre-annealed DPD samples (Annl-DPD), solid solution treated samples after DPD (SST-DPD) and aging at 400 ◦ C for 3 h (aged DPD), respectively.
ductivity, in contrast to conventional nanostructured alloys mostly with mostly the common HAGBs [4]. The corresponding tensile curves of the solid solution treated coarse grained samples (SST-CG), solid solution treated samples after DPD (SST-DPD) and aging at 400 ◦ C for 3 h (aged DPD) are listed in Fig. 5. A curve of pre-annealed nanostructured CuCrZr (Annl-DPD) from the previous work [9] is also shown for reference. The DPD deformation brings obvious increment in the strength of the solid solution treated CuCrZr samples. It is mainly attributed to the introduction of NTs and NGs in the matrix (Fig. 1), which is consistent with the pre-annealed ones (Annl-DPD) [9].The tensile strength of SST-DPD samples (746 MPa) is higher than that of Annl-DPD samples (700 MPa). This is attributed to the difference in the sizes of nanostructures, which had been analyzed based on Fig. 1 and the size statistics. After aging at 400 ◦ C for 3 h, i.e. at the peak aging state, the tensile strength further rises to 832 MPa. The microstructures had been characterized and statistically measured before and after aging. Most of the NT bundles and NG regions remained in nanometer scales with only slight coarsening. It is the foundation why there is no softening after aging at 400 ◦ C for such a long time. Due to the few SRX region after aging (Fig. 2), the contribution of the nanostructured matrix on strengthening must be reduced compared with that in the SST-DPD samples. Therefore, it is reasonable that the contribution of nanoprecipitation on strength is substantial, with an increment even larger than 86 MPa after aging treatment. By introducing nanostructures and nanoprecipitates in the CuCrZr alloys, a good combination of strength (832 MPa) and elec-
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Fig. 6. Combinations of the electrical conductivity and tensile strength of SST-DPD and aged DPD CuCrZr alloy in comparison with that of the reported CuCr(Zr/Ag) alloys.
trical conductivity (71.2 % IACS) is obtained. Higher electrical conductivity about 79.4 % IACS could be obtained with increasing the aging duration time, with the strength of 704 MPa. Fig. 6 summaries the electrical conductivities vs. tensile strengths synergy of CuCrZr alloys with the solid solution treated nanostructures and aged nanostructures, including the pre-annealed nanostructure, solution state ultrafine-grained ones and aged UFGs from literatures for comparison [5–9,29–31]. The five structures are marked in the Fig. 6 as NS (SST), NS + P (aged), NS (Pre-Annl), UFG (SST) and UFG + P (aged), respectively. The electrical conductivities recover remarkably after aging treatments, in both of the UFGs and NSs. However, the tensile strength of the aged UFGs is usually less than 700 MPa. The finer nanostructures provide stronger matrix. The combination of nanostructure strengthening and precipitation hardening is the key point to break through the strength limit of 800 MPa. The TBs with better thermal stability and lower electrical resistivity are the essential microstructures here, comparing with conventional HAGBs in the UFGs or conventional NGs. The NTs and NGs provide high strength and survive after long time aging treatment when precipitation hardening and recovery of conductivity take place. They also tune the finely dispersed precipitates precipitation and suppress coarsening of the precipitates. The combined strengthening from nanotwins and nanoprecipitates is the chosen way for electrical conductive alloys. 4. Conclusion In this work, a combined strengthening method was reported to strengthen the alloys with nanostructured matrix and a high density of nanoprecipitations. It provides an alternative way to further strengthening the high strength alloys with nanostructures by a high density of nanoprecipitates tuned by nanostructured matrix. In the CuCrZr alloy, the high strength of 832 MPa was obtained with the matrix of nanotwin/nanograin and high density of the nanoprecipitates locating at the high density of boundaries. The electrical conductivity of 71.2 % IACS and substantially improved thermal stability were achieved simultaneously. Acknowledgments This work was financially supported by the Ministry of Science & Technology of China (No. 2017YFA0204401), the Chinese Academy of Sciences (No. zdyz201701), the Liaoning Revitalization Talents Program (No. XLYC1808008), the National Natural Science Foundation of China (Nos. 51501192 and 51771196), the Fundamental Research Funds for the Central Universities (No. 3072019CF1017) and the Key Research Program of Frontier Science, Chinese Academy of Sciences.
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