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Precipitation in a Cu–Cr–Zr–Mg alloy during aging
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
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Precipitation in a Cu\Cr\Zr\Mg alloy during aging J.Y. Cheng⁎, B. Shen, F.X. Yu School of Materials Science and Engineering, Nanchang University, Nanchang 330031, PR China
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Article history:
The precipitation processes in a Cu-0.69Cr-0.10Zr-0.02Mg alloy aged at 450 °C and 550 °C
Received 20 February 2013
have been investigated by transmission electron microscopy and high resolution trans-
Received in revised form
mission electron microscopy. The precipitation sequence in this alloy aged at 450 °C is:
14 April 2013
supersaturated solid solution → Guinier–Preston zone (fcc Cr-rich phase) → ordered fcc
Accepted 15 April 2013
Cr-rich phase → ordered bcc Cr-rich phase. The precipitation sequence in this alloy aged at 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich
Keywords:
phase. In the evolution of decomposition, the orientation relationship between the
Precipitation sequence
precipitates and the Cu matrix changes from cube-on-cube to Nishiyama–Wassermann
Aging
orientation. The ordering of Cr-rich precipitates facilitates the formation of the bcc precipitates
Ordering
and promotes the development of Nishiyama–Wassermann orientation.
Orientation relationship
© 2013 Elsevier Inc. All rights reserved.
Cu\Cr\Zr\Mg alloy
1.
Introduction
Owing to their excellent electrical conductivity, high strength and fatigue resistance, aged dilute Cu\Cr alloys and their minor modifications have been considered as candidate materials for plasma facing components, divertor plates of Internal Thermonuclear Experimental Reactor [1–4], railway contact wires, integrated circuit lead frame, and electrode of resistance welding [5–12]. The high strength and electrical conductivity attributes to the nanosized Cr-rich precipitates after aging. Precipitation of chromium in dilute binary Cu\Cr alloys has been investigated extensively in the last three decades. However, nanosized Cr precipitates induce large lattice distortions that make them difficult to observe by transmission electron microscopy (TEM). Thus, there are still some ambiguities about the morphology, crystallographic structure and orientation relationship (OR) of Cr precipitates, especially in the early stage of nucleation. Some researchers [3,13–15] proposed that the metastable precipitate have the same face-centered cubic (fcc) lattice structure as that of the Cu ⁎ Corresponding author. Tel./fax: + 86 791 83969553. E-mail address: [email protected] (J.Y. Cheng). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.04.008
matrix, while others [16,17] suggested the same bodycentered cubic (bcc) lattice structure as that of pure Cr. In the overaging condition, it was generally agreed that the rod-like and lath-like precipitates have the same bcc structure as that of pure Cr and possess a Kurdjumov–Sachs (K–S) OR with the Cu matrix. These disputes were ended temporarily until Fujii and his co-authors [18] investigated the morphology and crystallographic structure and OR of Cr precipitate later through high resolution transmission microscopy (HRTEM). They established that the crystal structure of the chromium particles in Cu-0.2Cr alloy after aging at 773 K for various time is bcc regardless of their sizes. Also, two distinct ORs between the Cr particles and the Cu matrix were found: one near Nishiyama– Wassermann (N–W) and the other near Kurdjumov–Sachs (K–S). After prolonged aging, only the K–S particles remained in the Cu matrix (i.e. particles with K–S OR grew at the expense of those having an N–W OR). In the past decade, modified Cu\Cr alloys, especially Cu\Cr\Zr system, have been widely studied due to excellent combination of mechanical and electrical properties. Small additions of Mg and Zr impart to Cu\Cr alloys improved
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
high-temperature strength and ductility in both creep and low-cycle fatigue without impairing their good electrical and thermal conductivities [19]. The addition of Fe or Co has also been proposed by some authors [9,20]. At least four other precipitation products have also been observed in Cu\Cr\Zr based alloys: intragranular ordered CrCu2(Mg, Zr) (DO3) precipitates [21], grain boundary orthorhombic Cu4Zr phase with a = 0.504 nm, b = 0.492 nm and c = 0.664 nm [19,22], intragranular hcp Cu51Zr14 with a = 1.125 nm and c = 0.8275 nm [23] and fcc Cu5Zr with a = 0.687 nm [1,23,24]. These structures were observed in Cu\Cr\Zr alloys with different composition. Thus, the overall transformation kinetics appears to be very complex. Another issue is that there is no consensus on the evolution of precipitation in these alloys. Batra and co-authors [2,3] found that a metastable ordered phase with a possible B2 (CsCl-Type) structure formed through fcc ordered Cr particles with a cube-on-cube OR. However, their study focused on the Cu-1.0 wt%Cr-0.1Zr alloy solutionized plus aged at 480 °C for 5 h and only gave limited information about the general picture of the precipitation process. It is well known that different heat treatment process will give rise to different precipitate phase and its structure. In the present study, attempts were made to exhibit an entire precipitation sequence of Cu\Cr\Zr\Mg alloy and the orientation relationships between precipitates and Cu matrix in the early stages of aging at 450 °C and 550 °C, respectively.
2.
Experimental procedures
The Cu\Cr\Zr\Mg alloy was prepared using electrolytic copper, pure chromium, magnesium, and copper-10wt.% zirconium master alloys in a vacuum medium-frequency induction furnace, and then cast in an iron mould with a size of 20 × 100 × 150 mm. Table 1 lists composition of the alloy analyzed by ICP-AES Optima 5300DV. The ingot was hot rolled to a plate with a thickness of 10 mm at 850 °C and then solution treated at 960 °C for 2 h, followed by water quenching. The specimens cut from the quenched plate were aged at 450 °C and 550 °C for various time, respectively. Vickers hardness test was performed on HV-50 Vickers-hardness tester using a 1 kg load for 10 s loading time. Electrical conductivity was measured at room temperature by FQR7501 eddy current conductivity meter. The conductivity was measured and evaluated according to the international annealing copper standard (IACS, 100% IACS = 1.7241 μΩ cm). Thin foils for TEM examination were sliced from the aged samples and further mechanically thinned to 30–40 μm. Discs of 3 mm in diameter were punched out of these pieces and then ion-beam thinned by Gatan 600 Duomill. The foils were examined in JEOL 2100F (HRTEM) and JEOL 2010 microscopy operating at 200 kV.
Table 1 – Composition of Cu\Cr\Zr\Mg alloy (wt.%). Cr
Zr
Mg
Al
Cu
0.691
0.101
0.017
0.006
Bal.
3.
Results
3.1.
Precipitation during aging at 450 °C
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Fig. 1 summarizes the main TEM results obtained from the samples aged at 450 °C for 5 min. It can be found in Fig. 1(a) that high density of fine particles with an average size of 1– 2 nm distribute homogeneously in the matrix. There exist bright contrasts around these precipitates which result from the lattice distortion. Its corresponding selected area electron diffraction (SAED) pattern with zone axis of [112]Cu in Fig. 1(b) exhibits that no reflections other than those from the matrix are visible. It can only be concluded that these Guinier– Preston (GP) zones with the same structure and lattice as that of Cu matrix have formed at the stage of aging. This means that the Cr-rich precipitates [25] have the fcc structure rather than the equilibrium bcc, representing a low interfacial energy difference between the fcc Cr-rich phase and the copper matrix. And the reflections from fcc Cu matrix are streaked slightly. Fig. 1(c) and its corresponding SAED pattern observed along the zone axis [001]Cu (Fig. 1(d)) also reveal the formation of GP zones. Fig. 1(c) shows the streaks along <110 >* Cu around every fcc reflections of Cu. It is thought that this streaking is a result of the high degree of elastic anisotropy of the Cu matrix, which allows considerable shear strain along < 110 > direction [19,26]. Most precipitates take the form of nearly sphere-like shown in Fig. 1(c)) and few are plate-like (marked by arrow P3). Most GP zones show the black or white dots contrast. Some (marked by arrow P1 and P2) seem to exhibit strain-field contrast of the black/black lobe, white/white lobe (namely, lobe–lobe contrast). These deviations of the strain contrast associated with the GP zones from the classical Ashby–Brown rules [27] for spherical precipitates are considered a consequence of their very small size. McIntyre and Brown [28] have reported that the precipitate radius will strongly influence the type of contrast observed. With the increase of precipitate radius, the contrast for coherent precipitates imaged in two-beam conditions, can vary from black-white lobes and black dots to black-black lobes. Therefore, Fig. 2 illustrates the typical lobe–lobe contrasts of some coherent spherical precipitates in a sample aged for 4 h at 450 °C. The average size of coherent precipitates is about 5 nm according to the length of extinction line. Fig. 3 summarizes the main TEM results obtained from the samples aged at 450 °C for 8 h. With the prolonging time of aging, these precipitates tend to grow up and coarsen (Fig. 3(a)). The corresponding SAED pattern (Fig. 3(b)) with zone axis of [110]Cu shows the extra reflections from the ordered fcc precipitates having a cube-on-cube OR with the Cu matrix. A superlattice reflection midway between {220} fcc reflection from precipitates and the transmitted spot is also found (marked by arrow). This implies solute enrichment on alternate {220} planes of the precipitate. For more clarity, the spots are indexed in the Fig. 3(c). The reflection spots (such as {200}, {220} and {111}) from ordered fcc precipitates are streaked in different directions. The streaking seen in the pattern in the Fig. 3(b) is a consequence of the strain arising in the matrix due to the formation of the ordered product phase
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Fig. 1 – TEM images taken from a specimen aged at 450 °C for 5 min: (a) bright field (BF) image of GP zones (I), (b) SAED pattern of (a) with zone axis [112]Cu, (c)BF image of GP zones (I), (d) SAED pattern of (c) with zone axis [001]Cu. in a very fine state of subdivision. Such strains can influence the geometry of the decomposition product and account for the observed streaking [3]. The lattice parameter of the fcc Cr-rich phase is slightly larger than that of the fcc Cu matrix. Its lattice parameter a is measured to be 0.408 ± 0.003 nm, which is close to that of fcc Cr nanocluster in literature (a = 0.413 nm) [10,29]. Another kind of fine plate-like precipitates with different OR is also found in Fig. 3(d). Compared to the ordered fcc
Cr-rich phase (~ 5 nm), the size of these precipitates is slightly coarser (10–15 nm in length). It appears to be the products of the decomposition of supersaturated solid solution in the advanced stage of aging. The corresponding [112]Cu SAED pattern in Fig. 3(e) and the relevant keys in Fig. 3(f) exhibit N–W OR. The superlattice reflections at {100}P and other equivalent positions reveal that the Cr-rich precipitates maybe possess B2 (CsCl-type) structure (subscript P refers to precipitate, the same below). Assuming that the precipitates have a bcc unit cell with the extra reflections arising from ordering superlattices, these precipitates are found to be bcc ordered Cr-rich phase and oriented with respect to the Cu matrix according to the N–W OR: 110 == 111 P
ð001ÞP ==ð110ÞCu ½110P ==½112Cu
Fig. 2 – BF TEM image taken from a specimen aged at 450 °C for 4 h showing coffee-bean contrast of coherent precipitates. Zone axis close to [001]Cu.
Cu
Its lattice parameter a is calculated to be 0.280 ± 0.002 nm, which is close to that of bcc Cr (a = 0.2885 nm) in literature [1]. In addition, it can be found that in this N–W OR, the close-packed planes (i.e. {111}Cu and {110}P) in the two phases are parallel. And d110 of ordered bcc phase is almost equal to d111 of the Cu matrix. There is only a small misfit (2.3%) between the two planes [25]. But the misfit between {220}Cu and {200}P is calculated to be 15.4%. It can be expected that the coherent–semicoherent interfaces will form between the Cu matrix and the ordered bcc Cr-rich phase. This can greatly reduce the free energy in this system. Tang et al. [19] reported that this ordered phase possessed Heusler structure. The composition of Heusler phase is likely
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
71
Fig. 3 – TEM micrographs and SAED patterns taken from a specimen aged at 450 °C for 8 h: (a) BF micrograph showing different contrast, (b) SAED pattern from (a) with zone axis of [110]Cu showing extra reflections from the ordered fcc Cr-rich phase which have a cube-on-cube OR with Cu matrix. (c) Key to (b), (d) BF micrograph showing the coarser plate-like precipitates near the grain boundary, (e) SAED pattern from (d) with zone axis of [112]Cu, (f) keys to (e). Keys c and f show positions of the fundamental (○) and superlattice (●) reflections from the Cr-rich phase and fcc matrix (○). Extra reflections caused by double diffraction have been omitted. to be CrCu2(Zr, Mg), which has fcc crystal structure with a large unit cell containing 8 Cu, 4 Cr, and 4 Zr or Mg atoms. The electron diffraction pattern of bcc sublattice is the same as Fig. 3(f). However, J.B. Batra et al. [2,3] proposed that this precipitate phase is a B2 type (CsCl-type) order. Whether it is the truth or not, further investigation is needed. It is worth noting that, even in this condition, the ordered fcc precipitates in the Cu matrix can be found. Fig. 4 shows the HRTEM image of ordered fcc Cr-rich precipitates with zone axis of [110]Cu. These ordered fcc Cr-rich precipitates present in the form of spheres with a size of 1–2 nm and display a darker contrast than the surrounding Cu matrix. More information about them is showed in Fig. 4(b). It can be seen
that the Cr-rich GP zones are fully coherent with Cu matrix. The corresponding SAED pattern shown in the inset of Fig. 4(a) also reveals that these Cr-rich precipitates have an ordered fcc structure. There is a small deviation angle between <−200>*Cu and <−200 >*P. The above facts suggest that certain precipitates enter into the coarsening stage when nucleation of new embryos continues. According to the results mentioned above, it could be summarized that the evolution of decomposition of the studied alloy appears to undergo the following stages during the early period of aging at 450 °C for up to 8 h, that is, forming of GP zone (fcc Cr-rich phase) and ordered fcc Cr-rich phase, and further transforming to ordered bcc Cr-rich phase
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Fig. 4 – HRTEM images taken from a specimen aged at 450 °C for 8 h showing fine ordered fcc precipitates: (a) SAED pattern in the inset obtained by Fast Fourier Transform (FFT) show that these precipitates have the ordered fcc structure and cube-on-cube OR, (b) enlarged image of the box in (a) obtained by FFT and Filtered and then inverse FFT techniques.
(B2, or Heusler structure). It is difficult to differentiate clearly because of overlapping of three stages. Accompanied by these transforming, in addition, the morphology of precipitates changes from fine spheres to larger plates. And the orientation relationship between the precipitates and Cu matrix changes from cube-on-cube OR to N–W OR.
3.2.
Precipitation during aging at 550 °C
Fig. 5 shows a TEM image taken from a specimen aged at 550 °C for 5 min. The Moiré fringes are found in some larger particles (5–10 nm), while the fine particles with a size of 2– 4 nm show faint dark contrast. The corresponding SEAD pattern in the inset reveals that the precipitates have an ordered fcc structure with cube-on-cube OR. In this specimen, no GP zone is observed. Absence of the GP zone arises from the higher aging temperature. When aging at the temperature below the solvus of ordered fcc phase and above the one of GP zone, the former will precipitate first. Fig. 6 summarized the TEM results obtained from the samples aged at 550 °C for 8 h. Fig. 6(a) and its insets reveal that the fine precipitates have an ordered fcc structure and
cube-on-cube OR with Cu matrix along zone axis of [110]Cu. BF TEM image (Fig. 6(b)) and its corresponding SAED pattern with zone axis of [100]Cu (Fig. 6(c)) illustrate that all the fcc Cu reflections are streaked along the <110>* directions. There are two sets of diffraction spots from precipitates in Fig. 6(c). The faint superlattice reflections (marked by notation □ in the Fig. 6(d)) in the first set of diffraction pattern (marked by P1) can be seen midway between one of these reflections and the transmitted spot. Its diffraction pattern is indexed to be the ordered bcc structure according to the literature [3,25]. And its lattice parameter is measured to be 0.321 ± 0.002 nm, which is larger than the value of 0.280 ± 0.002 nm calculated in the section above. The other set of diffraction pattern (marked by P2) with faint superlattice reflections (marked by notation ● and arrows) exhibit that the precipitates also have an ordered structure. It is difficult to judge that these precipitates belong to bcc or fcc structure because ordered bcc structure has the same diffraction pattern along [111] zone axis as that of fcc structure. However, it does not affect the indexing of the diffraction spots. The lattice parameter of the precipitates is measured to be 0.446 ± 0.003 nm. And the OR of these precipitates with Cu matrix is: ð022ÞCu ==ð022ÞP ½100Cu ==½111P
Fig. 5 – TEM image and [112]Cu SAED pattern taken from a specimen aged at 550 °C for 5 min.
The coarse precipitates are also found in the same specimen as shown in Fig. 7. Its SAED in the inset obtained by FFT demonstrates the precipitates have B2-type or Heusler structure and N–W OR along zone axis of [112]Cu. Compared with Fig. 5, the interparticle spacing in Fig. 6(a) and (b) become greater with aging time. It suggests that these coarse particles with N–W OR grow at the expense of those having a cube-on-cube OR. In summary, it can be considered that the evolution of decomposition of the studied alloy during isothermal aging 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase (B2, or Heusler structure).
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73
Fig. 6 – TEM images and SAED patterns in the inset taken from a specimen aged at 550 °C for 8 h: (a) fine precipitates exhibiting a cube-on-cube OR with Cu matrix, the inset showing [110]Cu SAED pattern, (b) BF TEM image showing coarser precipitates, (c) SAED pattern taken from (b) with zone axis [100]Cu showing streaks along <110>* directions around the reflections of fcc Cu, (d) key to (c) showing positions of the fundamental (○) and superlattice (□ or ●) reflections from the ordered fcc precipitates and fcc matrix reflections (○).
3.3.
Properties of aged alloy
The age-hardening behaviors of the alloy aged at 450 °C and 550 °C are shown in Fig. 8. The hardness of the alloy increases quickly at the initiate stage of aging at 450 °C. After aging at 450 °C for 4 h, the precipitation kinetics seems to slow down.
The hardness value of 120 HV is obtained after 8 h. While aging at 550 °C, it is evident that a very rapid hardening process occurs in the early stages until 2 h and the hardness decreases with increasing aging time. The peak hardness, 118 HV, is obtained in the alloy aged at 550 °C for 2 h. In the initial 4 h of aging, the specimens aged at 550 °C have higher
Fig. 7 – HRTEM image taken from a specimen aged at 550 °C for 8 h. [112]Cu. SAED pattern in the inset obtained by FFT showing that the precipitates have B2-type or Heusler structure and N–W OR with the Cu matrix.
Fig. 8 – Vickers hardness as a function of aging time at 450 °C and 550 °C.
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hardness than those aged at 450 °C. But when the aging exceeds 4 h, the former have lower hardness than the latter. As the alloy is aged at low temperature, it takes a longer time to achieve complete precipitation due to the low diffusion rate of solute atoms. Besides, since the degree of supersaturation at low temperatures is higher than that at high temperatures, a higher density of fine fcc Cr-rich particles are precipitated from Cu solid solution (as shown in Fig. 1) [30]. Higher aging temperature and longer aging time easily lead to overaging. The specimen aged at 550 °C for 8 h exhibit greater interparticle spacing and coarser bcc Cr-rich particles (as shown in Fig. 7) than that aged for 5 min (Fig. 5). On the whole, the hardness behavior of the alloy investigated in the present study is in good agreement with published data on similar alloys [31]. Fig. 9 presents the electrical conductivity of the alloy as a function of aging time at 450 °C and 550 °C. All specimens aged at 550 °C have an overall higher conductivity than those aged at 450 °C. Moreover, the conductivity of the former increases more quickly than the latter at the initiate stage of aging (less than 60 min). It reveals that higher aging temperature leads to a higher conductivity and a shorter time to reach a stable value.
4.
Discussion
The two factors, namely, coherent interfaces and ordering, play very important roles in the precipitation evolution of Cu\Cr\Zr\Mg alloy. The precipitation of the Cr-rich bcc phase initiates through the nucleation of fcc precipitates that are coherent with the Cu-rich fcc parent phase. Because the atomic radii of Cr and Cu (rCr = 0.127 nm, rCu = 0.128 nm) differ by only 0.78%, the GP zones in the studied alloy take the form of sphere to minimize the total interfacial energy and therefore lowers the nucleation barrier in the initial stage of aging, although the distortion energy exists. The coherent interfacial energy γ (for pure nuclei, γ = 86 mJm−2) [25] is much smaller than the Crbcc/ Cufcc interfacial energy reported in the literature for non-coherent
Crbcc/Cufcc interfaces (625 mJm−2) [32]. That is, the nucleation of fcc Cr-rich is much easier than that of non-coherent bcc nuclei. By the aid of fcc transition phase, the total free energy of this alloy lower faster compared to those by the direct formation of bcc precipitate from matrix. It is well known that the process of ordering is different with the ordinary precipitation. Because ordering involves only local atomic adjusting within a unit cell, it is kinetically much faster than ordinary precipitation which involves long range diffusion. Therefore, it can accelerate the process of precipitation and phase transformation of fcc → bcc. Moreover, ordering make the interplanar spacing d220 of ordered fcc Cr-rich phase enlarged to the d200 of ordered Cr-rich phase. This facilitates the formation of the bcc precipitates and promotes the development of N–W orientation relationship.
5.
Conclusions 1. The sequence of decomposition in the Cu\Cr\Zr\Mg alloy during the early stage of aging at 450 °C can be summarized as: supersaturated solid solution → GP zone (fcc Cr-rich phase) → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase (B2, or Heusler structure). The sequence of decomposition during aging at 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase. 2. Accompanied by the precipitation process, the OR between the precipitates and the Cu matrix changes from cube-on-cube to N–W OR. 3. Ordering occurred in the precipitation in this alloy facilitates the formation of the bcc precipitates and promotes the development of N–W orientation relationship.
Acknowledgments The authors are grateful to Professor M.P. Wang and Dr. Q. Lei at the Central-South University for TEM experimental assistance, to Dr. B.B. Tang at the Nanchang University for his helpful discussions. This work is financially supported by National Natural Science Foundation of China (Project No. 51161018).
REFERENCES
Fig. 9 – Electrical conductivity as a function of aging time at 450 °C and 550 °C.
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Available online at www.sciencedirect.com
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Precipitation in a Cu\Cr\Zr\Mg alloy during aging J.Y. Cheng⁎, B. Shen, F.X. Yu School of Materials Science and Engineering, Nanchang University, Nanchang 330031, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
The precipitation processes in a Cu-0.69Cr-0.10Zr-0.02Mg alloy aged at 450 °C and 550 °C
Received 20 February 2013
have been investigated by transmission electron microscopy and high resolution trans-
Received in revised form
mission electron microscopy. The precipitation sequence in this alloy aged at 450 °C is:
14 April 2013
supersaturated solid solution → Guinier–Preston zone (fcc Cr-rich phase) → ordered fcc
Accepted 15 April 2013
Cr-rich phase → ordered bcc Cr-rich phase. The precipitation sequence in this alloy aged at 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich
Keywords:
phase. In the evolution of decomposition, the orientation relationship between the
Precipitation sequence
precipitates and the Cu matrix changes from cube-on-cube to Nishiyama–Wassermann
Aging
orientation. The ordering of Cr-rich precipitates facilitates the formation of the bcc precipitates
Ordering
and promotes the development of Nishiyama–Wassermann orientation.
Orientation relationship
© 2013 Elsevier Inc. All rights reserved.
Cu\Cr\Zr\Mg alloy
1.
Introduction
Owing to their excellent electrical conductivity, high strength and fatigue resistance, aged dilute Cu\Cr alloys and their minor modifications have been considered as candidate materials for plasma facing components, divertor plates of Internal Thermonuclear Experimental Reactor [1–4], railway contact wires, integrated circuit lead frame, and electrode of resistance welding [5–12]. The high strength and electrical conductivity attributes to the nanosized Cr-rich precipitates after aging. Precipitation of chromium in dilute binary Cu\Cr alloys has been investigated extensively in the last three decades. However, nanosized Cr precipitates induce large lattice distortions that make them difficult to observe by transmission electron microscopy (TEM). Thus, there are still some ambiguities about the morphology, crystallographic structure and orientation relationship (OR) of Cr precipitates, especially in the early stage of nucleation. Some researchers [3,13–15] proposed that the metastable precipitate have the same face-centered cubic (fcc) lattice structure as that of the Cu ⁎ Corresponding author. Tel./fax: + 86 791 83969553. E-mail address: [email protected] (J.Y. Cheng). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.04.008
matrix, while others [16,17] suggested the same bodycentered cubic (bcc) lattice structure as that of pure Cr. In the overaging condition, it was generally agreed that the rod-like and lath-like precipitates have the same bcc structure as that of pure Cr and possess a Kurdjumov–Sachs (K–S) OR with the Cu matrix. These disputes were ended temporarily until Fujii and his co-authors [18] investigated the morphology and crystallographic structure and OR of Cr precipitate later through high resolution transmission microscopy (HRTEM). They established that the crystal structure of the chromium particles in Cu-0.2Cr alloy after aging at 773 K for various time is bcc regardless of their sizes. Also, two distinct ORs between the Cr particles and the Cu matrix were found: one near Nishiyama– Wassermann (N–W) and the other near Kurdjumov–Sachs (K–S). After prolonged aging, only the K–S particles remained in the Cu matrix (i.e. particles with K–S OR grew at the expense of those having an N–W OR). In the past decade, modified Cu\Cr alloys, especially Cu\Cr\Zr system, have been widely studied due to excellent combination of mechanical and electrical properties. Small additions of Mg and Zr impart to Cu\Cr alloys improved
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
high-temperature strength and ductility in both creep and low-cycle fatigue without impairing their good electrical and thermal conductivities [19]. The addition of Fe or Co has also been proposed by some authors [9,20]. At least four other precipitation products have also been observed in Cu\Cr\Zr based alloys: intragranular ordered CrCu2(Mg, Zr) (DO3) precipitates [21], grain boundary orthorhombic Cu4Zr phase with a = 0.504 nm, b = 0.492 nm and c = 0.664 nm [19,22], intragranular hcp Cu51Zr14 with a = 1.125 nm and c = 0.8275 nm [23] and fcc Cu5Zr with a = 0.687 nm [1,23,24]. These structures were observed in Cu\Cr\Zr alloys with different composition. Thus, the overall transformation kinetics appears to be very complex. Another issue is that there is no consensus on the evolution of precipitation in these alloys. Batra and co-authors [2,3] found that a metastable ordered phase with a possible B2 (CsCl-Type) structure formed through fcc ordered Cr particles with a cube-on-cube OR. However, their study focused on the Cu-1.0 wt%Cr-0.1Zr alloy solutionized plus aged at 480 °C for 5 h and only gave limited information about the general picture of the precipitation process. It is well known that different heat treatment process will give rise to different precipitate phase and its structure. In the present study, attempts were made to exhibit an entire precipitation sequence of Cu\Cr\Zr\Mg alloy and the orientation relationships between precipitates and Cu matrix in the early stages of aging at 450 °C and 550 °C, respectively.
2.
Experimental procedures
The Cu\Cr\Zr\Mg alloy was prepared using electrolytic copper, pure chromium, magnesium, and copper-10wt.% zirconium master alloys in a vacuum medium-frequency induction furnace, and then cast in an iron mould with a size of 20 × 100 × 150 mm. Table 1 lists composition of the alloy analyzed by ICP-AES Optima 5300DV. The ingot was hot rolled to a plate with a thickness of 10 mm at 850 °C and then solution treated at 960 °C for 2 h, followed by water quenching. The specimens cut from the quenched plate were aged at 450 °C and 550 °C for various time, respectively. Vickers hardness test was performed on HV-50 Vickers-hardness tester using a 1 kg load for 10 s loading time. Electrical conductivity was measured at room temperature by FQR7501 eddy current conductivity meter. The conductivity was measured and evaluated according to the international annealing copper standard (IACS, 100% IACS = 1.7241 μΩ cm). Thin foils for TEM examination were sliced from the aged samples and further mechanically thinned to 30–40 μm. Discs of 3 mm in diameter were punched out of these pieces and then ion-beam thinned by Gatan 600 Duomill. The foils were examined in JEOL 2100F (HRTEM) and JEOL 2010 microscopy operating at 200 kV.
Table 1 – Composition of Cu\Cr\Zr\Mg alloy (wt.%). Cr
Zr
Mg
Al
Cu
0.691
0.101
0.017
0.006
Bal.
3.
Results
3.1.
Precipitation during aging at 450 °C
69
Fig. 1 summarizes the main TEM results obtained from the samples aged at 450 °C for 5 min. It can be found in Fig. 1(a) that high density of fine particles with an average size of 1– 2 nm distribute homogeneously in the matrix. There exist bright contrasts around these precipitates which result from the lattice distortion. Its corresponding selected area electron diffraction (SAED) pattern with zone axis of [112]Cu in Fig. 1(b) exhibits that no reflections other than those from the matrix are visible. It can only be concluded that these Guinier– Preston (GP) zones with the same structure and lattice as that of Cu matrix have formed at the stage of aging. This means that the Cr-rich precipitates [25] have the fcc structure rather than the equilibrium bcc, representing a low interfacial energy difference between the fcc Cr-rich phase and the copper matrix. And the reflections from fcc Cu matrix are streaked slightly. Fig. 1(c) and its corresponding SAED pattern observed along the zone axis [001]Cu (Fig. 1(d)) also reveal the formation of GP zones. Fig. 1(c) shows the streaks along <110 >* Cu around every fcc reflections of Cu. It is thought that this streaking is a result of the high degree of elastic anisotropy of the Cu matrix, which allows considerable shear strain along < 110 > direction [19,26]. Most precipitates take the form of nearly sphere-like shown in Fig. 1(c)) and few are plate-like (marked by arrow P3). Most GP zones show the black or white dots contrast. Some (marked by arrow P1 and P2) seem to exhibit strain-field contrast of the black/black lobe, white/white lobe (namely, lobe–lobe contrast). These deviations of the strain contrast associated with the GP zones from the classical Ashby–Brown rules [27] for spherical precipitates are considered a consequence of their very small size. McIntyre and Brown [28] have reported that the precipitate radius will strongly influence the type of contrast observed. With the increase of precipitate radius, the contrast for coherent precipitates imaged in two-beam conditions, can vary from black-white lobes and black dots to black-black lobes. Therefore, Fig. 2 illustrates the typical lobe–lobe contrasts of some coherent spherical precipitates in a sample aged for 4 h at 450 °C. The average size of coherent precipitates is about 5 nm according to the length of extinction line. Fig. 3 summarizes the main TEM results obtained from the samples aged at 450 °C for 8 h. With the prolonging time of aging, these precipitates tend to grow up and coarsen (Fig. 3(a)). The corresponding SAED pattern (Fig. 3(b)) with zone axis of [110]Cu shows the extra reflections from the ordered fcc precipitates having a cube-on-cube OR with the Cu matrix. A superlattice reflection midway between {220} fcc reflection from precipitates and the transmitted spot is also found (marked by arrow). This implies solute enrichment on alternate {220} planes of the precipitate. For more clarity, the spots are indexed in the Fig. 3(c). The reflection spots (such as {200}, {220} and {111}) from ordered fcc precipitates are streaked in different directions. The streaking seen in the pattern in the Fig. 3(b) is a consequence of the strain arising in the matrix due to the formation of the ordered product phase
70
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
Fig. 1 – TEM images taken from a specimen aged at 450 °C for 5 min: (a) bright field (BF) image of GP zones (I), (b) SAED pattern of (a) with zone axis [112]Cu, (c)BF image of GP zones (I), (d) SAED pattern of (c) with zone axis [001]Cu. in a very fine state of subdivision. Such strains can influence the geometry of the decomposition product and account for the observed streaking [3]. The lattice parameter of the fcc Cr-rich phase is slightly larger than that of the fcc Cu matrix. Its lattice parameter a is measured to be 0.408 ± 0.003 nm, which is close to that of fcc Cr nanocluster in literature (a = 0.413 nm) [10,29]. Another kind of fine plate-like precipitates with different OR is also found in Fig. 3(d). Compared to the ordered fcc
Cr-rich phase (~ 5 nm), the size of these precipitates is slightly coarser (10–15 nm in length). It appears to be the products of the decomposition of supersaturated solid solution in the advanced stage of aging. The corresponding [112]Cu SAED pattern in Fig. 3(e) and the relevant keys in Fig. 3(f) exhibit N–W OR. The superlattice reflections at {100}P and other equivalent positions reveal that the Cr-rich precipitates maybe possess B2 (CsCl-type) structure (subscript P refers to precipitate, the same below). Assuming that the precipitates have a bcc unit cell with the extra reflections arising from ordering superlattices, these precipitates are found to be bcc ordered Cr-rich phase and oriented with respect to the Cu matrix according to the N–W OR: 110 == 111 P
ð001ÞP ==ð110ÞCu ½110P ==½112Cu
Fig. 2 – BF TEM image taken from a specimen aged at 450 °C for 4 h showing coffee-bean contrast of coherent precipitates. Zone axis close to [001]Cu.
Cu
Its lattice parameter a is calculated to be 0.280 ± 0.002 nm, which is close to that of bcc Cr (a = 0.2885 nm) in literature [1]. In addition, it can be found that in this N–W OR, the close-packed planes (i.e. {111}Cu and {110}P) in the two phases are parallel. And d110 of ordered bcc phase is almost equal to d111 of the Cu matrix. There is only a small misfit (2.3%) between the two planes [25]. But the misfit between {220}Cu and {200}P is calculated to be 15.4%. It can be expected that the coherent–semicoherent interfaces will form between the Cu matrix and the ordered bcc Cr-rich phase. This can greatly reduce the free energy in this system. Tang et al. [19] reported that this ordered phase possessed Heusler structure. The composition of Heusler phase is likely
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
71
Fig. 3 – TEM micrographs and SAED patterns taken from a specimen aged at 450 °C for 8 h: (a) BF micrograph showing different contrast, (b) SAED pattern from (a) with zone axis of [110]Cu showing extra reflections from the ordered fcc Cr-rich phase which have a cube-on-cube OR with Cu matrix. (c) Key to (b), (d) BF micrograph showing the coarser plate-like precipitates near the grain boundary, (e) SAED pattern from (d) with zone axis of [112]Cu, (f) keys to (e). Keys c and f show positions of the fundamental (○) and superlattice (●) reflections from the Cr-rich phase and fcc matrix (○). Extra reflections caused by double diffraction have been omitted. to be CrCu2(Zr, Mg), which has fcc crystal structure with a large unit cell containing 8 Cu, 4 Cr, and 4 Zr or Mg atoms. The electron diffraction pattern of bcc sublattice is the same as Fig. 3(f). However, J.B. Batra et al. [2,3] proposed that this precipitate phase is a B2 type (CsCl-type) order. Whether it is the truth or not, further investigation is needed. It is worth noting that, even in this condition, the ordered fcc precipitates in the Cu matrix can be found. Fig. 4 shows the HRTEM image of ordered fcc Cr-rich precipitates with zone axis of [110]Cu. These ordered fcc Cr-rich precipitates present in the form of spheres with a size of 1–2 nm and display a darker contrast than the surrounding Cu matrix. More information about them is showed in Fig. 4(b). It can be seen
that the Cr-rich GP zones are fully coherent with Cu matrix. The corresponding SAED pattern shown in the inset of Fig. 4(a) also reveals that these Cr-rich precipitates have an ordered fcc structure. There is a small deviation angle between <−200>*Cu and <−200 >*P. The above facts suggest that certain precipitates enter into the coarsening stage when nucleation of new embryos continues. According to the results mentioned above, it could be summarized that the evolution of decomposition of the studied alloy appears to undergo the following stages during the early period of aging at 450 °C for up to 8 h, that is, forming of GP zone (fcc Cr-rich phase) and ordered fcc Cr-rich phase, and further transforming to ordered bcc Cr-rich phase
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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
Fig. 4 – HRTEM images taken from a specimen aged at 450 °C for 8 h showing fine ordered fcc precipitates: (a) SAED pattern in the inset obtained by Fast Fourier Transform (FFT) show that these precipitates have the ordered fcc structure and cube-on-cube OR, (b) enlarged image of the box in (a) obtained by FFT and Filtered and then inverse FFT techniques.
(B2, or Heusler structure). It is difficult to differentiate clearly because of overlapping of three stages. Accompanied by these transforming, in addition, the morphology of precipitates changes from fine spheres to larger plates. And the orientation relationship between the precipitates and Cu matrix changes from cube-on-cube OR to N–W OR.
3.2.
Precipitation during aging at 550 °C
Fig. 5 shows a TEM image taken from a specimen aged at 550 °C for 5 min. The Moiré fringes are found in some larger particles (5–10 nm), while the fine particles with a size of 2– 4 nm show faint dark contrast. The corresponding SEAD pattern in the inset reveals that the precipitates have an ordered fcc structure with cube-on-cube OR. In this specimen, no GP zone is observed. Absence of the GP zone arises from the higher aging temperature. When aging at the temperature below the solvus of ordered fcc phase and above the one of GP zone, the former will precipitate first. Fig. 6 summarized the TEM results obtained from the samples aged at 550 °C for 8 h. Fig. 6(a) and its insets reveal that the fine precipitates have an ordered fcc structure and
cube-on-cube OR with Cu matrix along zone axis of [110]Cu. BF TEM image (Fig. 6(b)) and its corresponding SAED pattern with zone axis of [100]Cu (Fig. 6(c)) illustrate that all the fcc Cu reflections are streaked along the <110>* directions. There are two sets of diffraction spots from precipitates in Fig. 6(c). The faint superlattice reflections (marked by notation □ in the Fig. 6(d)) in the first set of diffraction pattern (marked by P1) can be seen midway between one of these reflections and the transmitted spot. Its diffraction pattern is indexed to be the ordered bcc structure according to the literature [3,25]. And its lattice parameter is measured to be 0.321 ± 0.002 nm, which is larger than the value of 0.280 ± 0.002 nm calculated in the section above. The other set of diffraction pattern (marked by P2) with faint superlattice reflections (marked by notation ● and arrows) exhibit that the precipitates also have an ordered structure. It is difficult to judge that these precipitates belong to bcc or fcc structure because ordered bcc structure has the same diffraction pattern along [111] zone axis as that of fcc structure. However, it does not affect the indexing of the diffraction spots. The lattice parameter of the precipitates is measured to be 0.446 ± 0.003 nm. And the OR of these precipitates with Cu matrix is: ð022ÞCu ==ð022ÞP ½100Cu ==½111P
Fig. 5 – TEM image and [112]Cu SAED pattern taken from a specimen aged at 550 °C for 5 min.
The coarse precipitates are also found in the same specimen as shown in Fig. 7. Its SAED in the inset obtained by FFT demonstrates the precipitates have B2-type or Heusler structure and N–W OR along zone axis of [112]Cu. Compared with Fig. 5, the interparticle spacing in Fig. 6(a) and (b) become greater with aging time. It suggests that these coarse particles with N–W OR grow at the expense of those having a cube-on-cube OR. In summary, it can be considered that the evolution of decomposition of the studied alloy during isothermal aging 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase (B2, or Heusler structure).
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
73
Fig. 6 – TEM images and SAED patterns in the inset taken from a specimen aged at 550 °C for 8 h: (a) fine precipitates exhibiting a cube-on-cube OR with Cu matrix, the inset showing [110]Cu SAED pattern, (b) BF TEM image showing coarser precipitates, (c) SAED pattern taken from (b) with zone axis [100]Cu showing streaks along <110>* directions around the reflections of fcc Cu, (d) key to (c) showing positions of the fundamental (○) and superlattice (□ or ●) reflections from the ordered fcc precipitates and fcc matrix reflections (○).
3.3.
Properties of aged alloy
The age-hardening behaviors of the alloy aged at 450 °C and 550 °C are shown in Fig. 8. The hardness of the alloy increases quickly at the initiate stage of aging at 450 °C. After aging at 450 °C for 4 h, the precipitation kinetics seems to slow down.
The hardness value of 120 HV is obtained after 8 h. While aging at 550 °C, it is evident that a very rapid hardening process occurs in the early stages until 2 h and the hardness decreases with increasing aging time. The peak hardness, 118 HV, is obtained in the alloy aged at 550 °C for 2 h. In the initial 4 h of aging, the specimens aged at 550 °C have higher
Fig. 7 – HRTEM image taken from a specimen aged at 550 °C for 8 h. [112]Cu. SAED pattern in the inset obtained by FFT showing that the precipitates have B2-type or Heusler structure and N–W OR with the Cu matrix.
Fig. 8 – Vickers hardness as a function of aging time at 450 °C and 550 °C.
74
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 1 (2 0 1 3) 6 8–7 5
hardness than those aged at 450 °C. But when the aging exceeds 4 h, the former have lower hardness than the latter. As the alloy is aged at low temperature, it takes a longer time to achieve complete precipitation due to the low diffusion rate of solute atoms. Besides, since the degree of supersaturation at low temperatures is higher than that at high temperatures, a higher density of fine fcc Cr-rich particles are precipitated from Cu solid solution (as shown in Fig. 1) [30]. Higher aging temperature and longer aging time easily lead to overaging. The specimen aged at 550 °C for 8 h exhibit greater interparticle spacing and coarser bcc Cr-rich particles (as shown in Fig. 7) than that aged for 5 min (Fig. 5). On the whole, the hardness behavior of the alloy investigated in the present study is in good agreement with published data on similar alloys [31]. Fig. 9 presents the electrical conductivity of the alloy as a function of aging time at 450 °C and 550 °C. All specimens aged at 550 °C have an overall higher conductivity than those aged at 450 °C. Moreover, the conductivity of the former increases more quickly than the latter at the initiate stage of aging (less than 60 min). It reveals that higher aging temperature leads to a higher conductivity and a shorter time to reach a stable value.
4.
Discussion
The two factors, namely, coherent interfaces and ordering, play very important roles in the precipitation evolution of Cu\Cr\Zr\Mg alloy. The precipitation of the Cr-rich bcc phase initiates through the nucleation of fcc precipitates that are coherent with the Cu-rich fcc parent phase. Because the atomic radii of Cr and Cu (rCr = 0.127 nm, rCu = 0.128 nm) differ by only 0.78%, the GP zones in the studied alloy take the form of sphere to minimize the total interfacial energy and therefore lowers the nucleation barrier in the initial stage of aging, although the distortion energy exists. The coherent interfacial energy γ (for pure nuclei, γ = 86 mJm−2) [25] is much smaller than the Crbcc/ Cufcc interfacial energy reported in the literature for non-coherent
Crbcc/Cufcc interfaces (625 mJm−2) [32]. That is, the nucleation of fcc Cr-rich is much easier than that of non-coherent bcc nuclei. By the aid of fcc transition phase, the total free energy of this alloy lower faster compared to those by the direct formation of bcc precipitate from matrix. It is well known that the process of ordering is different with the ordinary precipitation. Because ordering involves only local atomic adjusting within a unit cell, it is kinetically much faster than ordinary precipitation which involves long range diffusion. Therefore, it can accelerate the process of precipitation and phase transformation of fcc → bcc. Moreover, ordering make the interplanar spacing d220 of ordered fcc Cr-rich phase enlarged to the d200 of ordered Cr-rich phase. This facilitates the formation of the bcc precipitates and promotes the development of N–W orientation relationship.
5.
Conclusions 1. The sequence of decomposition in the Cu\Cr\Zr\Mg alloy during the early stage of aging at 450 °C can be summarized as: supersaturated solid solution → GP zone (fcc Cr-rich phase) → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase (B2, or Heusler structure). The sequence of decomposition during aging at 550 °C is: supersaturated solid solution → ordered fcc Cr-rich phase → ordered bcc Cr-rich phase. 2. Accompanied by the precipitation process, the OR between the precipitates and the Cu matrix changes from cube-on-cube to N–W OR. 3. Ordering occurred in the precipitation in this alloy facilitates the formation of the bcc precipitates and promotes the development of N–W orientation relationship.
Acknowledgments The authors are grateful to Professor M.P. Wang and Dr. Q. Lei at the Central-South University for TEM experimental assistance, to Dr. B.B. Tang at the Nanchang University for his helpful discussions. This work is financially supported by National Natural Science Foundation of China (Project No. 51161018).
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Fig. 9 – Electrical conductivity as a function of aging time at 450 °C and 550 °C.
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