Effect of dynamic strain aging on the deformation behavior and microstructure of Cu-15Ni-8Sn alloy

Accepted Manuscript Effect of dynamic strain aging on the deformation behavior and microstructure of Cu-15Ni-8Sn alloy Guangwei Peng, Xueping Gan, Yexin Jiang, Zhou Li, Kechao Zhou PII:

S0925-8388(17)31721-8

DOI:

10.1016/j.jallcom.2017.05.127

Reference:

JALCOM 41857

To appear in:

Journal of Alloys and Compounds

Received Date: 14 December 2016 Revised Date:

8 May 2017

Accepted Date: 12 May 2017

Please cite this article as: G. Peng, X. Gan, Y. Jiang, Z. Li, K. Zhou, Effect of dynamic strain aging on the deformation behavior and microstructure of Cu-15Ni-8Sn alloy, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.127. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effect of dynamic strain aging on the deformation behavior and microstructure of Cu-15Ni-8Sn alloy

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Guangwei Penga,b, Xueping Gan*a , Yexin Jiang c , Zhou Lic, Kechao Zhoua a. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. b. Hunan Automotive Engineering Vocational College , Zhuzhou, 412001,China. c. School of Materials Science and Engineering, Central South University, Changsha 410083, China.

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* Corresponding author: Tel.: +86 731 88836303;E-mail address: [email protected] (Xueping Gan).

Abstract: The compression deformation behavior of the Cu-15Ni-8Sn alloy prepared by powder

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metallurgy was investigated under a temperature range from 300 to 573K and a strain rate range of 5×10-5 ~1×10-2 s-1. The results showed that in a certain range of strain rates and temperatures, the stress-strain curve exhibited obvious serrated flows which were induced by dynamic strain aging (DSA). The activation energy calculations indicated that the DSA was mainly attributed to the interaction of Sn solute atoms with dislocations. In the microstructure of the alloy pre-deformed in

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the region of DSA, Sn atoms were more likely to aggregated along the slip bands and therefore resulted in the dynamic precipitation of Sn-rich phase at lower temperature than that of natural aging alloy without pre-deformation.

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1. Introduction

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Key words :Cu-15Ni-8Sn alloy; serrated flow; dynamic strain aging; dynamic precipitation.

Cu-15Ni-8Sn(wt.%) is a commercial alloy (designated as C72900 ASTM B740-80) and widely used in the electronic and mechanical industry due to its good electric conductivity, excellent age harden-ability and high corrosion resistance [1,2] . The high strength of Cu-Ni-Sn alloys derives from precipitation hardening. Many studies had been performed to understand the precipitation hardening mechanism of Cu-Ni-Sn alloys [3-6]. To attain higher strength, cold rolling or drawing are usually used before aging since cold pre-deformation lead to both work hardening and acceleration of aging strengthening process. So far, researchers have not reached an agreement about the influence mechanism of pre-deformation on aging process.

ACCEPTED MANUSCRIPT Plewes[7] considered that the prior cold work increases the kinetics of the precipitation, and confirmed that the lower the aging temperature, the higher the minimum level of prior cold work required to achieve a certain strengthening effect. S.Spooner and B. G. Lefevre[8] believed that the accelerated strengthening response in cold worked alloys cannot be attributed to an accelerated or altered decomposition process, and the observed strengthening was the result of the interaction

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between the coarsened spinodal structure and the dislocation substructure formed by the pre-deformation. Meng Shu-ken[9] observed that the pre-deformed alloy precipitates a large number of fine particles along the dislocation during spinodal decomposition, these particles can effectively prevent the movement of dislocations and improve the strength of the alloy. J. Balik et

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al[10] studied the dynamic strain aging (DSA) in CuNiSn alloys which contain about 10 at.% Ni and 0, 0.24, 0.99, 1.19 at.% Sn and found no remarkable DSA for Ni solution and strong DSA for Sn containing alloys.

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Up to now, there have been few works concerning the effects of DSA of Cu-15Ni-8Sn alloy, as well as the evolution of the microstructure during dynamic deformation. Based on the analysis of the previous studies, the author seeks to determine the temperature and strain rate range of DSA and investigate its effect on the microstructure of the alloy. The present study may be helpful for understanding the mechanism of the acceleration effect of pre-deformation on aging process.

2.1. Materials and specimen

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2. Experimental procedures

The material was prepared by powder metallurgy with the chemical compositions (wt%) : Ni 15.20, Sn 7.97. Alloy powders used were -100 mesh with oxygen content less than 290ppm.

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Pressed ingots with a diameter of 123mm and a length of 190mm were produced by isostatic cool pressing conducted at 200MPa. Vacuum-sintering was performed at 850

for 6h before hand .Hot extrusion with water-sealing was

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alloy ingots were homogenized at 830

for 4h and 10-3Pa. The

performed at 830℃ with an extrusion speed of 30mm/s and an extrusion ratio of 0.95. As extruded alloy rods with a diameter of 40mm were subjected to homogenization treatment at 850℃ for 1h and then water quenched. Finally, the cylindrical specimens with the diameter of 8mm and the height of 12mm were machined from the ingot. 2.2. Test procedures Compression tests were used instead of tensile testing because they are less time and material consuming. Compression tests were conducted using gleeble-3800 simulator under temperature ranges from 300 to 573K, and strain rates range from 5×10-5 to 5×10-2s-1. Resistance heating was adopted and heat preservation for two minutes. The temperature in all the tests was controlled

ACCEPTED MANUSCRIPT within ± 2 K. Pad graphite sheet was placed at both end surfaces of the specimens to reduce the friction in-between the die and work piece. 3. Results and discussions 3.1 Serrated flow in stress-strain curves Fig.1 shows the true stress-strain at 523K with different strain rates. Repeated load fluctuations

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in these stress-strain curves are called serrated flow which is one of the most important manifestations of dynamic strain aging. The onset of serrated flow is postponed with increasing of strain rate at the given temperature, suggesting higher strain rates lead to later happen of serrated flow. Since the DSA is mainly controlled by the movement speed of the mobile dislocations. The

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diffusion of solute atom is more difficult to keep up with the velocity of dislocations when the strain rate is higher, and can not form an effective pinning to mobile dislocations [11]. The moving dislocation density plays a leading role in the deformation process, only when enough dislocations

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are available to interact with solute atoms will serrated flows happen. So, the minimum strain

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required to achieve the corresponding dislocation density increases when strain rate increases.

Fig.1 The true stress-strain at 523K with different strain rates

Fig.2 illustrates the difference of stress-strain at different temperature with the same strain rate

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of 5×10-3s-1. At the temperature below 423K, the diffusion rate of Sn atoms is too small to catch mobile dislocations, serrated flow was suppressed. At the temperature range of 423-573K, the diffusion ability of solute atoms was accelerated and can pin the moving dislocations in some region with enough dislocation density. The multiplication rate of mobile dislocation increases with the increasing of the test temperature, and this may explain why the serrated flow appeared earlier at higher temperature.

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Fig.2 The true stress-strain curves at different temperature with strain rate of 5×10-3s-1

Serrated flow can occur in a certain range of temperature and strain rate. Nevertheless, they may not have the same shape. The different types of serrations are identified as types A, B and C

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according to the classification scheme in the literature [12-14] and summarized on the serration map shown in Fig.3. Type A serrations are often characterized by an abrupt rise in the loads followed by discontinuous drops to or below the general level of stress–strain curves, and they generally only occur at low temperature with high strain rate. Type B serrations band propagation is discontinuous and type B serrations are fine scale oscillations about the general or mean level of load values in the

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stress-strain curves. Type B serrations often develop from type A serrations with increasing strain or occur at the onset of serrated flows. Type C serrations are characterized by load drops always below the mean level of stress–strain curves, and they generally occur at high temperature with low strain rate[15,16]. Following the above classification, different types of serrations observed at various

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strain rates and temperatures are summarized in Table 1.

Fig.3 Various types of serrated flow in stress-strain curve

ACCEPTED MANUSCRIPT Table 1 Different types of serrations at various strain rates and temperatures in Cu-15Ni-8Sn. Type of serrations at different temperature (K) 300 373 423 473 523 553 -5 B+C B+C B+C C C C 5×10 -4 B B B+C C C C 1×10 --B B+C B+C C 5×10-4 -3 --MS B+C B+C B+C 1×10 ---B B B+C 5×10-3 ---MS A+B B 1×10-2 -----A+B 5×10-2 - - No serration; MS – Mild serrations

573 C C C B+C B+C B A+B

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Strain rate,s-1

3.2 Activation energy for serrated flow

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According to Cottrell’s theory[12], a critical strain ( ε c ) is necessary for initiation of serrated flow because a critical vacancy concentration created by plastic deformation is needed to enhance the diffusion coefficient of the solute species responsible for DSA. When the diffusion coefficient

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of solute atoms is high enough to pin the moving dislocation with solute atmosphere during the waiting time, serrations will start [17]. The critical strain ε c for initiation of serration was related to the strain rate( ε& ) and temperature by the following equations[18-20]: ε c( m+ β ) = Kε& exp(Q / RT ) ln ε& = (m + β ) ln ε c − (ln K + Q / RT )

(1) (2)

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Where m , β are exponents related to the variation of vacancy concentration and mobile dislocation density; K is a constant; Q is the activation energy at the onset of serration (kJ/mol); R is the universal constant(8.3144J/mol K) and T is absolute temperature(K). Equation (2) represents a straight line with a slope of (m + β ) , when ln ε& is plotted as a

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function of ln ε c at a constant T . Knowing the value of (m + β ) , activation energy Q can be determined from the slope of the plot of ln(ε cm+ β / T ) versus 1 / T for a given strain rate.

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Fig.4 plot showing the effect of strain rate on critical strain at 373K, 423K, 473K and 523K. It is obvious that the critical strain for onset of serrations is also increased as the rate of deformation is increased. The slope of 373K, 423K, 473K and 523K plot yields the value of (m + β ) as 2.02, 2.23, 2,52 and 2.74, respectively. With the values of (m + β ) at different temperature, the plot of ln(ε cm+ β / T ) versus 1 / T can be obtained, as shown in Fig.5. The slope of the plot of ln(ε cm+ β / T )

versus 1 / T is the value of Q / R . So, the calculated activation energy in this work is 68.84 kJ / mol , this value is close to the reported activation energy of Sn in Sn-0.7Cu alloy (60 kJ / mol ) [21]. So, it can be concluded that the serrated flow phenomenon associated with DSA was due to the interaction of Sn solute atoms with dislocations, which agree with the result of J. Balik’s study [10].

Fig.4 Relationship between critical strain and strain rate

Fig.5 The plot of ln(ε cm+β / T ) versus (1/T)

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3.3 Microstructure analysis

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The optical micrograph of solution-treated and pre-deformed specimens is shown in Fig.6.

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Fig.6(a) shows that the microstructure of the solid-solution appears many quenching twins and has a little precipitates within grains or at grain boundaries. The stress-strain curve at 300K with 5×10-3s-1 shows no serrated flow (in Fig.2), the microstructure of this specimen shows that the grains have been elongated and many slip bands appear in the grain, as shown in Fig.6(b). In Fig.6(c), there are many black dots in the elongated grains, indicating that DSA has led to aggregation of Sn atoms and

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accelerated the precipitation of Sn-rich phase.

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Fig.6 The optical micrographs of different specimens (a) solid-solution state;(b) pre-deformed at 300K with 5×10-3s-1;(c)pre-deformed at 373K with 5×10-5s-1

The microstructures of the specimens after compression tests were also observed by scanning electron microscope (SEM) .As shown in the Fig.7, many precipitates of Sn-rich phase were found in the grain, as well as at grain boundaries, but the former precipitates are usually smaller than the latter.

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Fig.7 Sn-rich phase precipitates of grain interior and grain boundary

Fig.8 shows the SEM micrographs of the solution-treated and pre-deformed specimens. Fig.8 (a) is the micrograph of the specimen in solid-solution state, crystal and grain boundaries are

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relatively clean and appear no slip bands and little Sn-rich precipitates. There exit obvious serrated flow in the stress-strain curve at 423K with strain rate of 5×10-5s-1 (as listed in Table.1) and Fig.8 (b) is the micrograph of the specimen pre-deformed under this condition. It can been seen that a large number of parallel slip bands appear on the grain surfaces at both sides of the grain boundary. The slip bands terminate at the grain boundaries, and the fine Sn-rich precipitates dispersed along them. There appear no serrations in the stress-strain curve at 423K with strain rate of 1×10-2s-1. In the

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microstructure of the specimen pre-deformed under this condition, there exit only some coarse concentrated precipitates, which are mainly inherited from solid-solution state rather than caused by

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DSA, as shown in Fig.8(c).

Fig.8 The SEM micrographs of various specimens(a) solid-solution state (b) pre-deformed at 423K with 5×10-5s-1; (c) pre-deformed at 423K with 1×10-2s-1.

The deformation dislocation density and the morphology of precipitates were also analyzed by transmission electron microscope (TEM). As shown in Fig.9, the dislocation density of the solid-solution state is very low, and there appear only a few dislocation lines, while a lot of dislocation pile-ups and deformation twins appear in the specimen pre-treated at 300K with

ACCEPTED MANUSCRIPT 5×10-5s-1. At temperature of 373K, modulated structures can be found in some regions with low dislocation density, as Fig.9(c) shows. As the temperature reach to 473K, dislocations pile up and tangle each other to form a large number of dislocation cells, due to the solute pinning effect. At temperature of 523K, some nano scale spherical particles were separated out, the strengthening mechanism is gradually transformed from dislocation strengthening to precipitation strengthening.

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While in natural aging, Sn-rich precipitates generally appear only in the temperature above 573K[22]. This dynamic precipitation is closely related to DSA, where the initial solute clustering required for precipitation is caused by the dislocation junctions of DSA process [23]. When the dislocations break free from the Sn atoms lock, the clusters left behind can nucleate precipitates

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which act as obstacles for the following dislocations. Therefore, nucleation required for dynamic precipitation is smaller than that of natural aging, and the Sn-rich phase can be precipitated at a lower temperature than that in natural aging. It has been reported that dynamic precipitation occur

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through nucleation of small (0.6 nm) zones, while in naturally aged material the main effect is

Fig.9

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growth if existing zones (in the range of 1 nm) [24].

The TEM micrograph of various specimens (a) solid-solution state;(b) 300K with 5×10-5s-1;(c)373K with 5×10-5s-1;(d)473K with 5×10-5s-1; (e)523K with 5×10-5s-1;

4. Conclusions The compression tests of Cu-15Ni-8Sn alloy prepared by powder metallurgy were performed under a temperature range from 300 to 573K and a strain rate range of 5×10-5 ~1×10-2 s-1, and the following conclusion were drawn: 1) The Cu-15Ni-8Sn alloy exhibits serrated flow behavior at temperature ranges from room

ACCEPTED MANUSCRIPT temperature to 573K. The higher the strain rates, the higher the temperature of DSA. 2) The activation energy of DSA in Cu-15Ni-8Sn alloy was estimated to be 68.84 kJ / mol , and equivalent to the activation energy for diffusion of Sn atoms in Cu matrix. Therefore it can be identified that Sn atoms play major role in the process of DSA. 3) The aggregation of Sn atoms was more likely to occur in the pre-deformed alloys in the

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region of DSA, and therefore resulted in the dynamic precipitation of Sn-rich phase at lower temperature than that of natural aging alloy without pre-deformation.

4) The strengthening effect of DSA on the Cu-15Ni-8Sn alloy was achieved not only by the

was inspired by DSA.

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interaction of solute atoms with dislocations, but also was related to dynamic precipitation, which

5) The DSA and dynamic precipitation during the pre deformation may be an important reason

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for the acceleration of subsequent aging process.

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Acknowledgements: This work was financially supported by The National Key Research and Development Program of China(No. 2016YFB0301402), and project of innovation-dirven plan in central south university is gratefully acknowledged. The authors would like to express their gratitude to the support by State Key Laboratory of Powder Metallurgy, Central South University, Changsha.

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Research Highlights 1) The Cu-15Ni-8Sn alloy exhibits serrated flow in a certain range of strain rates and temperatures.

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2) It can be identified that Sn atoms play major role in the process of DSA.

3) DSA resulted in the dynamic precipitation of Sn-rich phase at

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lower temperature.

4) The DSA and dynamic precipitation may be an important reason

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for the acceleration of aging.