Hot deformation behavior of Cu–Fe–P alloys during compression at elevated temperatures

Hot deformation behavior of Cu–Fe–P alloys during compression at elevated temperatures

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2892–2896 journal homepage: www.elsevier.com/locate/jma...

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2892–2896

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Hot deformation behavior of Cu–Fe–P alloys during compression at elevated temperatures Hui Zhang a,b,∗ , Honggang Zhang c , Luoxing Li a,b a b c

State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Changsha 410082, China College of Materials Science and Engineering, Hunan University, Changsha 410082, China College of Materials Science and Engineering, Beijing University of Science and Technology, Beijing 100083, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Hot compression tests of the Cu–Fe–P alloys (KFC alloy and C194 alloy) were performed

Received 13 August 2007

on Gleeble 1500 system at strain rates ranged between 0.01 and 10 s−1 and temperatures

Received in revised form

between 650 and 850 ◦ C. The results show that the flow stress and deformed microstructure

26 May 2008

strongly depend on the Fe contents. For the KFC alloy, the true stress–true strain curves

Accepted 21 June 2008

are characterized by multiple peaks or a single peak flow, followed by a steady state flow stress, mainly related to dynamic recrystallization. For C194 alloy, the true stress–true strain curves exhibit a peak stress in the initial stage of deformation, after which the flow stress

Keywords:

decreases monotonically, dynamic particles coarsening seemed to be responsible for flow

Cu–Fe–P alloys

softening. The values of deformation activation energy for the KFC alloy and the C194 alloy

Hot compression deformation

are 289 and 316 kJ/mol, respectively. Both are higher than that of the polycrystalline copper.

Flow softening

The increase of deformation activation energy of the Cu–Fe–P alloys is due to the existence

Dynamic recrystallization

of the Fe-rich particles, which act as obstacles for the dislocation movement, and lead to a

Dynamic particle coarsening

increase of the flow stress.

Hot deformation activation energy

1.

Introduction

With the recent development in the semiconductor industries, precipitation-hardenable copper alloy strips with high electrical conductivity and high strength, as well as excellent heat resistance, are widely used as leadframe materials. There are two processing routes for manufacturing of these kind strips. One is the conventional process including hot rolling, cold rolling and subsequent solution heat treatment and aging. Another is an advanced process combining hot rolling and solution heat treatment by controlled online water cooling. Norityuki Nomoto et al. (1999) have proposed a new process for manufacturing Cu–Fe alloy C194-ESH with high electrical conductivity and excellent heat resistance, including solution treatment at temperatures higher than 900 ◦ C using new



Corresponding author. Tel.: +86 731 8664086; fax: +86 731 8821483. E-mail address: [email protected] (H. Zhang). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.06.048

© 2008 Elsevier B.V. All rights reserved.

continuous annealing furnace. Systematic physical simulation of thermo-mechanical processing routes has been applied by Somani and Karjalainen (2004) on a Gleeble 1500 simulator to four copper alloys Cu–0.57Co–0.32Si, Cu–0.55Cr–0.065P, Cu–0.22Zr–0.035Si and Cu–1.01Ni–0.43Si (mass %) to clarify the influences of processing conditions on their final properties, such as strength and electrical conductivity. For metallic materials, their properties are greatly influenced by their microstructure. Microstructural control during hot deformation requires knowledge of how they respond to the deformation parameters, including deformation temperature, strain and strain rate. Recent observations have suggested that dynamic recrystallization may operate during high temperature deformation of pure copper and alloys. Impurity atoms (Gao and Belyakov, 1999), initial grain size

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Table 1 – Main chemical compositions of alloys used for tests (wt%) Alloys KFC C194

Fe

P

0.13 2.34

0.025 0.020

Zn 0.13

Cu Remaining Remaining

(Blaz and Sakai, 1983) and alloying (Sakai et al., 1995) influence greatly on flow stress behaviors and dynamic recrystallization of copper and alloys. Dynamic particles coarsening within shear bands was observed and assumed to affect further strain softening during hot deformation of a supersaturated Cu–3.45 wt% Ti alloy (Hameda and Blaz, 1998, 1997). Straininduced localized Ni2 Si-precipitate during hot deformation of Cu–Ni–Si–Cr–Mg alloy (Niewczas et al., 1992) and intensive coarsening of precipitates began at grain boundaries during hot deformation of Cu–Ni alloys (Blaz et al., 1994) were found to be a dominating reason for flow softening. It has been reported that (Zhang et al., 2006) the dynamic recrystallization (DRX) was the main reason for the flow softening during hot deformation of a KFC copper alloy and the dynamic spherical Fe-rich precipitates and successive dynamic particles coarsening has been assumed to be responsible for flow softening at high strains, especially when samples deformed at low temperatures and higher strain rates. In the present study, hot compression tests of two Cu–Fe–P alloy were preformed on Gleeble 1500 system at strain rates ranged between 0.01 and 10 s−1 and deformation temperatures between 650 and 850 ◦ C. Hot deformation behavior of the alloys, including their flow stress and associated structural changes were investigated.

2.

Experimental procedure

The experiments were carried out on the Cu–Fe–P alloys with chemical compositions given in Table 1. Cylindrical samples with 8 mm in diameter and 12 mm in height were machined from commercially hot-rolled sheets with 14 mm in thickness and subsequent homogenized at 750 ◦ C for 1 h. Convex depressions 0.2 mm deep were machined on both ends of the sample in order to maintain the lubricant of graphite mixed with machine oil during compression tests. Compression tests

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were performed on a computer servo-controlled Gleeble 1500 system at a strain rates of 0.01–10 s−1 and deformation temperatures of 650–850 ◦ C. The sample was resistance heated to deformation temperature by thermocoupled-feedbackcontrolled AC current at a heating rate of 10 ◦ C/s and held for 3 min before compression. The compressed samples were automatic water quenched immediately upon the completion of the tests. Water-quenched condition samples examined using POLYVER MET II metallographic microscope (OM) and H-800 transmission electron microscope (TEM). Specimens for microstructure examination were taken near the middle height of the pancake thickness where there is homogeneous large deformation.

3.

Results and discussion

3.1.

The stress–strain curves of Cu–Fe–P alloys

Typical true stress–true strain curves obtained during hot compression of the two alloys at a strain rate of 1 s−1 and deformation temperature 650–850 ◦ C are shown in Fig. 1. For the KFC alloy, at high temperatures and/or low strain rates, true stress–true strain curves show stress oscillations at early stages of compression, followed by a steady state flow at high strains, indicating that the occurrence of discontinuous dynamic recrystallization. For the C194 alloy, the true stress–true strain curves exhibit a peak stress in the initial stage of deformation, after which the flow stress decreases monotonically until high strains. The flow softening is probably subjected to the non-homogeneous localized plastic flow, dynamic coarsening of precipitates and intense dynamic recovery (DR) of the metal matrix within shear bands. The development of highly coarsened spherical Fe-rich particles in the vicinity of grain boundaries was also considered to produce a soft structural component in hot deformation of the Cu–Fe–P alloys. The softening mechanism will discuss in detail in the following section by comparison analysis with hot deformation activation energy calculations and microstructural observation. From Fig. 1a and b, it can also be seen that the strain corresponding to the maximal stress increases with increase in strain rate and/or with decrease in deformation temperature

Fig. 1 – Typical true stress–strain curves at strain rate of 1 s−1 for: (a) KFC alloy and (b) C194 alloy.

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for the KFC alloy (0.068–0.549), but almost keep low values for the C194 alloy (<0.05). The high hardening rate at initial deformation stages of the C194 alloy is due to the inhabitation of dynamic recrystallization and due to the existence of intensive dynamic precipitation of Fe-rich particles. It should be also noted that flow stress levels for the C194 alloy are always higher than those for the KFC alloy under similar deformation condition.

3.2.

Constitutive analysis

In hot working, it is commonly accepted that the relationship between the peak stress or steady state stress, strain rate and temperature can be expressed as (McQueen, 1993; McQueen and Ryan, 2002): Z = ε˙ exp

Q RT

= f1 () = A1  n1

Z = f2 () = A2 exp(ˇ)

(1) (2)

where A1 , A2 , n1 and ˇ are constants, Z is the Zener–Hollomon parameter, Q is an activation energy for hot deformation and R is the gas constant. The power law, Eq. (1), and the exponenttype equation, Eq. (2), break at a high stress and at a low stress, respectively. The hyperbolic sine type equation, Eq. (3), proposed by Sellars and McTegart (Sellars and McTegart, 1966) is more general form suitable for stresses over a wide range. Z = f () = A(sin h ˛)

n

(3)

where A and n are constants. ˛ is the stress multiplier, also the additional adjustable parameter and ˛=

ˇ ˇ ≈ n n1

Fig. 2 – Dependence of the activation energies on the total alloying contents.

(4)

It is easier to obtain values of n1 or n and ˇ by means of linear regression through Eq. (1) and Eq. (2), respectively. Then a value of ˛ was taken as a first approximation in Eq. (4) and the relationship of Eq. (3) was derived to obtain a new value of n, which was then iterated to obtain the optimum values of ˛, n, A and Q (Zener and Hollomon, 1944). The hot deformation activation energy Q is an important physical parameter serving as indicator of deformation difficulty degree in plasticity deformation. The values of Q for the KFC alloy and the C194 alloy are,

respectively, 289 and 316 kJ/mol, which are higher than that of polycrystalline copper with different purities (210 kJ/mol for 6N or 7NCu, 245 kJ/mol for 4NCu) (Gao and Belyakov, 1999). The higher Q of the Cu–Fe–P alloys may be associated with the addition of Fe atoms, which can play a role as extra obstacles for moving dislocations and hence increases the flow stress level during hot deformation. Simultaneously, higher value of Q for the C194 alloy indicates increasing difficulty of hot working operation when compared with the KFC alloy. Combining Gao’s results (Gao and Belyakov, 1999), the effect of alloying elements in the Cu–Fe–P alloys on the constitutive relationship is similar to the impurities in polycrystalline copper, the dependence of the activation energies on the total contents of alloying elements is shown in Fig. 2. Analysis indicates that the hot deformation activation energy Q for the Cu–Fe–P alloys can be approximately expressed as: Q = 304.72 + 10.76 ln



Ci

(5)

where Ci is the total contents of alloying elements.

3.3.

Microstructural evalution

Figs. 3 and 4 show the representative optical deformed microstructures of the KFC and the C194 alloy, respectively. It can be seen that dynamic recrystallization (DRX) occurs in

Fig. 3 – Representative optical microstructures of the KFC alloy deformed at: (a) deformation temperature of 650 ◦ C and strain rate of 1 s−1 and (b) deformation temperature of 850 ◦ C and strain rate of 1 s−1 .

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Fig. 4 – Representative optical microstructures of the C194 alloy deformed at: (a) deformation temperature of 650 ◦ C and strain rate of 1 s−1 and (b) deformation temperature of 850 ◦ C and strain rate of 1 s−1 .

evidence during hot compression deformation of the Cu–Fe–P alloys by bulging out of part serrated grain-boundary (Miura et al., 2004) and formation of twin (Miura et al., 2002). The dynamic recrystallization grain size is sensitively dependent on deformation temperature and strain rate, exactly on the Zener–Hollomon parameter Z (Miura et al., 2004; Miura et al., 2002). Grain size in the C194 alloy is much smaller than that in the KFC under the same deformation temperature and strain

rate. Working flow streamline is clearly seen in the deformed C194 alloy at temperature of 650 ◦ C and strain rate 1 s−1 . Transmission electron microscopy revealed that coarsen spherical precipitates (size about 0.5 ␮m) companying with a number of fine precipitates (size about 80 nm) were observed in the deformed KFC alloy at temperature of 650 ◦ C and strain rate 1 s−1 (Fig. 5(a)), while at temperature of 850 ◦ C and strain rate 1 s−1 in the deformed C194 alloy (Fig. 5(c)).

Fig. 5 – TEM deformed microstructures: (a) KFC alloy, deformation temperature of 650 ◦ C and strain rate of 1 s−1 , (b) KFC alloy, deformation temperature of 850 ◦ C and strain rate of 1 s−1 and (c) C194 alloy, deformation temperature of 850 ◦ C and strain rate of 1 s−1 .

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Precipitation takes place commonly in grain interior or twin grain interior and within high-energy zone, such as local shear bands, high-density dislocation zone or grain boundaries (Zhang et al., 2006). Shear banding and localized flow in the vicinity of grain boundaries was found to be a dominating feature of inhomogeneous deformation at lower deformation temperature, the phenomenon was also observed for hot deformed Cu-3.45 wt%Ti alloy (Hameda and Blaz, 1998, 1997), Cu–Ni–Cr–Si–Mg alloy (Niewczas et al., 1992) and Cu–Ni alloys (Blaz et al., 1994). For the KFC alloy, dynamic coarsening of Fe-rich particles within shear bands is the most efficient structural processes giving rise to the pronounced flow softening at temperature of 650 ◦ C, whereas precipitates apparently dissolve in the Cu matrix during deformation at 850 ◦ C and strain rate of 1 s−1 (Fig. 5(b)). In contrast, for the C194 alloy, dynamic coarsening of Fe-rich particles within shear bands is observed during deformation at 850 ◦ C and strain rate of 1 s−1 . The different dynamic precipitating behavior of the two alloys is related to the Fe content. The increasing in Fe content leads to the increasing of inhibition for bulging of the grain boundaries, and hence, retards of dynamic recrystallization and increases the driving force for dynamic precipitation during hot deformation of Cu–Fe–P alloys. By comparing the flow stress and microstructure of the KFC alloy with those of the C194 alloy, it is suggested that the promoting of the finished deformation temperature (i.e. finished rolling temperature) is required to retard the coarsening of spherical precipitates during hot deformation of high Fe content Cu–Fe–P alloys. Based on early studies, the precipitates in the aged Cu–Fe alloy C194-ESH (Norityuki et al., 1999) and Cu–1%Fe alloy with a small amount of phosphorous are considered to be spherical ␣-Fe particles (Hidemichi et al., 1996).

4.

Conclusions

Hot compression tests of Cu–Fe–P alloys (KFC alloy and C194 alloy) were carried out on Gleeble 1500 system at strain rates ranged between 0.01 and 10 s−1 and deformation temperatures between 650 and 850 ◦ C. Following conclusions are drawn from the study. (1) The flow stress and deformed microstructure behaviors strongly depend on the Fe contents for the studied alloys. For the KFC alloy, the true stress–true strain curves are characterized by multiple peaks or a single peak flow, followed by a steady state flow stress, mainly related to dynamic recrystallization. For the C194 alloy, the true stress–true strain curves exhibit a peak stress in the initial stage of deformation, after which the flow stress decreases monotonically, dynamic particles coarsening has been assumed to be responsible for flow softening. (2) The values of deformation activation energy for the KFC alloy and the C194 alloy are 289 and 316 kJ/mol, respectively, which are higher than that of polycrystalline copper. Fe-rich particles in the Cu–Fe–P alloys distinctly change the

activation energy, which can be approximately expressed  as Q = 304.72 + 10.76ln Ci , hence influence the flow stress value and increases difficulty of hot working operation. (3) Dynamic particles coarsening may occur at higher deformation temperature for the C194 alloy than that for the KFC alloy, it is suggested that the promoting of the finished deformation temperature is required to retard the coarsening of spherical precipitates during hot deformation of high Fe content Cu–Fe–P alloys.

references

Blaz, L., Evangelista, E., Niewczas, M., 1994. Precipitation effects during hot deformation of a copper alloy. Metall. Mater. Trans. A. 25A, 257–266. Blaz, L., Sakai, T., 1983. Effect of initial grain size on dynamic recrystrallization of copper. Met. Sci. 17, 609–616. Gao, W., Belyakov, A., 1999. Dynamic recrystallization of copper polycrystals with different purities. Mater. Sci. Eng. A265, 233–239. Hameda, A.A., Blaz, L., 1998. Microstructure of hot-deformed Cu–3.45 wt%Ti alloy. Mater. Sci. Eng. A254, 83–89. Hameda, A.A., Blaz, L., 1997. Flow softening during hot compression of Cu-3.45 wt%Ti alloy. Scripta Mater. 37, 1987–1993. Hidemichi, F., Tatsuo, S., Akihiko, K., 1996. Effect of the additions of a small amount of phosphorous on precipitation in Cu–1%Fe alloy. Nippon Kinzoku Gakkaishi. 59, 505–511. McQueen, H.J., 1993. Metal forming: Industrial, mechanical computational and microstructural. J. Mater. Process. Technol. 37, 3–36. McQueen, H.J., Ryan, N.D., 2002. Constitutive analysis in hot working. Mater. Sci. Eng. A322, 43–63. Miura, H., Hamaji, H., Sakai, T., 2002. Twin nucleation at triple junction during hot deformation of copper polycrystal. Mater. Sci. Forum 408–412, 755–760. Miura, H., Sakai, T., Mogawa, R., Gottsten, G., 2004. Nucleation of dynamic recrystallization at grain boundaries in copper bicrystals. Scripta Mater. 51, 671–675. Niewczas, M., Evangelista, E., Blaz, L., 1992. Strain localization during a hot compression test of Cu–Ni–Cr–Si–Mg alloy. Scripta Metall. Mater. 27, 1735–1740. Norityuki, N., Tong, C., Makoto, O., Katsuhiro, Y., 1999. A process for manufacturing Cu–Fe alloy C194-ESH with high electrical conductivity and excellent heat-resistance. Hitachi Cable Review 18, 61–66. Sakai, T., Miura, H., Muramatsu, N., 1995. Effect of small amount addition of Co on dynamic recrystallization of Cu–Be alloys. Mater. Trans. JIM 36, 1023–1030. Sellars, C.M., McTegart, W.J., 1966. On the mechanism of hot deformation. Acta Metall. 14, 1136–1138. Somani, M.C., Karjalainen, L.P., 2004. Improving the mechanical properties of copper alloys by thermo-mechanical processing. Acta Metall. Sin. (English Lett.) 17, 111–117. Zener, C., Hollomon, J.H., 1944. Effect of strain-rate upon the plastic flow of steel. J. Appl. Phys. 15, 22–27. Zhang, H., Zhang, H.G., Peng, D.S., 2006. Hot deformation behavior of a KFC copper alloy during compression at elevated temperatures. Trans. Nonferrous Met. Soc. China 16, 562–566.