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Production and development of copper-base alloys by the Osprey process
PM
SPECIAL
FEATURE
;
Production and d e v e l o p m e n t of copperbase alloys by the Osprey process R.H. Cookey (Open University, Milton Keynes, UK) and J.V. Wood (University of Nottingham, UK)
Spray deposition has been employed to develop a range of complex copper base-alloys including Cu-Ni-Cr and Cu-Ni-CrSi. These alloys exhibit high temperature stability, and refined and homogeneous microstructures. The effects of silicon additions, t h e r m o m e c h a n i c a l and ageing treatments on the performance of these alloys has been characterized in terms of hardness, tensile strength, electrical conductivity and corrosion properties. There is evidence to show that Cu- l Owt% Ni-3wt% C r - 3 w t % Si exhibits physical and mechanical properties which are superior to those of other copper base alloys that are currently used for electronic connectors, contacts, commutators and springs.
Paper presented at the 1st International Conference on Spray Forming. Reprinted with permission.
38
MPR
April
1991
n the l a s t few years, c o n s i d e r a b l e advances have been achieved in the development of s u p e r s a t u r a t e d copper base alloys t h a t offer improved properties compared with conventional wrought products. These materials exhibit high strengths, high conductivity, good resistance to corrosion, microstructural stability at elevated t e m p e r a t u r e s and are attractive for electronic and marine applications (17). Many of t h e s e d e v e l o p m e n t s have concentrated on atomized powders as a starting feed stock. The most recent publications (6-7) show t h a t Cu-Ni-Sn strips 0026-0657/91/$3.50 ©, ElsevierScience Publishers Lid
made from consolidated and worked atomized powders are of considerable interest as candidates for electronic contacts, connectors and springs. However, melt spinning and spray forming (Osprey processing) also permit the achievement of refined microstructures, solid solubility extensions and near-net shaped products similar to powders without many of the limitations and d i s a d v a n t a g e s a r i s i n g from s u b s e q u e n t consolidation steps. In fact, several high alloy copper base particles are difficult to consolidate to full density. A large amount of residual porosity has been observed in some copper-base alloys (i.e. Cu-15 Ni-8 Sn) which have been spray deposited using nitrogen gas (4). Although porosity in Osprey preforms is generally thought to be controlled by various processing p a r a m e t e r s such as the gas-liquid ratio, and cooling rate, some further porosity results from gas evolution during solidification. The pores vary in dimension and can subsequently lead to detrimental effects on mechanical properties. The aim of this work was to investigate the effects of adding strong nitride formers such as silicon and chromium to see whether nitrides would form in preference to gas evolution. The properties were characterized in terms of microstructure, hardness, tensile strength, corrosion resistance and electrical conductivity.
Experimental procedure - m
Based on a large number of previous results on rapidly solidified copper alloys (16) a single composition was chosen for spray deposition. Preforms with a nominal composition of Cu-10wt% Ni-3wt% Cr-3wt% Si were produced by Osprey Metals Ltd, of Neath, UK, u n d e r s t a n d a r d conditions. Nitrogen gas was employed for atomization.
Other specimens were (:old rolled to achieve a n 80% t h i c k n e s s r e d u c t i o n . They were s u b s e q u e n t l y homogenized at 950°C for 2 hours prior to ageing. A detailed description of the procedures for m e c h a n i c a l a n d m i c r o s t r u c t u r a l examin a t i o n s is p r e s e n t e d elsewhere (4). The electrical conductibility" was assessed by the four t e r m i n a l m e t h o d as per BS5714 with a c u r r e n t of 10 A (14). A D u P o n t 1090 e q u i p p e d with a t h e r r n o m e c h a n i c a l analyzer (TMA) w a s u s e d to m e a s u r e t h e coefficient of t h e r m a l e x p a n s i o n between 20°C a n d 300°C. The corrosion p e r f o r m a n c e was assessed by salt spray testing as per ASTM B l 1 7 for a d u r a t i o n of 168 h o u r s (15).
FIGURE 1: Optical rnicrograph of Osprey Cu-lONi-3Cr-3Si.
FIGURE 2: Optical micrograph of Osprey Cu-lONi-3Cr-3Si after cold rolling and homogenization.
Results w
l
Although the a d d i t i o n s of c h r o m i u m a n d silicon to cupro-nickel alloys in the p r e s e n t work was i n t e n d e d to m i n i m i z e the effect of porosity, studies by Sahoo a u d co-workers (13, 17, 18) have shown t h a t as cast Cu30wt% Ni-l.7wt% Cr-0.45wt% Si alloy (IN768) e x h i b i t s e n h a n c e d corrosion resistance in flowing sea water, good m e c h a n i c a l properties and weldability c o m p a r e d with other cupro-nickel casting alloys. This alloy is s t r e n g t h e n e d by a c o m b i n a t i o n of spinodal d e c o m p o s i t i o n a n d p r e c i p i t a t i o n of nickel a n d c h r o m i u m ,silicides. As the high nickel c o n c e n t r a t i o n in 70/30 cupro-nickel alloys leads to a low electrical conductivity of a b o u t 4.7% IACS the a u t h o r s decided to investigate only the Cr-Si-modified 90/10 cupro-nickel alloys in order to achieve a high strength, high conductivity copperbase alloy. A p r e l i m i n a r y study of the age h a r d e n i n g response of' various Cu-Ni-Cr-Si alloys (16) p r o d u c e d by melt s p i n n i n g i n d i c a t e d t h a t a m a x i m u m h a r d n e s s of 480 Hv can be o b t a i n e d from as cast Cu10wt% Ni-3wt% Cr-3wt% Si whilst a f u r t h e r i m p r o v e m e n t u p to 650 Hv was achieved by ageing this alloy at 400°C for 2 hours. The p e a k h a r d n e s s a n d t e n s i l e s t r e n g t h of rapidly solidified alloys were twice as high as those of Cr-Si-modified 70/30 cupronickel (IN-768) p r o d u c e d by t r a d i t i o n a l m e t h o d s a n d this was coupled with a fourfold increase in ductility.
FIGURE 3: Optical micrograph of Osprey Cu-1ONi-3Cr-3Si after cold rolling, homogenization and ageing at 800°C for 2 hours.
FIGURE 4: Microstructure near the tensile fracture surface of Osprey Ca-IONi-3Cr3Si aged at 4OO°C. The arrow idicates the tensile direction.
FIGURE 5: SEM fractograph of the tensile fracture surface of Osprey Cu-lONi-3Cr3Si that has been cold rolled, homogenized and aged at 40O°C for 2 hours.
Microstructure
Some r e c t a n g u l a r specimens cut from the preform were initially homogenized at 950°C for 2 hours u n d e r argon, q u e n c h e d in cold w a t e r t h e n e n c a p s u l a t e d in quartz t u b e s before ageing at various t e m p e r a tures between 20°C a n d 800°C for 2 hours.
Figure 1 shows the m i c r o s t r u c t u r e of as sprayed Cu-10wt% Ni-~,~vt% Cr-3wt% Si. It is a composite s t r u c t u r e c o n t a i n i n g over 20% by volume of a second p h a s e a n d with virtually no porosity. The second phase is m a r k e d a n d is p r e s e n t at grain b o u n d a r i e s as either films or discrete particles, or w i t h i n grains as fine precipitates. Analysis of these areas shows t h a t they comprise a
MPR April 1991 39
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tion. S u b s e q u e n t t h e r m o m e c h a n i c a l t r e a t m e n t s u p to 800°C led to t h e f o r m a t i o n of m a t r i x a n n e a l i n g twins a n d b r e a k i n g u p of t h e g r a i n b o u n d a r y p h a s e . The s i l i c i d e p a r t i c l e s a p p e a r to p i n grain b o u n d a r i e s r e s u l t i n g in a fine g r a i n e d c o m p o s i t e (Figures 2 a n d 3). Figure 4 shows t h e m i c r o s t r u c t u r e n e a r t h e f r a c t u r e surface of tensile t e s t e d s a m p l e s a g e d at 400°C for 2 h o u r s in o r d e r to o b t a i n p e a k m e c h a n i c a l p r o p e r t i e s . It can be seen t h a t f r a c t u r e is i n i t i a t e d w i t h i n t h e m i x e d silicides at t h e grain b o u n d a r i e s , while t h e c o p p e r rich m a t r i x p h a s e is ductile. The fracture surface ( F i g u r e 5) d e m o n s t r a t e s t h e l i m i t e d d u c t i l i t y of t h e m a t r i x w i t h silicide p a r t i c l e s sitting a t t h e b o t t o m of t h e dimples.
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FIGURE 6: The age hardening responses of Osprey Cu-lONi-3Cr3Si.
m i x t u r e of Ni2Si a n d Cr3Si. The s m a l l e r m a t r i x p a r t i c l e s are of s i m i l a r d i m e n s i o n s to t h o s e o b s e r v e d in r a p i d l y s o l i d i f i e d r i b b o n s a n d r e s u l t from p r e c i p i t a t i o n during solid s t a t e cooling. The l a r g e r p a r t i c l e s are t h o u g h t to solidify d i r e c t l y from t h e melt. The m e a n grain size of t h e c o p p e r rich grains is less t h a n 7~m which is c o m p a r a b l e to t h o s e of m o s t r a p i d l y solidified p r o d u c t s . E n e r g y d i s p e r s i v e X-ray (EDX) a n a l y s i s showed t h a t t h e c o p p e r m a t r i x c o n t a i n e d 0.77wt% Si, 0.18wt% Cr a n d 3.7wt% Ni in solid solution while the bulk analysis c o m p a r e s well w i t h t h e n o m i n a l c o m p o s i -
Mechanical and physical properties Figure 6 shows t h e age h a r d e n i n g r e s p o n s e s of t h e as s p r a y e d c a s t a n d rolled samples. It c a n be s e e n t h a t t h e r e is a d r a m a t i c i m p r o v e m e n t in m e c h a n i c a l p r o p e r t i e s as t h e ageing t e m p e r a t u r e w a s i n c r e a s e d from 20°C to 400°C. A l t h o u g h p e a k p r o p e r t i e s a r e achieved a t 400°C for b o t h t h e as s p r a y e d a n d t h e rolled alloys, t h e r m o m e chanical p r o c e s s i n g e n a b l e d t h e achievem e n t of t h e h i g h e s t m i c r o h a r d n e s s v a l u e of 540 Hv, c o u p l e d w i t h a t e n s i l e s t r e n g t h of 810 MPa a n d 7.0% elongation. These a r e close to t h e r e s u l t s o b s e r v e d for m e l t s p u n ribbons. In t h e as s p r a y e d condition, t h e alloy can be s u b j e c t e d to significant d e f o r m a t i o n ( u p to 80% t h i c k n e s s r e d u c t i o n ) w i t h o u t interm e d i a t e annealing. The electrical conductivity of t h e m a t e r i a l is n o t affected by t h e t h e r m o m e c h a n i c a l t r e a t m e n t b u t it increases from 10% IACS a t 20°C to a p e a k value of 18% IACS at 600°C ( F i g u r e 7). This level of c o n d u c t i v i t y is s u p e r i o r to t h o s e of w r o u g h t Cu-15wt% Ni-8wt% Sn m a d e from b o t h a t o m i z e d p o w d e r s (6, 7) a n d s p r a y d e p o s i t i o n (4) w h e r e 7.8% IACS a n d 10% IACS were achieved respectively. Figure 8 shows t h e influence of ageing
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400 temperature,
600
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FIGURE 7: The plot of electrical conductivity of Osprey Cu-lONi3Cr-3Si against ageing temperature.
40 MPR April 1991
200 Ageing
400 Temperature,
60C
800
C
FIGURE 8: The corrosion rate of Osprey Cu-lONi-3Cr-3Si after seven-day salt spray test.
A
1 2 3 4 5 6 7 8
B
Treatments As-cast Microhardness (Hv25g) 285 Tensile strength (MPa) 420 Elongation (%) 11 Coefficient of thermal expansion (}.tm/m C) 149 Electrical conductivity al 20~C (%IACS) 15 Density at 20~C (Mg/m 3) 859 Corrosion rate (mpy) 41
C
D
Aged at 400°C Rolled+aged at 400°C 460 540 760 810 4 7 14.9 149 15 15 859 8 59 51 51
TABLE 1: The mechanical and physical properties of Osprey CuIONi-3Cr-3Si alloy.
t e m p e r a t u r e on the corrosion rate of the alloys. It can be seen t h a t a p a r t from a slight increase to 5.1 m p y at 400'~C the corrosion rate r e m a i n e d at 4.1 mpy for most of the t e m p e r a t u r e s investigated. E q u i v a l e n t resuits were also o b t a i n e d from the thermomechanically treated specimens. For comparison, the corrosion rate of Osprey Cu-10wt% Ni-3wt% Cr-awt% Si is half of t h a t for pure copper (8.2 mpy) u n d e r identical conditions. A summary, of t h e m e c h a n i c a l a n d physical properties of the alloy is p r e s e n t e d in Table 1. It is evident t h a t rolling a n d ageing at 4 0 0 C has a significant effect on both the m e c h a n i c a l a n d electrical properties. The coefficient of t h e r m a l e x p a n s i o n a n d d e n s i t y r e m a i n c o n s t a n t a t 14.9 ~ m / m ° C a n d 8.59 Mg/m a respectively even after the t h e r m o m e c h a n i c a l t r e a t m e n t a n d confirms t h a t the material was fully dense at the start.
Discussion A significant a m o u n t of residual porosity has been observed in a previous s t u d y on spray deposited Cu-15wt% Ni-8wt% Sn (4). These gas pores were t h o u g h t to result from evolution of nitrogen d u r i n g solidification. The achievement of zero porosity in the p r e s e n t study along with the fact t h a t the t h e r m o m e c h a n i c a l t r e a t m e n t had no signifi c a n t effect o n t h e d e n s i t y , e l e c t r i c a l conductivity, coefficient of t h e r m a l e x p a n sion and corrosion rate, show t h a t full density has been obtained. An identical effect has also been observed in sprayformed .M-20Si-X alloys (where X = Mg a n d Fe) where a low level of porosity of a b o u t 1.3% is reported (12). Again the proposed e x p l a n a t i o n is t h a t nitrides have formed d u r i n g spray forming. The age h a r d e n i n g b e h a v i o u r of spray deposited Cu-10 wt% Ni-3wt% Cr-3wt% Si is similar to t h a t of Cu-31.6wt% Ni-1.7wt% Cr0.46wt% Si reported by 8ahoo and Boyd (13). On the basis of extensive t r a n s m i s s i o n electron microscopy (TEM) studies a n d Xray analysis, Sahoo a n d Boyd showed t h a t a
c o m b i n a t i o n of s p i n o d a l d e c o m p o s i t i o n a n d p r e c i p i t a t i o n of n i c k e l a n d c h r o m i u m silicides a c c o u n t s for t h e c o n s i d e r a b l e s t r e n g t h e n i n g effect. ,Mthough Sahoo a n d Boyd p r o d u c e d their alloy by a c o n v e n t i o n a l casting technique, we believe t h a t a similar e x p l a n a t i o n can a c c o u n t for the age hardening response in the p r e s e n t work. The results also show t h a t a n electrical conductivity of 15% IACS coupled with a coefficient of t h e r m a l e x p a n s i o n of 14.9 I~m/m°C can be achieved after ageing at 400°C. These are superior to those of w r o u g h t powder metallurgy (PM) Cu-15wt% Ni-8wt% Sn developed for electronic a p p l i c a t i o n s (6, 7) which has reported values of 7.8% IACS a n d 16.4 p m / m ° C respectively, The high level of electrical conductivity achieved in the p r e s e n t alloy is due to the lower level of nickel in solid solution (3.71wt% Ni) a n d is c o m p a r a b l e to t h a t predicated for a Cu4wt% Ni (11). Despite the lower nickel concentration, spray deposited Cu-10wt% Ni-3wt% Cr-3wt% Si also exhibits excellent m e c h a n i c a l properties a n d good corrosion resistance coupled with good formability.
Conclusions Osprey processing can be used to produce a high alloy copper m a t e r i a l (e.g. Cu-10wt% Ni-3wt% Cr-3wt% 8i) with r e f i n e d a n d h o m o g e n e o u s microstructures. These alloys can e x h i b i t good c o m b i n a t i o n s of physical a n d m e c h a n i c a l p r o p e r t i e s s u p e r i o r to those of t r a d i t i o n a l alloys t h a t are currently used for electronic hardware applications. There is evidence to show t h a t the a d d i t i o n of strong nitride formers e n h a n c e s the minim i z a t i o n of porosity in copper-base alloys p r o d u c e d by the Osprey process.
Acknowledgements The a u t h o r s are gratelhl to SERC a n d BNF Metals Technology Centre for providing the financial s u p p o r t for this project; to Osprey Metals Ltd for p r o d u c i n g the m a t e r i a l and to Keith D i n s d a l e of t h e U n i v e r s i t y of N o t t i n g h a m for this i n v a l u a b l e assistance with the salt spray corrosion testing.
References (1). R.H. Cookey, and J.V. Wood, International Journal of Rapid Solidification, Vol 3 (1987), p.319-337. (2). R.H. Cookey a n d J.V. Wood, Materials" Science and Enginee~:ng, Vol 99, (1988), p. 513-516.
MPR April 1991 41
(3). R.H. Cookey and J.V. Wood, Proceedings of BNF 7th Inter. Conf. on the Materials Revolution Through the '90's, (1989), 3-5 July 1989. Vol, 2, Paper 23. (4). R.H. Cookey and J.V. Wood, Proceedings of the PM Group Meeting, 'PM '89', (1989), Paper 2.5, section A, (5). R.H. Cookey and J.V. Wood, 'Microstructure andmechanical properties of rapidly solidified Cu-15wt% Ni-8wt% Cr-8wt% Sn alloys', to be published in the proceedings of PM90, London. (6). C.R. Scorey et al, J. of Metals (1984), p. 52-54. (7). C.R. Scorey, Metal Powder Report (1988) January, p. 33-34. (8). A,G. Leatham et al, Modern Development in Powder Metallurgy (1985), Vol 15, Metal Powder Industries Federation, Princeton, NJ, USA. (9). D. Apelian et al, ASM Conf. Proc. on 'processing of structural metals by rapid solidification' (1987), Orlando, Florida, Ed. by F.H. Froes and S~I. Savage, p. 107-125.
42 MPR April 1991
(10). P. Mathur et al, Acta Metall (1989), Vol 37, (2), p. 429-443. (11). Smithels Metals Ref. Book. Sixth Edition. Edited by E.A, Brandes, published by Butterworth and Co Ltd, 1983, p. 19. (12). J.L. Estrada and J. Duszczyk, J. of Materials Science (1990), Vol. 25, p. 13811391. (13). M. Sahoo and J.D. Boyd Canadian Metall Quarterly (1982), Vol 21, No 3, p. 281-288. (14). BS5714 (1979) Electrical conductivity measurement. (15). ASTM Bl17, Salt spray testing of copper base alloys. (16). R.H. Cookey and J.V. Wood (1988), ~l~e Effect of Silicon on the properties of Cu-10% Ni-3% Cr produced by melt spinning' unpublished worlc (17). INCO preliminary data, Copper-nickel-chromium-silicon casting alloy IN-768 (1970).(18). M. Sahoo et al, AFS Trans (1979), Vol 87, p. 529.
SPECIAL
FEATURE
;
Production and d e v e l o p m e n t of copperbase alloys by the Osprey process R.H. Cookey (Open University, Milton Keynes, UK) and J.V. Wood (University of Nottingham, UK)
Spray deposition has been employed to develop a range of complex copper base-alloys including Cu-Ni-Cr and Cu-Ni-CrSi. These alloys exhibit high temperature stability, and refined and homogeneous microstructures. The effects of silicon additions, t h e r m o m e c h a n i c a l and ageing treatments on the performance of these alloys has been characterized in terms of hardness, tensile strength, electrical conductivity and corrosion properties. There is evidence to show that Cu- l Owt% Ni-3wt% C r - 3 w t % Si exhibits physical and mechanical properties which are superior to those of other copper base alloys that are currently used for electronic connectors, contacts, commutators and springs.
Paper presented at the 1st International Conference on Spray Forming. Reprinted with permission.
38
MPR
April
1991
n the l a s t few years, c o n s i d e r a b l e advances have been achieved in the development of s u p e r s a t u r a t e d copper base alloys t h a t offer improved properties compared with conventional wrought products. These materials exhibit high strengths, high conductivity, good resistance to corrosion, microstructural stability at elevated t e m p e r a t u r e s and are attractive for electronic and marine applications (17). Many of t h e s e d e v e l o p m e n t s have concentrated on atomized powders as a starting feed stock. The most recent publications (6-7) show t h a t Cu-Ni-Sn strips 0026-0657/91/$3.50 ©, ElsevierScience Publishers Lid
made from consolidated and worked atomized powders are of considerable interest as candidates for electronic contacts, connectors and springs. However, melt spinning and spray forming (Osprey processing) also permit the achievement of refined microstructures, solid solubility extensions and near-net shaped products similar to powders without many of the limitations and d i s a d v a n t a g e s a r i s i n g from s u b s e q u e n t consolidation steps. In fact, several high alloy copper base particles are difficult to consolidate to full density. A large amount of residual porosity has been observed in some copper-base alloys (i.e. Cu-15 Ni-8 Sn) which have been spray deposited using nitrogen gas (4). Although porosity in Osprey preforms is generally thought to be controlled by various processing p a r a m e t e r s such as the gas-liquid ratio, and cooling rate, some further porosity results from gas evolution during solidification. The pores vary in dimension and can subsequently lead to detrimental effects on mechanical properties. The aim of this work was to investigate the effects of adding strong nitride formers such as silicon and chromium to see whether nitrides would form in preference to gas evolution. The properties were characterized in terms of microstructure, hardness, tensile strength, corrosion resistance and electrical conductivity.
Experimental procedure - m
Based on a large number of previous results on rapidly solidified copper alloys (16) a single composition was chosen for spray deposition. Preforms with a nominal composition of Cu-10wt% Ni-3wt% Cr-3wt% Si were produced by Osprey Metals Ltd, of Neath, UK, u n d e r s t a n d a r d conditions. Nitrogen gas was employed for atomization.
Other specimens were (:old rolled to achieve a n 80% t h i c k n e s s r e d u c t i o n . They were s u b s e q u e n t l y homogenized at 950°C for 2 hours prior to ageing. A detailed description of the procedures for m e c h a n i c a l a n d m i c r o s t r u c t u r a l examin a t i o n s is p r e s e n t e d elsewhere (4). The electrical conductibility" was assessed by the four t e r m i n a l m e t h o d as per BS5714 with a c u r r e n t of 10 A (14). A D u P o n t 1090 e q u i p p e d with a t h e r r n o m e c h a n i c a l analyzer (TMA) w a s u s e d to m e a s u r e t h e coefficient of t h e r m a l e x p a n s i o n between 20°C a n d 300°C. The corrosion p e r f o r m a n c e was assessed by salt spray testing as per ASTM B l 1 7 for a d u r a t i o n of 168 h o u r s (15).
FIGURE 1: Optical rnicrograph of Osprey Cu-lONi-3Cr-3Si.
FIGURE 2: Optical micrograph of Osprey Cu-lONi-3Cr-3Si after cold rolling and homogenization.
Results w
l
Although the a d d i t i o n s of c h r o m i u m a n d silicon to cupro-nickel alloys in the p r e s e n t work was i n t e n d e d to m i n i m i z e the effect of porosity, studies by Sahoo a u d co-workers (13, 17, 18) have shown t h a t as cast Cu30wt% Ni-l.7wt% Cr-0.45wt% Si alloy (IN768) e x h i b i t s e n h a n c e d corrosion resistance in flowing sea water, good m e c h a n i c a l properties and weldability c o m p a r e d with other cupro-nickel casting alloys. This alloy is s t r e n g t h e n e d by a c o m b i n a t i o n of spinodal d e c o m p o s i t i o n a n d p r e c i p i t a t i o n of nickel a n d c h r o m i u m ,silicides. As the high nickel c o n c e n t r a t i o n in 70/30 cupro-nickel alloys leads to a low electrical conductivity of a b o u t 4.7% IACS the a u t h o r s decided to investigate only the Cr-Si-modified 90/10 cupro-nickel alloys in order to achieve a high strength, high conductivity copperbase alloy. A p r e l i m i n a r y study of the age h a r d e n i n g response of' various Cu-Ni-Cr-Si alloys (16) p r o d u c e d by melt s p i n n i n g i n d i c a t e d t h a t a m a x i m u m h a r d n e s s of 480 Hv can be o b t a i n e d from as cast Cu10wt% Ni-3wt% Cr-3wt% Si whilst a f u r t h e r i m p r o v e m e n t u p to 650 Hv was achieved by ageing this alloy at 400°C for 2 hours. The p e a k h a r d n e s s a n d t e n s i l e s t r e n g t h of rapidly solidified alloys were twice as high as those of Cr-Si-modified 70/30 cupronickel (IN-768) p r o d u c e d by t r a d i t i o n a l m e t h o d s a n d this was coupled with a fourfold increase in ductility.
FIGURE 3: Optical micrograph of Osprey Cu-1ONi-3Cr-3Si after cold rolling, homogenization and ageing at 800°C for 2 hours.
FIGURE 4: Microstructure near the tensile fracture surface of Osprey Ca-IONi-3Cr3Si aged at 4OO°C. The arrow idicates the tensile direction.
FIGURE 5: SEM fractograph of the tensile fracture surface of Osprey Cu-lONi-3Cr3Si that has been cold rolled, homogenized and aged at 40O°C for 2 hours.
Microstructure
Some r e c t a n g u l a r specimens cut from the preform were initially homogenized at 950°C for 2 hours u n d e r argon, q u e n c h e d in cold w a t e r t h e n e n c a p s u l a t e d in quartz t u b e s before ageing at various t e m p e r a tures between 20°C a n d 800°C for 2 hours.
Figure 1 shows the m i c r o s t r u c t u r e of as sprayed Cu-10wt% Ni-~,~vt% Cr-3wt% Si. It is a composite s t r u c t u r e c o n t a i n i n g over 20% by volume of a second p h a s e a n d with virtually no porosity. The second phase is m a r k e d a n d is p r e s e n t at grain b o u n d a r i e s as either films or discrete particles, or w i t h i n grains as fine precipitates. Analysis of these areas shows t h a t they comprise a
MPR April 1991 39
600-
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as cast
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tion. S u b s e q u e n t t h e r m o m e c h a n i c a l t r e a t m e n t s u p to 800°C led to t h e f o r m a t i o n of m a t r i x a n n e a l i n g twins a n d b r e a k i n g u p of t h e g r a i n b o u n d a r y p h a s e . The s i l i c i d e p a r t i c l e s a p p e a r to p i n grain b o u n d a r i e s r e s u l t i n g in a fine g r a i n e d c o m p o s i t e (Figures 2 a n d 3). Figure 4 shows t h e m i c r o s t r u c t u r e n e a r t h e f r a c t u r e surface of tensile t e s t e d s a m p l e s a g e d at 400°C for 2 h o u r s in o r d e r to o b t a i n p e a k m e c h a n i c a l p r o p e r t i e s . It can be seen t h a t f r a c t u r e is i n i t i a t e d w i t h i n t h e m i x e d silicides at t h e grain b o u n d a r i e s , while t h e c o p p e r rich m a t r i x p h a s e is ductile. The fracture surface ( F i g u r e 5) d e m o n s t r a t e s t h e l i m i t e d d u c t i l i t y of t h e m a t r i x w i t h silicide p a r t i c l e s sitting a t t h e b o t t o m of t h e dimples.
.
.
+ 1¢0c,
T~mper~ture,°C
FIGURE 6: The age hardening responses of Osprey Cu-lONi-3Cr3Si.
m i x t u r e of Ni2Si a n d Cr3Si. The s m a l l e r m a t r i x p a r t i c l e s are of s i m i l a r d i m e n s i o n s to t h o s e o b s e r v e d in r a p i d l y s o l i d i f i e d r i b b o n s a n d r e s u l t from p r e c i p i t a t i o n during solid s t a t e cooling. The l a r g e r p a r t i c l e s are t h o u g h t to solidify d i r e c t l y from t h e melt. The m e a n grain size of t h e c o p p e r rich grains is less t h a n 7~m which is c o m p a r a b l e to t h o s e of m o s t r a p i d l y solidified p r o d u c t s . E n e r g y d i s p e r s i v e X-ray (EDX) a n a l y s i s showed t h a t t h e c o p p e r m a t r i x c o n t a i n e d 0.77wt% Si, 0.18wt% Cr a n d 3.7wt% Ni in solid solution while the bulk analysis c o m p a r e s well w i t h t h e n o m i n a l c o m p o s i -
Mechanical and physical properties Figure 6 shows t h e age h a r d e n i n g r e s p o n s e s of t h e as s p r a y e d c a s t a n d rolled samples. It c a n be s e e n t h a t t h e r e is a d r a m a t i c i m p r o v e m e n t in m e c h a n i c a l p r o p e r t i e s as t h e ageing t e m p e r a t u r e w a s i n c r e a s e d from 20°C to 400°C. A l t h o u g h p e a k p r o p e r t i e s a r e achieved a t 400°C for b o t h t h e as s p r a y e d a n d t h e rolled alloys, t h e r m o m e chanical p r o c e s s i n g e n a b l e d t h e achievem e n t of t h e h i g h e s t m i c r o h a r d n e s s v a l u e of 540 Hv, c o u p l e d w i t h a t e n s i l e s t r e n g t h of 810 MPa a n d 7.0% elongation. These a r e close to t h e r e s u l t s o b s e r v e d for m e l t s p u n ribbons. In t h e as s p r a y e d condition, t h e alloy can be s u b j e c t e d to significant d e f o r m a t i o n ( u p to 80% t h i c k n e s s r e d u c t i o n ) w i t h o u t interm e d i a t e annealing. The electrical conductivity of t h e m a t e r i a l is n o t affected by t h e t h e r m o m e c h a n i c a l t r e a t m e n t b u t it increases from 10% IACS a t 20°C to a p e a k value of 18% IACS at 600°C ( F i g u r e 7). This level of c o n d u c t i v i t y is s u p e r i o r to t h o s e of w r o u g h t Cu-15wt% Ni-8wt% Sn m a d e from b o t h a t o m i z e d p o w d e r s (6, 7) a n d s p r a y d e p o s i t i o n (4) w h e r e 7.8% IACS a n d 10% IACS were achieved respectively. Figure 8 shows t h e influence of ageing
20 <:
E
~4
16
-~
~3
14
o ~2
~ 2 1
--
8
200 Ageing
400 temperature,
600
800
C
FIGURE 7: The plot of electrical conductivity of Osprey Cu-lONi3Cr-3Si against ageing temperature.
40 MPR April 1991
200 Ageing
400 Temperature,
60C
800
C
FIGURE 8: The corrosion rate of Osprey Cu-lONi-3Cr-3Si after seven-day salt spray test.
A
1 2 3 4 5 6 7 8
B
Treatments As-cast Microhardness (Hv25g) 285 Tensile strength (MPa) 420 Elongation (%) 11 Coefficient of thermal expansion (}.tm/m C) 149 Electrical conductivity al 20~C (%IACS) 15 Density at 20~C (Mg/m 3) 859 Corrosion rate (mpy) 41
C
D
Aged at 400°C Rolled+aged at 400°C 460 540 760 810 4 7 14.9 149 15 15 859 8 59 51 51
TABLE 1: The mechanical and physical properties of Osprey CuIONi-3Cr-3Si alloy.
t e m p e r a t u r e on the corrosion rate of the alloys. It can be seen t h a t a p a r t from a slight increase to 5.1 m p y at 400'~C the corrosion rate r e m a i n e d at 4.1 mpy for most of the t e m p e r a t u r e s investigated. E q u i v a l e n t resuits were also o b t a i n e d from the thermomechanically treated specimens. For comparison, the corrosion rate of Osprey Cu-10wt% Ni-3wt% Cr-awt% Si is half of t h a t for pure copper (8.2 mpy) u n d e r identical conditions. A summary, of t h e m e c h a n i c a l a n d physical properties of the alloy is p r e s e n t e d in Table 1. It is evident t h a t rolling a n d ageing at 4 0 0 C has a significant effect on both the m e c h a n i c a l a n d electrical properties. The coefficient of t h e r m a l e x p a n s i o n a n d d e n s i t y r e m a i n c o n s t a n t a t 14.9 ~ m / m ° C a n d 8.59 Mg/m a respectively even after the t h e r m o m e c h a n i c a l t r e a t m e n t a n d confirms t h a t the material was fully dense at the start.
Discussion A significant a m o u n t of residual porosity has been observed in a previous s t u d y on spray deposited Cu-15wt% Ni-8wt% Sn (4). These gas pores were t h o u g h t to result from evolution of nitrogen d u r i n g solidification. The achievement of zero porosity in the p r e s e n t study along with the fact t h a t the t h e r m o m e c h a n i c a l t r e a t m e n t had no signifi c a n t effect o n t h e d e n s i t y , e l e c t r i c a l conductivity, coefficient of t h e r m a l e x p a n sion and corrosion rate, show t h a t full density has been obtained. An identical effect has also been observed in sprayformed .M-20Si-X alloys (where X = Mg a n d Fe) where a low level of porosity of a b o u t 1.3% is reported (12). Again the proposed e x p l a n a t i o n is t h a t nitrides have formed d u r i n g spray forming. The age h a r d e n i n g b e h a v i o u r of spray deposited Cu-10 wt% Ni-3wt% Cr-3wt% Si is similar to t h a t of Cu-31.6wt% Ni-1.7wt% Cr0.46wt% Si reported by 8ahoo and Boyd (13). On the basis of extensive t r a n s m i s s i o n electron microscopy (TEM) studies a n d Xray analysis, Sahoo a n d Boyd showed t h a t a
c o m b i n a t i o n of s p i n o d a l d e c o m p o s i t i o n a n d p r e c i p i t a t i o n of n i c k e l a n d c h r o m i u m silicides a c c o u n t s for t h e c o n s i d e r a b l e s t r e n g t h e n i n g effect. ,Mthough Sahoo a n d Boyd p r o d u c e d their alloy by a c o n v e n t i o n a l casting technique, we believe t h a t a similar e x p l a n a t i o n can a c c o u n t for the age hardening response in the p r e s e n t work. The results also show t h a t a n electrical conductivity of 15% IACS coupled with a coefficient of t h e r m a l e x p a n s i o n of 14.9 I~m/m°C can be achieved after ageing at 400°C. These are superior to those of w r o u g h t powder metallurgy (PM) Cu-15wt% Ni-8wt% Sn developed for electronic a p p l i c a t i o n s (6, 7) which has reported values of 7.8% IACS a n d 16.4 p m / m ° C respectively, The high level of electrical conductivity achieved in the p r e s e n t alloy is due to the lower level of nickel in solid solution (3.71wt% Ni) a n d is c o m p a r a b l e to t h a t predicated for a Cu4wt% Ni (11). Despite the lower nickel concentration, spray deposited Cu-10wt% Ni-3wt% Cr-3wt% Si also exhibits excellent m e c h a n i c a l properties a n d good corrosion resistance coupled with good formability.
Conclusions Osprey processing can be used to produce a high alloy copper m a t e r i a l (e.g. Cu-10wt% Ni-3wt% Cr-3wt% 8i) with r e f i n e d a n d h o m o g e n e o u s microstructures. These alloys can e x h i b i t good c o m b i n a t i o n s of physical a n d m e c h a n i c a l p r o p e r t i e s s u p e r i o r to those of t r a d i t i o n a l alloys t h a t are currently used for electronic hardware applications. There is evidence to show t h a t the a d d i t i o n of strong nitride formers e n h a n c e s the minim i z a t i o n of porosity in copper-base alloys p r o d u c e d by the Osprey process.
Acknowledgements The a u t h o r s are gratelhl to SERC a n d BNF Metals Technology Centre for providing the financial s u p p o r t for this project; to Osprey Metals Ltd for p r o d u c i n g the m a t e r i a l and to Keith D i n s d a l e of t h e U n i v e r s i t y of N o t t i n g h a m for this i n v a l u a b l e assistance with the salt spray corrosion testing.
References (1). R.H. Cookey, and J.V. Wood, International Journal of Rapid Solidification, Vol 3 (1987), p.319-337. (2). R.H. Cookey a n d J.V. Wood, Materials" Science and Enginee~:ng, Vol 99, (1988), p. 513-516.
MPR April 1991 41
(3). R.H. Cookey and J.V. Wood, Proceedings of BNF 7th Inter. Conf. on the Materials Revolution Through the '90's, (1989), 3-5 July 1989. Vol, 2, Paper 23. (4). R.H. Cookey and J.V. Wood, Proceedings of the PM Group Meeting, 'PM '89', (1989), Paper 2.5, section A, (5). R.H. Cookey and J.V. Wood, 'Microstructure andmechanical properties of rapidly solidified Cu-15wt% Ni-8wt% Cr-8wt% Sn alloys', to be published in the proceedings of PM90, London. (6). C.R. Scorey et al, J. of Metals (1984), p. 52-54. (7). C.R. Scorey, Metal Powder Report (1988) January, p. 33-34. (8). A,G. Leatham et al, Modern Development in Powder Metallurgy (1985), Vol 15, Metal Powder Industries Federation, Princeton, NJ, USA. (9). D. Apelian et al, ASM Conf. Proc. on 'processing of structural metals by rapid solidification' (1987), Orlando, Florida, Ed. by F.H. Froes and S~I. Savage, p. 107-125.
42 MPR April 1991
(10). P. Mathur et al, Acta Metall (1989), Vol 37, (2), p. 429-443. (11). Smithels Metals Ref. Book. Sixth Edition. Edited by E.A, Brandes, published by Butterworth and Co Ltd, 1983, p. 19. (12). J.L. Estrada and J. Duszczyk, J. of Materials Science (1990), Vol. 25, p. 13811391. (13). M. Sahoo and J.D. Boyd Canadian Metall Quarterly (1982), Vol 21, No 3, p. 281-288. (14). BS5714 (1979) Electrical conductivity measurement. (15). ASTM Bl17, Salt spray testing of copper base alloys. (16). R.H. Cookey and J.V. Wood (1988), ~l~e Effect of Silicon on the properties of Cu-10% Ni-3% Cr produced by melt spinning' unpublished worlc (17). INCO preliminary data, Copper-nickel-chromium-silicon casting alloy IN-768 (1970).(18). M. Sahoo et al, AFS Trans (1979), Vol 87, p. 529.