Materials Science and Engineering, A 108 (1989) 97-104 97

Flow Stress and Structure of Age-hardened Cu-0.4wt.%Cr Alloy after Large Deformation

H. DYBIEC, Z. RDZAWSKI* and M. RICHERT

Institute for Metal Working and Physical Metallurgy, University of Mining and Metallur~©,. al. Mickiewieza 30, 30-059 ("racow (Poland)

(Received February 23, 1988; in revised form June 9, 1988)

Abstract

In this paper the results of an investigation of the structure and mechanical properties of age-hard- ened and heavily deformed Cu-O.4wt.%Cr alloy are presented. It was found that the structure and shape of the o0. 2 vs. tr curve is strongly influenced by the dispersion of second-phase particles. The presence of small, highly dispersed particles causes the development of a uniformly distributed dis- location structure as a result of retarding the re- arrangement dislocation process. The development of this structure leads to a passive dislocation back- ground and causes the localized flow bands to take the main role in the deformation. As a result a step-like 0o. 2 vs. ~ curve in which each step on the curve corresponds" to the formation and develop- ment of one-band family is obtained. For a low dispersion of particles the recovery process leads to a cellular dislocation structure and to partition of the strain between the cellular dislocation back- ground and the bands. In the latter case the o~.2 vs. er curve is smooth. The appearance of localized flow bands causes a rapid decrease in the strain- hardening rate.

1. Introduction

The structure and properties of metallic mater- ials after large deformations have been the subject of many papers in recent years. The interest in these problems results from the need to be acquainted with deformation effects at strains comparable with those usually applied in techno- logical processes [1, 2].

The appearance of strain heterogeneity after a large deformation has been observed in pure metals and single-phase alloys. It is manifested by

*Present address: Institute of Nonferrous Metals, ul.Sowiriskiego 5, 44- l 01 Gliwice, Poland.

the presence of localized plastic flow bands which take the main role in the deformation. The onset of strain localization in bands depends on the type of the material and the deformation condi- tions [1]. The influence of the second phase on the structure and metal properties after large deformations has been more poorly documented. Hornbogen and coworkers [3, 4] described the appearance of shear bands in age-hardened material and associated them with local softening processes caused by the shearing and dissolving of coherent precipitations inside the shear bands. The presence of bands in a material age hardened by undeformable second-phase particles was also observed [5]. In contrast, investigations of age- hardened materials after small and medium strains showed that the second phase essentially influences the homogeneity of deformation and makes the recovery processes more difficult [6, 7].

It appears interesting to investigate the struc- ture development and changes in flow stress at large deformations in materials with both deformable and undeformable second-phase par- ticles in an unchanged matrix composition. Cu-Cr alloy is a suitable material for this pur- pose. Many research studies have been carried out to investigate the properties and structure of this alloy and to describe the influence of many factors on its morphology, growth, properties and arrangement of particles [8, 9]. It is possible to obtain coherent and incoherent precipitation networks by special heat treatment on the unchanged matrix composition (similar to pure copper) [10]. This is of great advantage when interpreting the results.

2. Material and experimental procedure

Specimens of cross-section 12 m m x 8 mm were cut from Cu-0.4wt.%Cr alloy rolled sheet.

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5001 Cu-0.36% Cr

(o 1.S Er Fig. 1. Relation between the flow stress 00. 2 determined in a tensile test and the true rolling strain er for aging times of 30 min (o), 120 min (D) and 3000 min ( zx ).

They were heated in a salt bath to a temperature of 1270 K for 60 rain and then quenched in water. Subsequently the material was aged at 773 K for 30, 120 and 3000 min to obtain mater- ials of the same matrix composition and precipi- tated phase volume but of different dispersions and coherences of particles.

The accuracy of this treatment was controlled by measuring the value of electrical conductivity. This parameter was 47 MS m -t for all speci- mens, which is evidence of the unchanged matrix composition [10]. Heat-treated specimens were cold rolled to obtain a thickness of 2.0 mm. Dur- ing the rolling process, some specimens were sub- jected to structural observations and tensile tests. The microstructural observations were carried out using optical and electron microscopes with the samples cut perpendicular to the transverse direction. Tensile tests were carried out on an Instron machine.

3. Experimental results

Figure 1 shows the relation between the flow stress 00. 2 determined in a tensile test on pre- rolled material and the true rolling strain. The first stage of this relation is characterized by a considerable slope of the curve for all aging times. The slope decreases rapidly at strains e, greater than 0.2. For a material aged for 3000 min an almost linear gradient with o0.2 vs. e~ relation was observed and the slope of the curve for e, above 0.2 is about 20 times smaller than that at the start of the deformation.

The o0.2 vs. e, relation for materials aged for 30 and 120 min has a "step-like" character for deformations higher than 0.2. There are three distinct "plateaux" on the curve for the following deformation ranges: 0.2-0.4, 0.5-0.95 and above 1.05. The structure of material aged for 30 min and deformed to er = 0.24 (first plateau) is shown in Fig. 2(a). Slip lines were observed inside the grains, and the angle between the slip line and the rolling direction depends on grain orientation. At a similar deformation the same effects can be seen in materials aged for 120 and 3000 rain. In the second plateau a r e a (er = 0 . 6 5 ) for materials aged for 30 and 120 rain, shear bands were observed (Figs. 2(b) and 3(b)). They make angles of about 32°-40 ° with the rolling direction. The volume of material occupied by the shear bands increased with increasing deformation. Shear bands retaining identical directions in all pene- trated grains created very distinctive offsets at the grain boundaries (Figs. 2(b), 2(c) and 3(b)). In the material aged for 3000 min and deformed to e r = 0.65, shear bands were also observed. In this case, however, some lines in the band tangles are more broadened although the size of the offsets at the grain boundaries is comparable with that of the offsets left by "sharp" bands in materials aged for a short time (Fig. 4(a)). Deformation corre- sponding to the third plateau on the 00. 2 vs. er

curve produces structural effects similar to those shown in Figs. 2(d), 3(c) and 4(b). Two shear band families can be seen. One of them makes an angle of about 35 ° with the rolling direction, and the other, showing distinct cutting traces, forms an angle of about 10°-15 °. Figures 2(c) and 3(a) show structures corresponding to transient deformations between the "plateaux".

Examples of structures of the material aged for 30 min, obtained by the transmission electron microscopy technique for deformations corre- sponding to the individual stages on the oo.2 vs. er

curve are shown in Fig. 5. After deformation at t r=0.17 (Fig. 5(a)) a large density of uniformly distributed dislocations is observed. At er = 0.24 (Fi~. 5(b)), many deformation microbands are visible, which cause corrugation of the grain boundary. In the neighbouring grains, micro- bands with a changed direction can also be seen. An increase in deformation to er = 0.63 causes the appearance of shear bands (Fig. 5(c)). In the vicinity of these a uniformly distributed disloca- tion structure is observed (Fig. 5(d)) which is identical with that for er = 0.17. Two families of

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Fig. 2. Micrographs of alloy aged for 30 min and deformed to the following strains: (a) e~=0.24; (b) e,=0.65, (c) e~=0.95; (d) E, = 1.3s.

shear bands are characteristic of the structure at the third plateau (Fig. 5(e)). The shear bands intersect at an angle of 20 ° but the band-free areas retain the same structure as at er = 0.17. In a material aged for 3000 min and deformed to e r=0.18 , a cellular dislocation structure is observed (Fig. 6(a)). The mean diameter of cells is about 0.4 pm. After deformation at e r = 0.24, in some areas, microbands similar to those obtained in a material aged for 30 min can be seen (Figs. 6(b) and 6(c)). Figure 6(d) shows the structure of a material after deformation at e~ = 0.65. Shear bands with subboundaries which are the effects of a significantly advanced recovery process are visible. The cell structure could still be observed in areas outside the bands (Fig. 6(e)). The param- eters of the cells are similar to those for e~ = 0.18 but the density of dislocation in the cell walls is larger. An increase in the deformation to e~ = 1.35 causes an increase in the number of the shear bands in the structure (Fig. 6(f)). Simultaneously, in areas closer to the shear bands, strong recovery effects are observed (Fig. 6(g)).

4. Discussion

A characteristic feature of the alloy tested is the sudden decrease in the slope of the oo.2 vs. er

curve for deformation at about er = 0.2. This is accompanied by the appearance of a new element in the structure (microbands or shear bands after further deformation). Thus, it appears that the activity and development of these structural com- ponents should be related to the decrease in slope of the 0o.2 vs. er curve.

The observations of the microbands and shear bands in our experiments do not provide suffi- cient information for their complete analysis. However, some features of the bands are worth noting. Both the microbands and the shear bands are localized shear deformation areas. This is manifested by the large offsets at the grain bound- aries cut by the bands or by the boundary corrug- ations due to microbands. In the region of strains where a specific structural element (a band or microband) is active, an increase in deformation leads to increase in the material volume occupied

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Fig. 3. Micrographs of an alloy aged for 120 min and deformed to the following strains: (a) er = 0.42; (b) e r = 0.65; (c) e r = 1.35.

by this structural element. However, the filling of the whole material volume by structural elements of the same family has never been observed. When a certain critical volume is occupied by the bands of the same family, it became more advantageous for the deformation process to mobilize the suc- cessive family of bands rather than to continue the concentration of those already existing. This could be evidence of the long-range interaction of the bands. Figure 6(c) illustrates an important feature of the microbands. The presence of dislo- cation pile-ups and deformation microtwins in places where the microbands stop indicates the existence of long-range stress at the head of the microbands. This is also confirmed by observa- tions of the microband interaction in the neigh- bouring grains. The shear bands are of a more complex nature. The appearance of mutually mis- oriented lines in the shear band tangles could be evidence of the secondary importance of crystal- lographic relations for band propagation. This is also confirmed by the propagation of bands

Fig. 4. Micrographs of an alloy for 3000 min and deformed to the following strains: (a) er = 0.65; (b) er = 1.35.

through several grains in the same direction. Th e bands are created at constant angles to the rolling direction and then they rotate towards it. This indicates the importance of stress in the band formation and propagation. Leaving open the problem of the mechanism of the microband and shear band formation, we can say that these two structural elements transfer large strains and that they are one of the forms of localized defor-

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Fig. 5. Dis locat ion s t ruc ture of a material aged for 30 min and de fo rmed to the fol lowing strains: (a) er = (I.17; (b) er = (I.24; (c) er = 0.65; (d) e, = 0.65; (el er = 1.35.

mation. From the viewpoint of material deforma- tion analysis, they can be treated as similar elements, according to the proposal of Korbel and coworkers [ 11-13].

The other important structural element is the dislocation background against which the bands appear. For short aging times the dislocations are distributed densely and uniformly. This type of

structure may occur as a result of slip dislocation reaction with dislocation debris created when bypassing the precipitations [6, 14]. It causes marked dislocation pinning which with a great number of small precipitates retards the disloca- tion rearrangement down to low energy states [15]. Observations of this deformation structure within the e~ range 0.2-1.35 show its stability;

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Fig. 6. Dislocation structure of a material aged for 3000 min and deformed to the following strains: (a) er=O.18; (b) er = 0.24; (c) er= 0.24; (d) er = 0.65; (e) er = 0.65; (f) er= 1.35; (g) e~ = 1.35.

thus it can be assumed that, after its formation at small strains, this structure becomes passive and does not bear deformation. The background structure in materials aged for 3000 min shows different features. In this case a small number of large precipitates in comparison with material aged for a shorter time leads to a decreasing amount of dislocation debris and a larger free path for dislocation redistribution. As a result the cell structure is developed. With increasing defor- mation from at=0.2 to er = 1.35, this structure becomes denser and the recovery effects become greater. This makes the background able to bear deformation as well as to sustain systematic strain hardening. The superposition of these localized deformation effects on two different backgrounds must as a consequence lead to differently shaped o()2 vs. er curves. Regardless of the deformation mechanism active during the tension of the for- merly rolled material, the stress necessary to initi- ate the deformation will be determined by the structural element most susceptible to deforma- tion. When the strain hardening in the bands is taken into account, the background is more favourable to deformation. In materials aged for a short time, only one structural element bears the deformation within specific strain ranges and the general structural changes in such areas consist only of an increase in the material volume occu- pied by this element together with its interaction zone. As long as the whole material volume is not occupied by such elements, there exist some areas where the initiation of deformation requires the same stress. Consequently, 0o.2 is here indepen- dent of the strain. Filling the material volume by one of the band families causes their further elimination from deformation and their further role is limited to the steady background quality change in the whole volume. In this case a passive background and active bands can be considered.

In a material where the partitioning of the deformation into the background and bands is observed, a different behaviour can be expected. On account of the hardening of the active back- ground, plateaux are not expected in the Oo2 vs. e~ curve and the slope of this curve will be deter- mined by the interaction of strains in the back- ground and bands and by their hardening abilities.

The influence of the second phase on the structure and flow stress in a Cu-0.4wt.%Cr alloy is substantial and it depends on the dispersion of this phase. For small deformations, the presence

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of strongly dispersed particles favours a highly uniform dislocation distribution, as was observed earlier [6]. A decrease in dispersion leads to cell structure formation although the size of the cells is smaller than that of the cells in pure copper [5]. The first effects of localized deformation (micro- bands) are observed at a strain of about er = 0.2, while in pure copper such effects occur at er = 0.1 [ 1 ]. The suggestion of the homogenizing influence of small precipitates on the deformation is thus confirmed. In contrast, the localization effects which are shifted to higher strains in a material with precipitates develop more rapidly than in pure copper, where distinct shear bands are not observed until a strain about 1.2 is attained and the decrease in the %2 vs. ce r curve slope is not so rapid [ 1 ].

The appearance of the same localization effects at the same deformation in materials with coherent and incoherent particles contradicts the suggestion of Hornbogen and coworkers about local dissolution of cut particles in the shear bands. Rather, the observations indicate the dominating influence of the increase in the local stress on the process of development of localized deformation bands. However, the explanation of the band formation and development needs further investigation.

Acknowledgments

The authors wish to thank Professor A. Korbel for his helpful advice. They also wish to thank Ing. M. Orkisz and Dr. L. BtaZ for their assistance in electron microscopy observations.

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