The effect of textures and grain shape on the shear band formation in rolled F.C.C. metals

The effect of textures and grain shape on the shear band formation in rolled F.C.C. metals

Vol. 37, No. 7, pp. 2031-2033, Printed in Great Britain. All rights reserved Acta metal/. 1989 Copyright 0 [email protected]/89 $3.00 + 0.00 1989 Pergamon ...

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Vol. 37, No. 7, pp. 2031-2033, Printed in Great Britain. All rights reserved Acta metal/.

1989 Copyright

0

[email protected]/89 $3.00 + 0.00 1989 Pergamon Press plc

THE EFFECT OF TEXTURES AND GRAIN SHAPE ON THE SHEAR BAND FORMATION IN ROLLED F.C.C. METALS SHIH-CHIN CHANG, DUEN-HUEI HOU and YUN-KIE CHANG Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, R.O. China (Received 29 June 1988; in revisedform

20 October 1988)

Abstract-In

rolled f.c.c. metals, the commonly observed textures are {110}(001), {110)(112), and { 112}( 111). A theoretical Taylor factor calculation was made to give the influence of these three textures on plastic deformation. The combined effect of textures and grain shape on the shear band formation and fracture in rolled f.c.c. metals were discussed and compared with experimental results. It is concluded that among the three textures studied, the Goss {110}(001) textured grains in rolled f.c.c. metals are easier deformed than the others, which gives an explanation of the form of shear bands usually observed in rolled f.c.c. metals. R&um&Dans les metaux c.f.c. lamines, les textures les plus communiment observees sont {1lO}(OOl), { 110}( 112) et { ll2}( 111). Un calcul thiorique du facteur de Taylor a et& realis pour ivaluer l’influence de ces trois textures sur la deformation plastique. L’effet combine des textures et de la forme des grains sur la formation des bandes de cisaillement et la rupture dans les mttaux c.f.c. lamines est discutd et compare aux resultats exp&imentaux. On conclut que parmi les trois textures ttudites, les grains de texture de Goss {1lO}(OOl) sont plus facilement deformts que les autres, ce qui donne une explication pour la forme. des bandes de cisaillement observees dans les metaux c.f.c. laminis. Zusammenfassung-In

gewalzten kfz Metallen sind die iiblicherweise beobachteten Texturen {1lO}(OOl), { 110)( 112) und {112) ( Ill ) Mit einer theoretischen Berechnung des Taylor-Faktors wurde der EinfluD dieser drei Texturen auf die plastische Verforrnung untersucht. Der kombinierte Effekt der Texturen und der Komform auf die Scherbandbildung und den Bruch in gewalzten kfz Metallen werden diskutiert und mit experimentellen Ergebnissen verglichen. Es folgt, da13 unter den drei untersuchten Texturen die Goss-Kiimer mit {1lO}(OOl)-Textur leichter als die anderen verformt werden konnen. Damit lll3t sich die tiblicherweise beobachtete Form der Scherblinder in gewalzten kfz Metallen erkllren.

1. INTRODUCTION Prior to fracture, metals always undergo some plastic deformation. So, the localization of plastic flow is a necessary condition for fracture of metal. The scale of the localization of plastic flow can vary, from the slip band in a grain [l], through various kinds of shear band [2-81, to the necking in a tensile bar. It is well recognized that the slip band is a crystallographic phenomenon related to the slip systems in the grains while the necking is a macroscopic manifestation of inhomogeneous deformation which could be rationalized totally based on the continuum mechanics. Shear bands could happen with scale of a single grain or twin in the grain [2, 31 to the adiabatic shear [4] in metals deformed under high rate, the shear bands in heavily rolled metals [5,6] and tensile stressed sheet specimens [7,8]. The behaviour of a polycrystalline material is essentially determined by that of the component grains. In a heavily textured material, the influence of crystal orientation on the mechanical properties should be more evident than in a random textured material.

The purpose of this paper is to discuss the effect of some common textures and grain shapes observed in rolled f.c.c. metals on the shear band formation in them.

2. THEORY When a simple applied to a f.c.c. stress on any of its be calculated from ri=u

tensile or compressive stress is single crystal, the resolved shear 12 { 11 l}( 110) slip systems could the equation cosu cos/? =ma

where the subscript i indicates the ith slip system, u is the angle between the tensile axis and the slip direction and /I is the angle between the tensile axis and the normal of slip plane. The factor m = cos CI cos/3is the Schmid factor. For f.c.c. crystal in which the Schmid law is obeyed, among all equivalent slip systems, the one with the largest Schmid factor activated first. If the crystal is not constrained in tensile or compressive test, a single slip system is sufficient to accommodate the prescribed tensile

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strain. However, if the deformation of a crystal is constrained, such as in plane strain compression or an “embedded” grain in a polycrystalline material generally, more slip systems should be activated for the deformation to happen. Thus, the stress needed for constrained deformation is usually higher than that predicted by Schmid factor and a Taylor factor calculation [9, IO] is more appropriate. Figure 1 shows the Taylor factors M for plane strain deformation (L,, = - cxx, Lag, = cY== c,~ = 0), with directions [OOl], [llO], [ill] and [I121 as X axis and all the possible Z axis as indicated in the figure. For rolled f.c.c. metals and alloys the predominate textures are {110}(112) brass texture, {112}(111) copper texture and { 1lo} (00 1) Goss texture. Among them, the brass and copper textures were generally recognized. The Goss texture, which some people do not think of as an important texture component in rolled high stacking fault energy f.c.c. metals [ll], is observed in some recent research on pure Al [12], X7091 Al alloy (131 and 8090 Al-Li Alloy [14]. As shown in Fig. 1, for {112) ( 111) copper oriented grains in plane strain rolling, X = (11 l), Z = (112) and the Taylor factor M = 3.674. Similarly, for (1 lO}( 112) brass oriented grains, M = 3.266. For {1lO}(OOl) Goss oriented grains, M = 2.440 which is significantly smaller than those for copper and brass oriented grains. Therefore, it is evident that in rolling of f.c.c. metals, the Goss textured grains are more easily deformed than both brass and copper textured

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grains. In fact, within an angular range of 17.63”, this statement keeps correct. Moreover, for an (1 lO)[OOl] oriented grain, the plane strain condition along the [ITO] transverse direction is automatically fulfilled when either [Oil] and [TO11on (111) or [Oil] and [loll on (771) is operating. Since the grain shape in rolled metal is generally a strip elongated in the rolling direction and thinned in the thickness direction, the glide on the above four slip systems produce no transverse displacement and relatively small displacement in the thickness direction which could be easily accommodated by its neighboring grains. On the other hand, for {110}(112), {112}(111) textured grains to compress in thickness direction while keeping the transverse strain zero, multiple slip in more than two slip planes are necessary, which will spread the deformation and make it homogenized through the grain. That is, the shear localization (shear band) will be not as likely to happen as in {110}(001) textured grains. 3. OBSERVATIONS

AND DISCUSSION

In aluminum foil the shear bands are running at approximately 35” to the foil surface [5]. Similar observations were found in Al-&-Li-Mg-Zr alloys [15], Al-Cu alloys and many other heavily rolled f.c.c. metals [6]. In every case, the shear bands are running the same 35” angle to the rolling direction. The angular relationship is exactly that of the

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(0)

FORMATION

(bl

4-

4-

3-

X=

coo11

I 0

30

I 60

I 90

z t; z ii z c

5

cc

1

r

1 X-Cl123

t 0

30

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90

0

I

I

I

I

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Fig. 1. Taylor factor M for plane strain deformation (c, = - L,, ty = E = exY= 0) based on {11l}( 1IO) ship. With X = (a) [OOl](b) [l lo] (c) [l 1l] (d) [112] and 2 = the dftections shown in the figure.

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It is well known that shear band formation is affected by many metallurgical and crystallographic parameters such as the shearable 6’ phase formed in Al-Li alloys [17]. Nevertheless, the effect of Goss oriented grains on the formation of shear band in rolled f.c.c. metals can not be over-emphasized. 4. SUMMARY

:exture (111)

POLE

FIGURE

A Goss

texture

0 Brass

texture

TD

A theoretical calculation has been made to give the influenceof{110}(001),{110}(112)and{112}(111) textures on plastic deformation. The combined effect of textures and grain shape on the shear band formation and fracture in rolled f.c.c. metals were discussed and compared with experimental results. Among the three textures studied, the Goss {1lO}(OOl) oriented grains in rolled f.c.c. metals are easier deformed and the deformation is easier to be localized than the others, which gives an explanation of the formation and the angle relationship of shear bands usually observed in rolled f.c.c. metals. Acknowledgement-The authors are grateful for the support of this work by the National Science Council. Republic of China under grant NSC77-0405-EOO7-04. REFERENCES

(200)

POLE

FIGURE

Fig. 2. The (111) and (200) pole figures of 60% cold rolled Al-3.2 Cu-1.6 Li-1.1 Mg-0.2 Zr alloy. slip planes in ( 1lO}(OOl ) Goss textured grains. As shown in Fig. 2, Goss component is observed to be one of the major texture components in a cold rolled and aged Al-Cu-Li-Mg-Zr Alloy. For pure Al, the deformation texture can be characterized as a pure metal rolling type (i.e. brass and copper) plus a Goss component 1121. In X7091 Al alloy 1131,Goss texture was observed. In other f.c.c. material, the formation of {llO}(OOl) Goss components in an annealing texture may appear unfavorable, however, they do exist in {1lO}( 112) deformation textures as twin component [16]. Therefore, it is believed that {110}(001) Goss texture does exist in rolled f.c.c. metals as an important or, at least, not negligible component. Based on the Taylor factor calculation, the {llO}(OOl) Goss oriented grains in rolled f.c.c. metals are easier to deform than both the {110) (112) brass oriented and {112}( 111) copper oriented grains. From the compatibility consideration, the deformation of Goss oriented grains are most easily localized to form shear bands. This prediction is consistent with the shear band observations in rolled f.c.c. metals. activated

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16. I. L. Dillamore and H. Katoh, Metal Sci. 8, 73 (1974). 17. T. H. Sanders Jr and E. A. Starke Jr, in AluminumLithium Alloys II (edited by T. H. Sanders Jr and E. A. Starke Jr). Metal1 Sot. A.I.M.E. (1984).