A study of group variants of martensite in Fe-Mn-Al-C alloy by transmission electron microscopy

Scripta Materialia, Vol. 35, No. 6, pp. 739-147, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. F’rinted in the USA. All rights reserved 1359-6462/96 $12.00 + .OO

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A SlTUDY OF GROUP VARIANTS OF MARTENSITE IN Fe-Mn-Al-C ALLOY BY TRANSMISSION ELECTRON MICROSCOPY H.Y. Chu*, F.R. Chen’ and T.B. Wu* *Department of Materials Science and Engineering National Tsing Hua University, Hsinchu, Taiwan, R.O.C. +Materials Science Center National Tsing Hua University, Hsinchu, Taiwan, R.O.C. (Received October 9, 1995) (Accepted April 26, 1996) Introduction Martensitic transformation in a BCC matrix in duplex Fe-Mn-Al-C alloys had been studied by several researchers.( l-4) The structure of martensite was identified to be close to a long period stacking order (LPSO) structure of 18R(42),. The crystallography of martensite such as orientation relationship, habit plane, shape strain and lattice invariant shear was verified by transmission electron microscopy (TEM), and the experimental crystallography was in good agreement with the theoretical results.(4) Twenty-four crystallographic martensite variants are possibly formed in Cu base shape memory alloys.(5-8) These twenty-four varjants consist of six plate groups, each of which consists of four habit plane variants that are near to and symmetrically clustered about (01 1 } .poles of the parent phase. Variants within a single group could form diamond, folk, wedge or spear shape.(S) However, morphology and structure of martensite variants in Fe&In-Al-C alloy have not been studied intensively yet. Especially, application of high resolution transmission electron microscopy (HRTEM) in this area of study is rare so far. In this paper, we present a HREM study of group variants of martensite in an Fe-Mn-Al-C alloy. The HRTEM could reveal local structures between variants in nanometer scale which may not be detectable by conventional TE:M technique. Exneriment An ingot of Fe-29.4Mn-8.03Al-0.3C alloy was prepared in an air induction furnace. This ingot was then homogenized at 1200°C for 4 hours, and forged from 5 cm to 2 cm thick at the same temperature. Specimens of 100 mm x 20 mm x 2 mm were cut from the forged plate. Each piece of specimen was subsequently heated at 1300 OCfor 1 hour in argon atmosphere, and followed by water quenching. Thin slices for TEM study were ground mechanically to 0.15 mm and punched into 3 mm diameter disks. TEM specimens were obtained using double jet polish in a solution containing 10 % perchloric acid, 20 % acetic acid and 70 % ethanol. The polishing temperature was kept between 10 and 20°C and the operation voltage was at 30 V. The TEM specimens were examined with a JOEL-200CX STEM and JOEL 4000.

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Figure. 1. Stereographic projection showing the calculated habit plane poles of 4 variants of martensite near (01 l)& pole, the relative stacking fault planes are denoted as As, Bs, Cs and Ds.

Results and Discussions Crystallographic relationship between four martensite variants in one (01 1} plate group for 1SR martensite is plotted in a stereographic projection shown in Figure 1. In Figure 1, A, B, C and D are the poles of four habit planes in this group and the traces of their associated stacking fault planes are also given and denoted as As, Bs, Cs, and Ds, respectively. The calculated of martensitic crystallography of these four variants using CRAB theory (5)are listed in Table 1. Four variants of one plate group could form diamond, spear-, wedge- or folk-pair martensites in Cu-based alloy. (6-9) Basically, two different shapes of group variants were observed in Fe-Mn-Al-C alloy which are shown in Figure 2(a) and (b). Figure 2(a) shows a kink martensite and (b) shows a lenticular shape of martensite TABLE 1 The Results from CRAB Phenomenological Calculation of Lenticular Martensites Shear system Habitplane Shape strundim&n

D

A

B

C

(10i)[101] ( I. -63O.e7.70) [O.OS,-0.72,-O 691

(Oll)[OlT] ( I.-7.70.~6.30) [O.OS,-0.69,0.721

(011)[OlT] ( 7.70,6.30)

(IlO)*

Oriemaion relarimrhip

5.53'

(0OlS)v

[ii 11~ hlapnitudcofrhape strain MsSnitudcofrhea

0.2522 00144

Shape deformation matrix

[4.017 0 cl,*

[ilila [iTo],

0.98'

[iiolw

0.9Se

0.109 0.886

0

[ -0.017

1.134

,010 1

1.010 4.030 -0.03 0

0

[iTIle [ilo),

5.53'

0.98'

(iol), (0018)~

5.53'

[iii]*. IIiojr

0.98'

0.2522 0.0144

-0.017 -0.014

Average of shapedefomwtion matrix

0 IW2

(lie), (0018)~

(Tol)[ioi] ( I, 6.30,7.70) [0 09. -0.72,0.691

1.m 0017 cl.014 1.002 0.014 0.017 0.886 -0 018 -al40 0.886 0.011 0 109 0.011 -0.140 1[ 1 [ ILL31 1 LWL

1.134 I -0.140

[0 09, 0.69..0.721

25'2 0.0144

025220. 00144

I.002 -0.014 -0.011

I,

0.109

0017

1.134 0.109

4.0111 0.w

-&MO

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A STUDY OF GROUP VARIANTS

Figure 2(a). The morphology of kink martensite and the correspondingdiffractionpattern,the zone axis is near [OlO],.

OF MARTENSITE

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Figure 2(b). The morphology of lenticular martensite and corresponding diffraction pattern, the zone axis is near [TIT],.

variants. The kink morphology was reported in this alloy system.(lO) As it is viewed from the [OlO],,J/[OlO],,,, the stacking faults of both variants in the kink martensite are edge-on that can be seen from the inset diffraction pattern in the Figure 2(a). These two variants in the kink martensite can be denoted as the A and D variants. In order to obtain the high resolution images of both martensites and matrix, the electron beam has to be parallel to the [TIT],,, - [OTl],,,. Figure 3(a) shows a high resolution image of kink martensite viewing from the [TlT],, direction. As it can be seen from the stereographic projection in Figure 1, only the stacking faults of D variant in the kink martensite are edge-on in the [TlT],, direction. The angle between the habit plane of A and D variants in the kink martensite is close to 10 o in good agreement with the phenomenological theory of martensite (10). The size of one of the variants could be as small as 10 nm shown in the Figure 3(b). The angle of shape strain directions of two A and D variants is also about 10”. Formation of the kink variant is thought to reduce the overall shape strain(6) Formation of four variants in one martensite plate could reduce the net shape strain to nil (6). Figure 4(a), a high resolution image of lenticular martensite from the marked square in Figure 2 (b), shows the coexistence of A, B, C and D variants. The direction of electron beam is parallel to the [TIT],,,. All the habit planes of martensite variants are nearly edge-on. However, only the stacking faults of B and D variants are edge-on in this direction. From Table 1, the average of the shape deformation matrix is close to a unity matrix. Net shape strain can be also averaged out by martensite variants from other plate groups. As shown in Figure 4 (b), spear-shape martensites with B and D variants from two different plate groups are close to each other. Figure 4(c) shows a C variant of about 10 nm in size nucleated within the D variant. The C variant possibly grows within the D variant probably by the same reason of reducing the net shape strain of the martensite. It is worth mentioning that the twin boundary between variants B and D is (0 11}bccwhich is aperiodic stacking in an atomic scale. A magnified view of twin boundary is shown in Figure 5(a). The LPSO in 18R structure can also be described as a FCC structure containing high density of stacking faults. The variant B and D have a relative rotation of 50” about the beam direction, in the FCC coordinate. The twin boundary of variant B and D can therefore that is [OT1],8Ro!r ,,, be described as a Cl 1, which is associated with a rotation <110>/50.48”. The aperiodic feature in the twin boundary or 1 11 may be due to this boundary containing high density of stacking faults. A hypothetic model of this boundary is depicted in Figure 5(b). Martensite variants from different plate groups could possibly intersect with each other. Figure 6 (a) shows two intersected martensite plates, marked as Ml and M2. Figure 6(b) is the corresponding diffraction pattern with, the zone axis near [TlT],,,. The schematic diagram of Figure 6(a) is shown in Figure 6(c). From the diffraction patterns, the stacking fault of these two martensites are almost edge-on. Unfortunately, this area of specimen is realtively thick in which the HRTEM image is difficult to obtain. The angle of the stacking fault between these two martensites is measured to be near 70”. The angle

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Figure 3(a). The high resolution image of kink martensite, the zone axis is near [TIT],,. The variant with stacking fault plane edge-on is variant D, the other is variant A.

between the interface of matrix and martensite of these two martensites is measured to be near 50”. It is worth noting that that the intersection region has only two stacking faults observed. The results of martensitic crystallography of these two martensites, which is calculated by the CRAB phenomenological theory, are listed in Table 2. The angle between shape strain direction, [0.09,-0.72,0.69],, and [-0.72,0.09,-0.69],,, is 52”. The stacking fault planes of both M 1, (00 1LX),,, and M2, (00 18),,, are close to (01 I}&=,and the angle between the stacking fault and (01 l}bccplanes is 5.53”. The angle of stacking fault planes of Ml and M2 is, therefore, close to 7 1O. The observed result in this case is similar to the intersected twins in silicon. (11) The intersected region is produced by two identical homogeneous shears successively applied along the two conjugate twin shear

Figure 3(b). The high resolution image of kink martensite, the zone axis is near [TIT],. The variant with stacking fault plane edge-on is variant D,which is about 5nm in size, the others are variant A.

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Figure 4(a). The high resolution image of lenticular martensite, the zone axis is near [TlT],,. the marked A, B, C and D are 4 varianl Y of martenstie which form the lenticular martensite.

Figure 4(b). The high resolution image of spear martensite, which compose two variants, B and D in the same plate. The zone axis is near [TlT],.

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Figulre 4(c). The high resolution image of martensite plate, which compose two variants, C and D in the same plate. The axis jis near [TIT],,.

Figure 5(a). The high resolution image of twin boundary of variant El and D.

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bee

OF MARTENSITE

745

bee

Figure 5(b). The schemate of 111 boundary.

planes. The combined effect of these two conjugate shear results in that the structure of tbe intersected region can be described by a reversed conjugate shear of a second twin (11). In the case of intersected martensite plates, the intersected region can be repoduced by two shape deformations associated with two intersected martensites. If we denote the Fl as the shape deformation matrix describing a transformation from BCC to Ml and F2 as the one transforming BCC to M2, the matrix F = F2 x Fl-’ x F2-’ describes a transformation of M2 to the intersected region. The resultant F, Fl and F2 are given in Table 3. Matrix F is associated with a shear plane (0.20,0.68,0.71L and a shear direction [-0.17,0.67, -0.721kThe shear magnitude is 0.2537. The shear plane and shear direction are very close to (01 l)bW and [OlT],,,,, respectively. The plane of lattice invariant strain and the lattice invariant shear of M2 are also (0 1IJu: and [OTllbcc. It implies that the transformation from matrix to intersection region is very close to a reverse lattice invariant shear of M2. It is known that the 18R structure is a complex FCC structure with long periodic stacking faults. Under the operation of reversed lattice invariant shear, the stacking fault will be eliminated. The structure of the intersection region will become perfect FCC structure. The diffraction pattern of FCC overlaps with that of 18R structure.

Figure 6(a). The morphology intersected martensil;e.

of

Figure 6(c). The schemate of Figure 6(a). M, Figure 6(b). The composed diffractin and M, are the two different variants of pattern of Figure 6(a). The zone axis is martensite, the intersection region is denoted as “1”. near [TlL

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TABLE 2 The Results from CRAB Phenomenological Calculation of Intersection

M2 (01 i)[oli] ( 6.30, 1 , -7.70) [-0.72, 0.09, -0.691

Ml

(iol)pol]

Shear system Habit plane Shape strain direction

( 1 , 6.30, 7.70) to.09, -0.72, 0.69]

(lio)b,

Orientation relationship

Martensites

5.53”

(0018)M

Magnitude of shape strain Magnitude of shear

Shape deformation matrix

[iiilb, 0.98” [iiojM)

[iiilb, 0.98’ I1 iolM

0.2522 0.0144

0.2522 0.0144

1.002

0.017

0.014

1.002

0.014

0.017

-0.018 0.017

-0.140 1.134

0.886 0.109

0.017 -0.018

0.109 0.886

1.134 I -0.140

1. Lenticular, spear and kink martensites were found in the Fe-Mn-Al-C alloy.Variants of nanometer in size could grow within other variants to reduce the total shape strain. 2. Stacking fault could be eliminated in an intersection of two 18R martensites. The structure of the intersected region transforms to a FCC structure which is formed by the reversed lattice invariant shear of the second martensite as in the case of the twin intersection in silicon. 3. Twin boundary in martensite plate can be described as a Cl 1 in terms of FCC structure. The twin boundary is aperiodic in an atomic scale which could be due to a high density of stacking fault within the martensite plate.

The Shape Deformation

TABLE 3 Matrix of Intersection

0.88557

-0.140007

0.109162 0.0141548

1.13356 0.0173186

0.99141

-0.02990

F2 x Fl-’ x F2-’ = i -0.03566 0.03342

-0.12418 1.11638

Martensite and Resultant Matrix

1[ F2 =

0.0141548

-0.109162 0.88557

-0.03099 0.87128 0.12063 I

1.00225 -0.0173186 -0.0181545

-0.0173186 1.12256 0.140007

1

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