RELATIONSHIP BETWEEN SURFACE MARTENSITE, THIN FOIL AND BULK MARTENSITE

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RELATIONSHIP BETWEEN SURFACE MARTENSITE, THIN FOIL AND BULK

MARTENSITE

F. Lovey

To cite this version:

F. Lovey. RELATIONSHIP BETWEEN SURFACE MARTENSITE, THIN FOIL AND BULK MARTENSITE. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-585-C4-590.

�10.1051/jphyscol:1982492�. �jpa-00222212�

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CoZZoque C4, supptdment au n o 1 2 , Tome 43, de'cembre 1982 page C4-585

RELATIONSHIP BETWEEN SURFACE MARTENSITE, THIN FOIL AND BULK MARTENSITE

F.C. Lovey

Centro Ato'mico BariZoche, Comisi6n NacionaZ de ~ n e r g c a ~ t b i c a 8400-S.C.

de Bariloche, Argentina

(Revised text accepted 23 September 1982)

Abstract.- The possibility that surface martensite can serve as nucleus for bulk martensite is investigated by transmission electron and light microscopy in ternary Cu-Zn-A1 alloys. For this reason the relationship of the variants and structures between surface martensite, thin foil and bulk martensite is studied. It is known that the surface martensite can serve as nucleus of bulk martensite in samples with the surface close to a (011)fi pole. The necessity to compensate the shape deformation induces, in some cases, additional variants which were not present on the surface.

In samples with the surface far away from the (011)~ pole it is found that the surface martensite can grow only in the thinnest region of the foil.

I. Introduction.- It was established recently (1-6) that the extra diffraction effects in many 13 phase noble metal alloys are not due to premartensitic or premonitory phenomena but arise from phases with 18R, 9R and 2H type of structures coexisting with the B phase. Moreover, it has been shown (1) that indeed they are surface martensites which exist several hundred degrees above the transformation temperature Ms in the bulk. By transmission electron microscopy (TEM) the structure and orientation of the surface martensite has been determined as a function of the orientation of the foil. It has been shown (3) that the martensite adjusts its structure and chooses the variants by the condition of having an interface with the B phase, of low distortion parallel to the surface.

The fact that martensite regions exist prior to the transformation in bulk may have a profound influence on the latter, if the surface martensite can serve as nucleus for the bulk. Whether this indeed is true, and how it depends on the orientation of the sample has been studied in Cu-Zn-A1 single crystals and is the subject of the present paper.

11. Experimental procedure and results.- Single crystals of Cu-Zn-A1 alloys were prepared in the usual way by the Bridgman method after encapsulating in vycor. The composition and the corresponding transformation temperature (Ms) in bulk is given in Table 1. As can be seen alloy 1 is martensitic at room temperature whereas alloy 2 remains 13 in the bulk. From the crystals, samples with the required surface orientation were spark machined and afterwards polished. They were observed by TEPi after thinning or by optical microscopy using polarized light. During observation the samples could be cooled or heated in an appropriate stage.

The surface orientations which were selected are shown in Fig. 1. They are subdivided into group A, for which the surface coincides or is close to a possible bulk habit plane orientation, group B around the line (011)-(023)

(Indices with reference to the bcc lattice when not otherwise indicated), group C any other orientations indicated in the fundamental triangle except orientations around the pole (100) which form group D and do not show any surface martensite prior to the bulk transformation (1).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982492

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Table 1

Figure 1: Stereographic. projection showing habit plane, hi, for 18R (A) and approximate habit plane hk for 2H (*). The full line joining hb-hA represents the locus of the least distorted Ofmartensite interface planes (see Ref. 3).

The regions indicated by A,B,C and D represent groups of surface orientation.

The line surrounding these regions must be taken as an approximate limit.

1. Group A samples: a) TEM results: Group A surface martensite generally consists of the two variants (denoted by 1 and 2 in the following) which have their bulk habit plane closest to the surface, when the bulk is in the 8 phase.

The dark field images corresponding to the two variants in alloy 2 are shown in Figs. 2a and 2b. Moire fringes can be present due to the superposition of the same variants from the top and bottom surfaces. The thickness of the surface martensite is estimated to range from 70 to 100 A (details on this subject and concerning the microstructure will be discussed elsewhere). On cooling an alloy 2 in the electron microscope, neither the dark field image nor the intensity of the diffraction spots due to the surface martensite is noticed to change until the transformation in bulk starts. Below Ms the bulk is martensitic, consisting consisting of the same variants and having a similar structure as the previous surface martensite. Large regions with a single variant of surface martensite at room temperature, shown in Fig.3a, transform after cooling to the same variant and a similar structure as can be seen in Fig. 3b. Notice that Moir6 fringes characteristic of surface martensite (in Fig. 3a) have disappeared after bulk transformation (Fig. 3b). These regions of the foils transform as a whole. The observations do not indicate that a plate of the same variant and structure as the surface martensite has nucleated and grown from inside the foil. This suggests that the surface martensite has indeed grown inward into the foil. In regions like those shown in Fig. 2 the variants 1 and 2 do not grow simultaneously into the bulk. This is evident in Fig. 4, where platelets of the surface martensite of one variant are completely embedded in foils transformed to the other variant. On heating again above Ms the samples return to 8 with the same surface martensite morphology as before cooling. When the sample is heated to 800°C, the surface martensite does not appear on cooling prior to the transformation in bulk (more details will be published elsewhere). However, on cooling below Ms the sample transforms to the same variants (1 and 2) which also have been found when surface martensite is present.

b) Light microscopy results: In order to see whether the TEN results apply also for thicker samples, the transformation has been studied by polarized light microscopy. It is found that discs with less than 1 mm thickness transform easily to the xrariants 1 and 2 on cooling (alloy 2), but that thicker samples are more difficult to transform in the same way. Samples of alloy 1, which consist of the normal self-accommodating bulk martensite initially, were heated to 800°C in order to retransform to I3 and eliminate any debris associated with the martensite. On cooling, variants 1 and 2 are observed distributed in an

Figure 2: Dark field electron micrographs of surface martensite in a (155) sample of alloy 2, taken with 122,8R reflection. a) Corresponding to variant 1 present on top and bottom surfaces. b) Corresponding to variant 2 present here only on top surface. Notice that the images are complementary.

Figure 3: Electron diffraction patterns and dark field micrographs taken with 1 2 2 L 8 ~ reflection in a (155) sample of alloy 2. a) Shows a large region of 8 foil with variant 1 of surface martensite on top and bottom surfaces at room temperature. b) After cooling below Ms the foil has been transformed to the same variant and structure as the surface martensite in a). Zone axis -[111]~

"[210118R.

Figure 4: Bright field show Figure 5: Optical kicrographs using polarized ing platelets of surface light in a (155) sample of alloy 1. a) (155) martensite of variant 2 on face showing large regions transformed to the foil transformed to variants 1 and 2. b) Cross section of the variant 1. sample showing a sandwich type accommodation

between variants 1 and 2.

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Figure 6: Optical micrograph using Figure 7: Electron diffraction patterns polarized light in a (023) sample of and dark field micrograph taken with alloy 1. Four variants corresponding 2 0 1 2 ~ reflection showing a single to the self-accommodating group are variant of 2H surface martensite on top present. The variants are identified and bottom surfaces of the foil. (223) by the numbers indicated in Fig. 1. sample of alloy 2 at room temperature.

Zone axis -[001]~ -[010]~~.

Figures 7b and 7c: Same sample as in Fig. 7a, but after cooling below Ms.

7b) Shows diffraction pattern and dark field image taken with 20128 reflection of the thinnest region. In 7c is shown a bright field image and diffraction pattern corresponding to the thicker region of the foil with 18R structure.

Figure 8: Optical micrograph using polarized light in a (223) sample of alloy 2. Normal transformation not in relation with the surface is observed.

irregular fashion, see Fig. 5a. Looking at a cross section of the disc a sandwich type of accommodation is found. Variant 1 alternates with variant 2, consistent with a (155) interface between them (Fig. 5b). Occasionally other variants belonging to the same or other accommodating groups are present.

2. Group B samples: Electron and light microscopy results: Although the surface martensite consists of the two variants (denoted 1 and 3, Fig.1) with their bulk habit plane close to the surface (3), the subsequent cooling to below Ms leads to four variants with their habit plane centered around the (011) pole, see Fig.6. The interface orientations between varisnts 1 and 2 are consistent with (155), those between variants 3 and 4 with (155) and finally those between 1 and 4 or 2 and 3 with (011). These four variants crystals are found to grow easily in disc shaped samples of 8 m in diameter and up to 3 m thick. In thicker samples the appearance of other self-accommodating groups occurs more frequently.

3. Group C samples: Electron and light microscopy results: Depending on the surface orientations these samples have one, two or three variants of surface martensite (3). In addition, when moving the orientation from that near (011) towards the (111)-(001) line, the structure changes from 18R to the hexagonal 2H(3). It may be mentioned that after polishing the sample in the 0 phase a thin foil martensite always forms spontaneously in the very thinnest regions near the edge of the hole. Its structure has been found to be 18R in most cases and is independent of the orientation of the surface of the foil. Occasionally 2H and fct structures have also been observed. These thin foil marternsites are not considered in the present report, instead we concentrate our attention to that part of the foil which initially remains in the I3 phase and which is not affected by the thin foil martensite.

In Fig.7a is shown a (223) sample with a 2H surface martensite. By TEM it is noticed that on cooling slightly below Ms the thin regions of the foils still remain I3 while the thicker regions have already been transformed to the accommodating groups of the 18R martensite, which are not related to the surface, see Fig.7~. This was confirmed by light microscopy to occur in all of the thick region of the sample, see Fig.8. With a further cooling below Ms the thin regions of the foils transform frequently, as a whole, to the same variant and a similar structure as the original surface martensite, see Fig.7b.

Thus the surface martensite needs a certain amount of undercooling for its growth.

The surface martensite in general is absorbed by the bulk martensite below Ms and thus disappears, but it comes back again when the bulk returns to 0 on heating above Ms (more details will be published elsewhere).

4. Group D samples: It has been found that the bulk transforms in the normal way and not in relation with the surface.

111. Discussion.- Although it has not yet been possible to transform a large 0 phase single crystal into a martensite having the same variant and structure as the original surface martensite, the results have shown several interesting features, as to how surface martensite can affect the subsequent growth of bulk martensite.

It is clear that the surface has an influence on the growth of bulk martensite, if the I3 phase surface orientation is near (011). When the surface is nearly parallel to a possible habit plane for the bulk (group A) two stages of growth below Ms can be discerned. An initial growth of one of the two variants into the interior and around islands of surface martensite of the second variant, and then the formation of a sandwich structure of both variants instead of the four which are found in normal martensite in an accommodating group. The reason for this structure is most likely due to the fact that one variant produces a net shape deformation, which can be compensated, at least partially, by the formation of the second variant (7), and viceversa. The

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difficulty of extending the sandwich structure into thicker samples is possibly related to the lower accommodation strain for the group of four martensite variants which nucleate in bulk, than the bivariant structure, which requires supercooling to temperatures below which normal growth otherwise occurs.

When the surface orientation is displaced from the (011) pole towards (023) (group B), the two variants of the surface martensite (variants 1 and 3 have approximately the same shape deformation associated with them (7). This large strain prevents the growth of both variants into the interior and instead favours the nucleation of two others which compensate the shape deformation of the former.

These considerations are in agreement with the experimental evidence that the four variants in group B are easier to extend in thicker samples than the two variants in the group A samples.

Although the influence of the heating to 800°C on the formation of surface martensite is difficult to interpret (it can be due to metal evaporation or a change in the surface oxide layer), the fact that the bulk transformation consists of the same surface variants indicates the strong role played by the surface orientation also in this case, maybe due to a thinner layer of surface martensite which escapes its observation by TEM.

When the surface is far away from a (011) orientation, surface martensite does not grow, except in the thinnest regions of the foils. This result cannot be explained by relating it to the appearance of a more hexagonal martensite structure, since it has been found that hexagonal and 18R martensites have the same energy (8). Instead it can be due to the problems associated with the nucleation of another variant in order to compensate for the shape deformation associated with the first one, or to a higher friction stress necessary to move the interface, which would require a too high amount of supercooling, compared to the bulk nucleation. The latter mechanism is supported by the observation that growth in the thinner parts of the foils requires some supercooling, and that hexagonal bulk martensite generally needs a higher friction for moving.

Acknowledgements: The author would like to thank Prof. M. Ahlers for valuable discussion and critical reading of the manuscript and Dr. M. Chandrasekaran for very useful advice.

References

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%

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4

8. BARCELO, G. and AHLERS, M., Scripta Met.

16