STRUCTURE PECULIARITIES OF MARTENSITE, FORMED BY THE EFFECT OF MAGNETIC FIELD

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1 STRUCTURE PECULIARITIES OF MARTENSITE, FORMED BY THE EFFECT OF MAGNETIC FIELD V. Sadovsky, V. Schastlivtsev, L. Romashev To cite this version: V. Sadovsky, V. Schastlivtsev, L. Romashev. STRUCTURE PECULIARITIES OF MARTEN- SITE, FORMED BY THE EFFECT OF MAGNETIC FIELD. Journal de Physique Colloques, 1982, 43 (C4), pp.c4-529-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1982 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE CoZZoque C4, suppze'ment au no 12, Tome 43, de'cembre 1982 page C4-529 STRUCTURE PECULIARITIES OF MARTENSITE, FORMEDBY THE EFFECT OF MAGNETIC FIELD V.D. Sadovsky, V.M. Schastlivtsev and L.N. Romashev Institute of Metal Physics, UraZs Scientific Centre, Acadeq of Sciences U. S. S.R., SverdZovsk, U. S.S. R. (Accepted 9 August 1982) Abstract.- Structural changes in alpha-martensite f orning by the action of a magnetic field have been investigated for high-nickel steel which remains in an austenitic state even when subjected to very low temperatures. It is shown that the thin plate martensite produced by exposure to a pulsed magnetic field becomes lenticular in the course of subsequent isothermal soaking without application of a magnetic field. Electron microscope studies of the structure of martensite plates at different stages of their formation are also presented. Introduction.- One of the factors exerting an initiating influence on the martensite (3-d) transformation in steel is a magnetic field [I 1. Application of a magnetic field whose intensity amounts to seve- ral hundreds of kilooersteds to an austenitic specimen is liable to lead to the inception of a ( 3-d) transformation therein at a temperature tens and sometimes hundreds of degrees higher than that Ms at which this transformation would begin in the absence of a magnetic field [2, 31. The martensite that has formed under the influence of a magnetic field at a temperature considerably exceeding & remains, after the field has vanished, in the austenite matrix whose physical properties may differ appreciably from those inherent therein at lower temperatures, i. e., when martensite forms under normal cooling conditions without exj?osure to field. This difference of the physical state of the austenite matrix is the more substantial the higher is the temperature at which the field has brought about the f ormation of martensite, as compared to Ms. Thus, in causing the occurrence of a martensitic transformation at a temperature above Us, a magnetic field enables one to trace the behaviour of the martensite phase under unusual physical conditions. Using this circumstance we have recently succeeded in revealing a number of interesting features peculiar to the mechanism of the formation of martensite crystals in chromium-nickel steel [4, 51. It is these peculiarities that will be briefly discussed in the present paper. E erimental rocedure.- Martensite formation has been investigated f% steel congaining C wt.56, Cr wt.%, Ni wt.T. Martensite transformation does not occur in specimens made of ths steel, quenched in water from 1273 K, when these are cooled down to 4.2 K. However, by applying a pulsed magnetic field it was possible to cause d-martensite to form in them even at comparatively high temperatures (ca 200 K). A pulsed magnetic field was produced by Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE means of a conventional high-voltage capacitor unit [6 1. The field pulse length was 10-4 sec. The experiments were conducted using cylindrical specimens 3 mm in diameter and 12 mm long. Results and discussion.- Fig. la shows the structure of the martensite forming in specimens at a temperature of 77 K by the action of a ~ulsed mametic field with a critical intensib7 R* corresponding to- the generation of martensite after the initial cooling. +he me& value of H* at 77 K for the series of specimens was equal to 100 He. Upon being subjected to such a field (H*) the specimen was immediately warmed in water to room temperature, and then underwent polishing and etching. The structure observed evidences that a magnetic field pulse gives rise to the formation of thin martensite plates. These plates have smooth interfaces and often branch and cross each okher. Judging by the morphology of the crystals, the martensite that has formed by the action of a pulsed magnetic field is similar to the thin plate martensite arising when Fe-Ni-C alloys with very low Ms temperatures are cooled under normal conditions (without application of a magnetic field) [7, 8, 91. Subsequent to photographing the martensite structure,. i.e., two or four hours later, the specimens were again immersed mto liquid nitrogen and held there for some hours, but without applying a magnetic field. Fig. Ib presents a photograph of the surface relief which has formed on the surface of a specimen (Fig. la) subjected after treatment in a magnetic field H*, to isothermal soaking at 77 K for 18 hr. It is seen that during the period of soaking at 77 K, the thin martensite plates have become much wider. Metallographic studies have shown that the specimens developed a martensite which, in its morphology, is similar to the usual lenticular martensite. With the help of magnetic measurements it was found that the isothermal formation of 'additional' martensite in the specimen proceeds gradually and lasts several hours. Fig. 2 demonstrates in what manner the amount of d-martensite in the specimen varies in the course of isothermal soaking. As is evident, the increase in the amount of the mhensite phase is very large. It was of interest to ascertain how thin martensite plates transform (widen) to lenticular plates. To this end, a study was made of the martensite structure in specimens subjected to isothermal soaking of different duration (from minutes to several hours). It turns out that soaking s ecimens even for a short time (in the absence of a magnetic field? leads to a change in the martensite structure. One can see from the electron microscope picture of the structure given in Fig. ga that after the specimen f ield-treated at 77 K has been subjected to soaking for a short time, some martensite plates display a complicated structure and consist of a thin martensite plate and a peculiar dislocation edging ('fringef). Such complicated 'fringed' martensite plates are of rare occurrence, approximately two or three per hundred. A dark-field analysis of different austenite and martensite reflections was performed. It is seen in Pig. 3b that the 'fringe' visible in an austenite 200g reflection. Fig. 3c depicts the same segment of the structure as that shown in Fig. 3b but obtained in a martensite 20% reflection. It is obvious that the peripheral part of the martensite plate, i. e., the 'fringet has the same contrast as the central part of the plate has. Thus one faces

4 a paradoxical situation when one and the same structural volume reveals itself in both the martensite and austenite reflections. These results evidence unambiguously that the dislocation edging of martensite plates contains both phases. The question arises as to what may be responsible for the appearance of a dislocation edging in martensite plates. In Ref. [ 91 the complicated dislocation structure of austenite around martensite crystals was attributed to dislocations arising from back transformation of martensite to austenite. In our view, the 'fringef observed is the initial stage of further lateral growth of the thin martensite plate. We draw this conclusion, in particular, from analysis of the places of collision of the martensite plates that have formed by the action of a pulsed magnetic field. Fig. 4 shows an electron microscope photograph of the place of collision of two martensite plates, one of which is 'fringed1. The plates are seen to collide at a spot which lies near the well-defined inner surf ace of the plate, i. e., the colliding plate may be said to have penetrated inside through the 'fringe1. This is so because, due to the lateral growth which has begun, the peripheral part of the martensite plate flows along the martensite plate which has collided against it and, as a result, the place of collision of the plates turns out to be within the 'fringe1. Different types of martensite plates may be found in specimens subjected to isothermal soaking for 5 to 10 kr, i. e., within the time when the martensite growth process in a specimen (see Fig. 2) is at the mediu stage of its development. Namely, there occur thin martensite plates which have undergone no appreciable changes, thin plates surrounded by a 'fringe', nonequilibrium-f orm plates, and wide lenticular plates in which the initial thin plates act as the midrib. In Fig. 5 is shown a photograph of a nonequilibrium-form mart ensite plate corre sponding to the intermediate stage of formation of a lenticular martensite plate. The right-hand portion of the plate produced in the process of isothermal soaking, is seen to be genetically related to the left-hand portion which has arisen in a magnetic field and to consist of successive layers rela-bed as twins. It is noteworthy that the irregular line of the twins projecting beyond the plate correlates well with the irregular martensite- -austenite interface occurring in 'fringed1 martensite crystals (see Fig. 3). Thus, using the initiating influence of a magnetic field on the martensite (2(+ d) transformation it has been possible to single out clearly the following lenticular martensite formation stages : 1 ) formation of thin plate mastensite by the action of a pulsed magnetic field; 2) occurrence of a mart;ensite-austenite edging around some martensite plates due to the inception of lateral plate growth resulting from the presence in austenite of unbalanced elastic stresses caused by the formation of thin plate martensite; 3) stepwise lateral martensite plate growth which, as the amount of twins diminishes, increasingly verges towards the growth of a late with a smooth (regular) martensite-austenite interface front ; 4y formation of a lenticular, frequently asymmetric plate in which the initial thin plate arising at the first stage acts as the midrib. Acknowledgments.- The authors aclmowledge with thanks the assistance of I. L. Yakovleva in electron microscope studies.

5 C4-532 JOURNAL DE PHYSIQUE References [I] SADOVSKY V. D., RODIGHIFJ N. M., SMIRNOV L.V., FILOMCI-TIK G.M.,. - FAKIDOV I.G., Fiz. Ketallov i Idetalloved. 12 (1961) J FAKIDOV I.G., VORONCHIKHIN L.D., ZAVADSKY E.A., B ~ ~ A N OA.K., V Fiz. Metallov i Metalloved. 2 (1965) 852. [3] KRIVOGLAZ ill.a., SADOVSKY V.D., SXI3NOV L.V., POKINA E.A., Zakalka stali v magnitnom pole, (ISaulra, 1.1oscou) [4] SADOVSKY V.D., ROJlSmV L.N., Dolrl. AN SSSR (1978) SCHASTLIVTSEV V.U., ROI'USmV L.N., YAKOVLEVU I.L., SADGVSIIY V.D.. Fiz. Metallov i PJetalloved. (1981) 773. [6] FAKIDOV I.G., ZAVAESKY X.A., Fiz. Ne-Gallov i IbTetalloved. 8 (1959: 562. [7] GHXOEGRIETA I. Ya., Yit'iKSIlAOVA 0.P., Fiz. Aleta1lo.r i IdIetalloved. 2 (1971) J.ILAKI T., SIIIl!.fOOIZA S., PUJIVImA S., TAlXURA I., Trans. Japan Inst. Metals 16 (1975) 35. [9] l;w(i T., VAYliUN C.M., in Proc. of the first JIM International Symposium on 'New Aspects of TvTartensitic Transformationf, 1976, p. 47. Fig. 1 : Optical micrographs of martensi-be crystals produced at 77 K by the action of a pulsed magnetic field (a), and mart ensite relief that has formed subsequent to undergoing isothermal soaking at 77 K (b). Pig. 2 : lliartensite volume fraction vs. soaking time at 77 K.

6 Fig. 3 : 'Fringed1 martensite plate (a) ; in austenite reflection - g = OOig, axis of area [130]~(b) ; in martensite reflection = 002~, axis of area [la01 (c). Fig. LC : Collision of two mmtensite plates, of which one is 'fringedf. 8,s pm I Fig. 5 : Gartensite plate upon soaking at 77 K within 7 hr.