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EFFECT OF APPLIED STRESS ON NUCLEATION RATE OF ISOTHERMAL MARTENSITIC
TRANSFORMATION
S. Kajiwara
To cite this version:
S. Kajiwara. EFFECT OF APPLIED STRESS ON NUCLEATION RATE OF ISOTHERMAL
MARTENSITIC TRANSFORMATION. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-97-
C4-102. �10.1051/jphyscol:1982407�. �jpa-00221951�
JOURNAL DF. P H Y S I Q U E
Col Zoque C 4 , suppLe'ment au n o 12, Tome 4 3 , d6csmbre 2982 page C4-97
EFFECT OF A P P L I E D S T R E S S ON NUCLEATION RATE OF ISOTHERMAL MARTENSITIC TRANSFORMATI ON
S. Kaj iwara
i'lationaL Research I n s t i t u t e for Metals, 2-3-12 iu'akameguro, Meguro-ku, Tokyo 153, .Japan
( R e v i s e d t e x t a c c e p t e d 4 O c t o b e r 1 9 8 2 )
Abstract. - T h e kinetic behavior of thc isothermal martensitic transformation under a constant load has been studied with Fe-Ni-Mn and FcNi-Mn-C alloys in order t o elucidate the rate controlling mechanism of the transformation. T h e results support the mechanism previously proposed by the present author that the transformation rate is controlled by thermally activated motion of lattice dislocations t o relax the shape strain of the transforming martensitc plate.
Int~n&uc&on, - A nucleation model for the martensitic transformation proposed by Olson and Cohen ( I ) seems to be supported by 1) some crystallographic evidence in the literature (2-4) that the martensite in the nucleation stage forms on close-packed planes o f the parent phase as predicted by the faulting mechanism and 2) the ability of the model to explain the observed kinetic features of the isothermal martensitic trans- format~on. However, recent studies on Fe-Ni-Mn and Cu-Zn alloys by the present author and his coworker (5-7) revealed that the habit plane at very early stages of martensite formation is the same as that of the fully grown martensite, indicating that the invariant plane strain condition on the habit plane is operating from the very beginning of martensitic transformation. As for the kinetics of isothermal martensitic trans- formation, the present author has proposed (6) that thc most important rate controlling factor is the thermal- ly activated motlon of lattice dislocations in austenitc t o relax the shape strain o f the transforming marten- site plate. T h e purpose o f the prescnt work is t o confirm this proposal experimentally by investigating the kinetic behavior of the isothermal martensitic transformation under constant loads. The other rate controll- ing mechanisms so far proposed are discussed in relation t o the prescnt experimental results.
Experimental method. - T h e alloys investigated are Fe-23Ni-3.8Mn, Fc-23Ni-4.OMn and Fc-22Ni-3.6Mn - -0. IC (wt. %). T h e former two alloys show a typical C-curve in the T.T.T. diagram with t h e maximum trans- formation rate a t 1 4 0 K , although the transformation rate for Fe-23Ni-4.OMn is much smaller. No martensitic transformation, either isothermal o r athermal, occurs in Fe-22Ni-3.6Mn-0.1C for the temperature range of 77-300 K unlcss an external load is applied. Specimens with 0.5 x 4 x 15 mm (gauge length) were austenized at 1373 K for 3 0 min o r at 1073 K for 1 hr to obtain two differcnt grain sizes (i.c. 55-80 p m grain and 10-15 urn). The isothermal runs wen: conducted a t 77 K under various constant loads. T h e constant load was applied by a conventional creep test apparatus. T h e volume fraction of the martensite transformed was measured in situ by a magnetic detector with the coil covering the specimen. T h e basic principle of this detector is t o utilize the change in magnetic property of the specimen when the rnartensitic transformation occurs, and its sensitivity is such that 0.2% martensite can be detected with a good accuracy. T h e elongation of the specimen was also measured at the same time by a differential transformer type detector. In order to measure the elongation from the very beginning of the application of the load, a small stress of 18 MPa (about 10% of the yield stress) had been applied before the experimental assembly was irnmersed in liquid nitrogen. Besides the three alloys mentioned above, an Fe-26Ni-3.8Mn alloy was uscd t o determine the temperature dependence of the yield stress and the deformation behavior of austenitc because this alloy is not transformed at any temperature down to 7 7 K even by plastic deformation.
Experimentalresults. - 1:igures I(a) and (b) show the amount of thc martensitc transformed and the elonga- tion of the spccimen for Fe-23Ni-4.OMn when the specimens were kept a t 7 7 K under various constant loads. T h e numeral on the curve shows the applied stress in MPa. Unless the load is applied, n o appreciable amount of transformation occurs in this alloy at 77 K even in the case of large grain size specimens. As seen in this figure, the effect of the applied stress is remarkable. First we note in (a) that the initial transformation
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982407
JOURXAT. D E PHYSIQUE
rate is drastically increased when the applied stress exceeds a certain level. This critical stress level is higher for the slnaller grain size specimen. It was found that this critical stress coincides with the yield stress of the specimen in the austenitic state. The yield stress, a, of austcnite a t 77 K for the alloys investigated arc listed in Table I. These values were estimated from the yield stress at room temperature and the temperature dependence of the yield stress of Fe-26Ni-3.8Mn (figure 5) by assunling the same temperature dependence.
Corresponding t o the high initial transformation ratc in (a), a high elongation rate is observed in (b). It is interesting t o note in figure ](a) that the transformation curves for 2 1 9 Mpa and 172 MPa are crossed over and the saturation level for 219 MPa is much lower than for 172 MPa. There are adrupt increases in both the amount of the transformation and the clongation when the load is increased suddenly. A similar effect of the sudden application of strcss was reported for re-29Ni alloy by Machlin and Cohen (8).
Figure 2 shows the transformation curve and the corresponding elongation for Fe-22Ni-3.6Mn-0. IC when thc load was increased stepwise. It is evidnet that an appreciable amount of the transformation starts to occur only when the applied strcss exceeds the yield stress ( 2 3 0 MPa). The plastic elongation begins t o appear with the start of the transformation. I'arallel phenomena were also observed for the specimens with a smaller grain size (14pm), in which case a stress above 3 0 0 MPa had to bc applied t o initiate the transfor- mation. These facts may give an impression that the observed transformation is of the strain-induced type.
However, this is denied by the following experiment. The elongation of the specimen of Fe-26Ni-3.8Mn under constant load was measured at 77K and compared with the corresponding one of Fe-23Ni-4.OMn in order t o know the plastic strain of the non-transforming specimen. ('The Fe-26Ni-3.8Mn alloy is not transformed in such an experiment). Specimens with the same grain size (about IOpnl) were used for both alloys. 'fhc result is shown in figure 3. At the applied strcss of 264 MPa which is nearly equal to the yield stress, there is n o measurable plastic elongation for Fe-26Ni-3.8Mn, while a considerable amount ofelonga- tion has occurred for Fe-23Ni4.OMn. 'The transformation curve for the latter alloy is shown by the broken line in the tigure. Thc elongation due to the shape strain of the niartensite plates formed during the test can be estimated from t h e a m o u n t of the transfor~nation and it is shown by a simple calculation that this amounts to, in unit of 70, about 1/10 o f the percentage of the martensite transformed. The overall elongation, including this term, expected for Fe-23Ni-4.OMn is shown by thc dotted line in figure 3. (We have assumed the same behavior o f plastic deformation for both alloys.) There is a large difference between the dotted curve and the actually observed elongation curve of Fe-23Ni-4.OMn. This extra plastic strain must have been stimulated by the transforming martensite plates, that is, dislocations generated t o relax the shape strain of the martensite plate become mobile and are multiplied under the applied stress nearly equal t o o, giving rise t o increasing plastic deformation. A parallel result was obtained for the specirnen with the larger grain size (about 65um): the plastic elongation under the appliedstrcssof 219 Ml'a was only 0.25% for Fe-26Ni-3.8Mn after holding the specimen a t 77K for 3hr. 'This elongation is smaller by an order of the magnitude than that for Fe-23Ni-4.OMn under the same condition (figure I(b)). Thus we can conclude that the plastic deforma- tion does not induce the martcnsitic transformation; on the countary, the transformation induces the plastic deformation.
Figure 4 shows the transformation curve and the corresponding elongation for Fe-23Ni-3.8Mn. An appreciable amount of isothermal martcnsitic transformation occurs at 77K in this alloy without any applied stress. T h e transformation rate is greatly incrcased by applying the strcss of 141 MPa which is far below the yield stress. An instantaneous incrcasc in the amount of the transformation, indicated by the arrow in the figure, was observed when the load was applied, which means that an atherma1 niartcnsitic transformation has taken place. It should be noted in figure 4 that the elongation is very small and its value (%) is nearly equal t o 1/10 o f the percentage of the martensile transformed. Tlds fact shows that the elongation in this casc is mostly due t o the shape strain of the martensitc plates formed: no "extra" plastic deformation other than t o relax the shape strain of the transforming martensite plate has occurred because the applied stress is much smaller than the yicld stress.
&sc_ussjon. - Before discussing possiblc ratc controlling mechanisms for the isothermal martensitic trans- formation, we summarize, as follows, the important features of the observed transformation kinetics under the stress.
1) There is a critical stress level above which the initial transformation ratc is drastically increased.
2) This stress level coincides with the yield strcss, a, of the austenitic specimen in the casc of the alloys where n o isothcrn~ai rnartcnsitic transformation occurs unless the external stress is applied, and accordingly it depcnds o n the grain size of the specimen: the critical stress level is much higher for a fine grain specimen.
3 ) I'he saturation level o f the transformation curve for an applied stress close to u is much lower than that for the stress below a.
F i g . Kinetic behavior of the isothennal martensitic transformation at 77K for Fe-23Ni-4.OMn alloy under various levels of the constant load. (a) Amount of martensite transformed, (b) corresponding clonga- tion. Numeral on each curve shows applied stress in MPa. Small h o r i ~ o n t a l arrow in (b) indicates a level of the instantaneous elongation. No appreciable transformation occurs in this alloy in the absence of applied stress.
J O U K N A L DE PHYSIQUE
Table 1 Yield stress of austenite a t 77K
7 6 , 1 3 4 , 1 6 3 , 1 9 3 , 2 2 2 ,
-. Alloys Yield stress (MPa)
.- . --
F e - 2 3 ~ i - 3 . 8 ~ n -.
-. - . -. . -
Fe-23Ni-4.OMn .- 1 2 255
65
1 -
220Fig..!: Kinetic behavior of
-
the transformation of F-22Ni-
4) A sudden increment of the load during the test causes an abrupt increase in both the transforma- tion rate and the elongation.
5) T h e enhanced transformation under load is not strain-induced transformation, although plastic deformation is stimulated by the transforming martensite plastes if the applied stress is close to a.
3.6Mn-0.1C when the load is
,:
,#
,#
'/ increased stepwisc..-
-
5 OI-
0W 1 3 ~ 163 193 222
0
0 1 2 3
Holding time ( h r )
T h e effects o f applied stress mentioned in ( I ) and (2) can not be explained by the usual approach that work done by applied stress on the trasforming martensite plate increases the driving energy for the transformation.
These features can be best explained by that the applied stress helps the thermally activated movement of lattice dislocations in austenite and consequently makes possible the plastic accommodation of the shape strain of nucleating martensite plate. This means that such dislocation motion in austenite controls thc rate of isothermal martensite formation as proposed by the present author in previous work (6). The observed feature in (3) is attributed t o a difference in work hardening of the y e t untransformed austenite, that is, in the case of an appliedstress close t o a , the "extra" plastic deformation other than t o accommodate the shape strain of the transforming martensite has occurred in a larger extent, resulting in much more work hardening.
This makes more difficult the plastic accommodation in the subsequent nucleation of martensite, which reduces t h e transformation rate in a latcr stage. We can not explain this feature by simple partitioning effect of the austenite d u e t o the martensite formation. The feature in (4) can be undcrstood as follows. A sudden increase in applied stress causes an abrupt increase in number of mobile dislocations in austenite, which makes it possible t h a t a number of potential nuclei grow i n t o martensite plates because the plastic accommoda- tion of the shape strain becomes much easier. Since the temperature dependence of yield stress is very large as shown in figure 5, work necessary for the plastic accommodation in austenite is greatly increased with decreasing temperature. This will cause a marked decrease in the transformation rate a t low temperature, producing the C-curvo in the T.T.T. diagram.
Thus all the observations in t h e present work show that the plastic accommodation of the shape strain in austenite is a very important necessary condition for martensitc nucleation. However, this point has n o t been taken into a c c o ~ i n t in any nuclcation models so far proposed. In order that a martcnsite plate is brought i n t o existence from a potential nucleation site, three kind of deformations must be taken place except for
Fe
-
23Ni-
4.0Mn 5 (Elongation 0 0 1 2 Holding llmr (hr) Fig. 3: Comparison of the behavior of plastic deformation at 77K between transforming (Fe-23Ni-4.OMn) and non- transforming (Fe-26Ni-3.8Mn) specimens. The amount of martensite transformed for Fe-23Ni-4.OMn is shown by broken line.Fe-23Ni-3.8Mn (77 pm grain) Fe -26 Ni- 3.8Mn 0 0
L
100 200 300 Temperature ( K ) Fig. 5: Temperature depcndence of -- yield stress of austenite for Fe-26Ni- 3.8Mn alloy. Holding time (hr) Fig. 4: Kinetic behavior of the transformation at -.--
77K for F-23Ni-3.8Mn alloy. Corresponding elongation is also shown..JOURNAT. DE PHYSIQUE
some special cases; that is, the "lattice deformation" which generates a martensite latticc from austenite, the "lattice invariant deformation" which produces a stain-free interface between the two phases, and the platic deformation in austenite t o accommodate the shape strain o f martensite plate. T o identify which of these deformations controls the rate of isothermal martensitic transformation is a vital step toward the under- standing of the kinetics of martensitic transformation. Kaufman and Cohen (9), later Raghavan and Cohen ( l o ) , proposed that the formation of the dislocation loops t o accomplish the latticc invariant deformation is the rate limiting step in isothermal martensitic transformation. Recently Olson and Cohen (1, 11) have reached a conclusion that the most probable ratc limiting step is the thermally activated motion of the partial dislocations bounding the semicoherent embryo proposed in their nucleation model. These proposals are equivalent to say that, in the former one, the lattice invariant deformation is the rate controlling, while, in the latter, the lattice deformation is the rate controlling. Both of the proposals, however, can not explain the observed features ( 1 H 3 ) mentioned above.
In conclusion, the experimental results in the present work support the present author's proposition that the rate of isothermal martensitic transformation is controlled by thermally activated motion of lattice dis- locations to accommodate the shape strain of the transforming martensite plate. A quantative treatment of the transfomration.kinetics based on this proposal will be published elsewhere together with more detailed experimental results.
References
1) OLSON G. B., and COHEN M., Met. Trans. 7 A ( 1 9 7 6 ) 1897, 1905, 1915.
2) DASH S., and BROWN N., Acta Met., l4(1966).595.
3) MAGEE C. L., Phase Transformation, ASM, (1970) p. 115.
4) FERRAGLIO P. L., and MUKHERJEE K., Acta Met., 22(1974) 835.
5) KAJIWARA S., Proc. 3rd Int. Conf. on Martensitic Transformations, Cambridge, USA, (1979) p. 362.
6) KAJIWARA S.. Phil. Mag., A,
9
( 1981) 1483.7) KIKUCHI T., and KAJIWARA S., t o appear In Proc. of this conference.
8 ) MACHLIN E. S., and COHEN M., Trans. AIME. p4_(1952) 489.
9) KAUFMAN L., and COHEN M.. Progr. Met. Phys. 1 ( 1 9 5 8 ) 165.
10) RAGHAVAN V., and COHEN M., Acta M e t . , x ( 1 9 7 2 ) 333.
1 1) OSLON G.B., and C O H t N M.. Proc. 3rd Int. Conf. on Martensitic Transformation, Cambridge, USA, ( I 979) p. 3 10.
