Metastable austenitic stainless steels undergo a strain-induced martensite transformation due to plastic deformation. The formation of strain-induced martensite is enhanced by larger grain size. The influence of grain size shows also in the Nohara equation commonly used to estimate the Md30 temperature which describes the stability of austenite against strain-induced martensite transformation:
Md30 = 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29Ni - 18.5Mo - 29Cu - 68Nb - 1.42 (GS - 8)
Why does large grain size promote this phase transformation?
What is the physical explanation?
Hi Timo,
As Hamed Mirzadeh said it can be indeed nontrivial phenomena involved: In a polycrystalline body under stress, grain boundaries are easy small-displacement sites (low nucleation barrier), places for stress localization (high driving force) but also obstacles for stress relaxation (build back stresses against nucleated phases on other boundaries); a combination of these effects can partially explain grain size effect on the mechanical stability of austenite
Hi Timo.
When a polycrystalline material is deformed, the strain will tend to localize in grains which are favorably oriented according to the loading direction. When the grain size is large, there will be fewer grains with favorable orientation and consequently strain localization will be more severe in the activated grains. The martensite transformation requires a certain amount of strain localization and therefore the large grain size material will be more prone to form strain induced martensite.
Martensitic transformation is realized by the occurrence of (1) lattice displacement from fcc to bcc structure, and (2) supplementary deformation such as dislocation slip and twinning. In special, the lattice displacement from fcc to bc causes crystallographic anisotropy in terms of the behavior of expansion and contraction. For promoting thermal martensitic transformation, both elastic strain energy for lattice displacement and chemical driving force are required
From the attached figure (Ref. Materials Transactions, Vol. 45, No. 7 (2004) pp. 2245), elastic strain energy greatly increases when the austenite grain size becomes smaller than 1 µm. This result demonstrates the difficulty of nucleation of martensite lath in ultrafine-grained austenite with the grain size below 1 µm. As the result, ultra grain refinement of austenite makes the occurrence of multi-variant transformation difficult and this leads to the suppression of martensitic transformation in metastable austenite.
For strain-induced martensite, see the following plot (our data/unpublished), we studied the austenite stability of 301LN with different grain sizes (UFG, FG, and CG) by interrupting tensile straining at given strains to determine the martensite fraction (as shown in the attached figure) in tensile samples. It is observed that the stability of the austenite varies slightly between the studied microstructures. As expected, the FG structure is the most stable structure exhibiting a retarded transformation rate and lower martensite fractions. In contrast, the UFG structure is more unstable than the CG structure. This is attributed to the precipitation of carbo-nitrides, which decrease the stability of the reversion-treated structures despite the structure becoming finer.
the larger the grain size, the larger the pores between the grains. hence, large values of joint dislocations
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