Dear colleagues,
I am aware of the amount of literature about martensite, steels etc. and maybe I am only weak in using google is so basic topic.
When we quench high carbon steel (>1.5 mass% ) we are obtaining martensite and in most of cases large amount of retained austenite. Due to high carbon content the martensite is Body Centered Tetragonal (a=b≠c, alpha=beta=gamma) (link 1). The martensite morphology may look like this (link 2). The martensite crystallography is related to parent martensite crystallography with crystallographic orientation relationships K-S, N-W...
If we consider one plate or "lens" of martensite (I am aware that it may be twinned or dislocated) as single crystal, can we say which direction is the c axis (a=b≠c)? Or maybe the tetragonality is somehow modulated (changes within volume of imaginary single crystal) and BCT lattice is a kind of approximation - average.
https://e-reports-ext.llnl.gov/pdf/329979.pdf
Article Martensite in steel: Strength and structure
If you consider how much carbon is in martensite then it turns out that only each third or fifth unit cell (are you talking about at% or mass%) contains a carbon atom. This means we have a very distorted structure, already without any dislocation. The microstructure however displays that this material is full of dislocations so that the 3D periodicity required for a nice diffraction does not exist, additionally to the statistical distribution of C and therefore a tetragonality which certainly changes even in a single martensite precipitate. The result in XRD are broad peaks which can be fitted by a tetragonal distortion. Whether this is an averaging of locally differently distorted grains, subgrains or simply the effect of the periodicity loss is hard do say since all other techniques either require ideal conditions (CBED) or are simply too low precise (SAED, EBSD). If you measure this martensite with a synchrotron I won't expect that you will get a much better peak resolution. At least not for all martensites since this broadening is from my point of view simply an effect of the inherent lattice and structure defects.
Thank you for answer Gert, I meant mass%. So we have maximum of about 7.5 at% of C (maximum solubility in austenite) and 2 atoms of Fe per bcc unit cell it gives only 15% of all unit cells will contain carbon atom.
What do you think about these XRD investigations (link 1)? Those peaks doesn't look like just broaden peaks?
https://doi.org/10.1016/0956-7151(91)90200-K
Well, if you have so much carbon inside I assume the structure really react in a more systematic way and distorts to an ordered tetragonal. The peaks look reasonable...but below the mentioned 0.6% I assume that the lattice simply is imperfect which broadens the peaks. I wonder how much stress produced by the remaining austenite will also support this lattice distortion...although the symmetry does not match (if the habit planes ate (111)fcc and (110)bcc). But around a particle this plane is anyway continuously changing.
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