Lath Martensite:
How many packets are in one prior austenite grain?
How many blocks are in one Packet?
How many laths are in one block (approximately)?
what is the role of orientation relationship?
Dear Vignesh,
Unfortunately, the first three questions cannot be answered unless all the details of processing conditions, heat treatment, cooling process etc. are known. Even with these details, there is no simple relation that will give a numerical size value (whether lath/packet/block). At best, a qualitative guess can be made based on general considerations. One of these considerations is, the lower you go below the Ms temperature, the finer is the resultant martensite. To complicate matters, the higher the cooling rate, the lower the Ms temperature can go -remember that Ms and Mf are not fixed temperatures(ref [1]).
As for orientation relationship, the austenite and martensite phases are crystallographically related, in the sense that a particular plane (& direction) of austenite will become a particular plane (& direction) of martensite. The relation between these two planes (& directions) is an orientation relationship (OR). The ORs are based on observation, and more than one OR has been observed; K-S and N-W are common ones. For a nice and detailed explanation, please have a look at ref [2].
Refs: [1]G. Krauss, Martensite in steel: strength and structure, MSEA 273-275 (1999) 40-57.
[2] http://pajarito.materials.cmu.edu/rollett/27750/OR-Trans-Textures-VT_28Apr2016.pdf
On should interpret K-S and N-W more as models and not as really existing relationships. The major problem is that the definition of parallel LATTICE planes and LATTICE directions do not take the reals player (atoms) into account (see link 1 which TRIES to show what happens when fcc (red) arranged iron changes to bcc (blue). The first slide displays a cube volume where atoms move and their rearrangement finally ends in bcc. The second slide focus on the interface since there the misfit becomes visible. Since it is hard to imagine how much strain then exist within the "interface plane" the third plane displays an arrangement of more atoms. However, there only the really adjacent atoms in fcc and bcc are shown, but what happens with the next layer? This is presented in the third plane. If there are any question don't hesitate to contact me.
The presentations also indicate that with the change of chemical composition also the OR changes.
Presentation Orientation relationships between fcc-bcc
I totally forgot to say something to your questions. I am not sure whether you are talking about the real number of lathes in a package or how many and which TYPES belong to one package or block? Both questions appear quite interesting. I did these OR observations more with respect to crystallography. I am no material scientist...and did not have the respective material in order to study it experimentally. Simply to risk an educated guess, I would assume that packages share either the same habit plane or direction, however, I have to admit that I did not check the literature. I cannot imagine that nobody else did this already...
There is no simple accurate answers to your questions. However, the quantitative analysis of packets and blocks numbers and their morphology changes mainly with carbon content and the grain size of prior austenite, in addition to the quenching rate. Please refer to this nice paper Article Quantitative analysis of three-dimensional morphology of mar...
for a 3D structural analysis of Martensite in low and high carbon steel which may answer some of your questions.
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