Abstract
The phase changes of steels in solid states may be classified into two kinds, the precipitation of ferrite (α) or cementite and the formation of martensite (α'). The mechanism of the one is very differ from other. The detailed interpretation has already been given by one of the present wriers. As regards the martensite, by the process of so called “ Umkloppen ” is generally accepted, although its n echanism has not yet been aufficiently interpreted.
On this paper, we shall consider the mechanism in detail; to make the differencns of nature, clear between two kinds stated above.
The facts hitherto obtained by the experiments on the formation of martensite show that this transformation consists of a great many minuites and impulsive change takis extremely short time. Hence it is concluded that the change occurs by the mechanism, does not need any diffusion of atoms as in the case of the precipitation. On the other hand, in view of the relation betwcen the crystallographic orientation of austenite (γ) and that of the martensite (α); the mechanism of the transformation should be like the mechanical slip in the case of twin-formation. Considering these facts, we support the theory that martensite forms through the process of Umklatypern. and proposes the following assumptions for its mechanism; so as to be possible to explain all the experimental facts.
i) In the Umklrppen at first an atom which activation energy is lowered dues to the defect of the crystal lattice displaces to a new position which is a lattice point of the new phase (α'). And then. this displacement propagates in the crystallographic directions with such a great velocity of elastic vibration, which results a slipping of atomic planes to form one needle of martensite.
ii) The elastic waves are accompanied by the formation of this martensite needle, other needles successively formed around it become a group.
iii) Many of such a group form at random in the specimen.
