Structural state and mechanical properties of polycrystalline Ni3Fe Alloys

The evolution of the degree of order on, isothermal ordering, of Ni3Fe was studied by X-ray determination of the long-range order parameter, S, and of the mean domain size ε. Nucleation and growth of ordered domains were clearly distinguished from the domain coarsening in the homogeneously ordered phase. This latter phenomenon follows a kinetic law analogous to that of grain coarsening εn – ε = kt, with n about two. A sharp decrease in short-range order in a 100 °C wide temperature range above the critical temperature, was observed by X-ray diffraction. The mechanical characteristics of samples with various states of order were determined by tensile tests at room temperature. Dislocation arrangements and slip lines were observed by electron microscopy. The following results may be pointed out: 1) The maximum yield stress in partly ordered alloys occurs just before the disappearance of the disordered phase, the role of which has been confirmed by experiments on alloys with a „stabilized”︁ two phase structure. 2) A yield drop and a yielding plateau develop on ordering. 3) In polycrystalline crystals short-range order induces a yield stress increase of 3 daN/mm2. 4) A linear work hardening stage occurs in the stress-strain curve as long-range order develops. During this stage (II), the length of which increases with order, most dislocations are paired. The pairing of dislocations disappears in the parabolic hardening stage (III).