Abstract
The serrated flow, the Lüders bands and flow stress observed in Fe-23%Ni-0.24%C martensite during tensile test were studied at various temperatures between-196 and 220°C. The results obtained were as follows:
(1) Temperature dependence of flow stress of the Fe-Ni-C martensite was explained in terms of the rearrangement of carbon atoms due to Snoek type or Cottrell type pinning, or of the clustering of carbon atoms or the precipitation of fine carbide particles on dislocations. The strain hardening at the early stage of deformation was remarkable in the martensite.
(2) Serrated flow was found at a certain range of the test temperature. The temperature range varied with the strain rate. For example, the serration appeared between 1 and 170°C for the strain rate of 5.6×10-4/sec. At this temperature range, the increment in 0.2%-flow-stress was observed, but the strain hardening during serrated region was not so much.
(3) The distinct Luders bands were observed during serrated flow. One Lüders band corresponded to one stress drop of the serration. The characteristic of Lüders bands in the martensite differed from that of annealed mild steel. The finer Liiders bands were uniformly distributed in the martensite, the larger total elongation was observed.
(4) From the analysis of the critical condition for the serration, it was found that (a) a certain degree of plastic strain was necessary for the serrated flow to occur and (b) each step of yielding during serrated flow was an independent process of dynamic strain aging and was associated with the occurrence of one Lüders band.
