It has been well established that there is a strong correlation between the magnetic behaviour of Fe based alloys and their austenite to martensite phase transformation. Despite the paramagnetic nature of the austenitic parent phase in these alloys, martensitic product phase exhibits a distinctive ferromagnetic character [1–4]. On the other hand, applied magnetic fields alter the austenite to martensite transformation kinetics by creating a free-energy difference between the two phases [1]. Tamarat et al. [2] examined the magnetic transitions during the austenite to a martensite transformation in an Fe–Mn–Si alloy by measuring the magnetic susceptibility variation, and observed a sharp change between different magnetic states at the transformation temperature (Ms). Yang et al. [3] also observed that similar changes occurred after the formation of a martensite in some Fe–Mn based alloys, and discussed the influence of austenite deformation on the magnetic properties of parent and product structures. In addition, they examined the influence of magnetic transition upon the transformation characteristics at various temperatures and described the strong dependence of the thermally induced martensite formation on the magnitude of this transition. However, in Fe based alloys, besides the thermal activation of martensitic transformation, martensite formation can, in general, also be induced by the plastic deformation of the matrix austenite [5]. Although the formation mechanism of thermally-induced martensite is different from that of the strain-induced type, there are still several similarities in their physical properties [6–8]. Despite the previous reports on the ferromagnetic transition occurring during the martensite formation and the effect of deformation upon the magnetic characteristics of the austenite and martensite phases, there has not yet been any study to examine the magnetic changes that accompany strain-induced martensite formation and to compare the magnetic behaviours of thermallyand strain-induced martensites. Therefore, the present study aimed to investigate the magnetic properties of these two martensitic products formed in an Fe–Cr–C alloy, by using magnetic susceptibility measurements. Fe–8.3%Cr–1.1%C alloy was prepared by vacuum induction melting. Samples were austenized at 1200 8C in vacuum for 12 h and furnace cooled to room temperature. The austenitic bulk samples were deformed plastically by compression at room temperature to obtain strain-induced martensite. The specimens for magnetic susceptibility measurements were prepared from the bulk material in the form of discs of 1.5 mm radius and 1.5 mm thickness, and the experiments were performed on a computerized a.c. susceptometer (Lake Shore model 7130) with a closed cycle refrigerator between 25 and y263 8C. The sample was kept in helium exchange gas for speedy thermal equilibrium with a controllable temperature resolution better than 10 mK and moved between the centres of the secondary coils in order to minimize the unwanted signals and hence to maximize the signal coming from the sample itself. The magnetic susceptibility measurements were taken by employing a lock-in amplifier with an input low pass filter. Thin foils for transmission electron microscope (TEM) observations were electropolished by using the double-jet polishing technique with a solution of methanol-15% perchloric acid at 0 8C and examined in a Jeol-100CX TEM operating at 100 kV. The effects of temperature and plastic deformation on the volume fractions of thermallyand straininduced martensites were determined metallographically. Figs 1 and 2 show the obtained change in the volume fractions of both martensites formed in the austenite matrix after cooling below the martensite start temperatures (Ms), and plastic deformations applied at room temperature. The Ms temperature of thermally-induced plate martensites was measured as y28 8C and it was found to be raised with the plastic deformation of the austenite matrix prior to the thermally-induced martensite formation, as expected for Fe based alloys [5]. However, strain-induced type martensites were initiated to form at room temperature and the minimum amount of deformation to start transformation was determined to be ,5%. Some austenitic samples were plastically deformed at room temperature after they were partly transformed to thermally-induced plate martensites to obtain strain-induced martensites in the same grains. Magnetic susceptibility measurements were then carried out with these samples to make quantitative comparisons with the samples that were only cooled or deformed. Fig. 3 is a TEM micrograph of thermally formed plate and lath shaped straininduced martensites, which were formed in an austenitic sample cooled at y40 8C and then plastically deformed to 17% at room temperature. Fig. 4 shows the magnetic susceptibility against temperature curve of an austenitic sample cooled to y263 8C. As shown in the figure, there is a distinct change at the Ms temperature, which indicates the