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The quenched M50NiL steel specimens have been plasma nitrided at temperatures from 460 to 590uC for 4 h under a constant mixture gaseous supply of 0?05N 2 -0?4H 2 L min 21 . The effects of the treatment temperature on the microstructure, microhardness and wear resistance of the surface nitrided layers are studied. The results show that the plasma nitrided layer includes only the diffusion layer without conventional compound layer. The surface hardness is significantly enhanced. a9-Fe and c9-Fe 4 N phases in surface layer are formed at relatively low nitriding temperatures (460-560uC), while a low nitrogen phase FeN 0?076 forms when the nitriding temperature exceeds 575uC. With the increase in nitriding temperature, the nitrided layer thickness increases, whereas both the surface and core microhardness of the nitrided specimens decreases. The wear resistance of specimens can also be improved significantly by plasma nitriding.

Ni-Ti-Nb alloy is a new type of shape memory alloy with a wide transformation hysteresis (A's-M~ = 130-150 °C) [1]. Parts made of the alloy can be stored and shipped at ambient temperature without keeping in liquid nitrogen, which is very convenient for engineering. Therefore, this new alloy has great potential usage as mechanical components for joining, fastening and sealing. The microstructure and effect of deformation on transformation hysteresis of the alloy have been studied [2][3][4][5]. However, the recovery stress of Ni-Ti-Nb shape memory alloy has not been reported. Therefore, the purpose of the present work was to investigate the effect of deformation and thermomechanical cycling on the recovery stress of Ni-Ti-Nb alloy.The composition of the alloy was 44.7 at % Ni, 46.3 at % Ti and 9 at % Nb. The Mr, Ms, As and Af temperatures of the alloy were determined to be -36, 14, 45 and 60 °C, respectively, by means of electrical resistance versus temperature measurement. The resultant ingot was hot-swaged and rolled at 850 °C to strips of approximately 0.5 mm thickness. Specimens for all tests having a tensile gauge length of 40 mm and a width of 5 mm were cut from the strips and annealed at 850°C for 1.8 x 103s under 0.1 MPa vacuum and then furnace cooled. For measuring recovery stress, tensile tests were performed on a Shimadzu Autograph DDS-10T-S machine with fixture to allow heating the specimen for temperature control with a strain rate of 4.1 x 10 -4 s -1. The schematic stress-strain curve in the experiments is shown in Fig. 1. The tensile specimens were deformed at various temperatures (Ta) to different total strain (eT) and then the load was removed. The specimen was then slowly heated from Td to T 1 above Af and the stress was increased to OR (recovery stress). In order to study the effect of thermomechanical cycling on recovery stress, following the above process, the specimens were cooled to T2 below Mf under constant strain and then heated to T 1 again. These heating and cooling processes were repeated 50 times and (Xg was recorded as a function of the cycling number N. The values of T 1 and T2 were 300 and -40 °C, respectively. Fig. 2 shows the relationships between tr R and eT for specimens deformed at room temperature (20 °C). It can be seen that when e T is < 9% o R increases with increasing eT and reaches a maximum value at eT = 9%, then decreases with increasing eT.It was previously found that the microstructure of Ni-Ti-Nb alloy consists of NiTi matrix and fi-Nb phase [2]. When the Ni44.7Ti46.3Nb 9 alloy was deformed at room temperature (a little higher than Ms) the stress-induced martensitic transformation occured in the Ni-Ti matrix [7]. Thus, Fig. 2 can be easily understood. When the total strain is < 9% no true plastic deformation occurs for the NiTi matrix of the specimen. The strain of the NiTi matrix is mainly attributed to the stress-induced martensitic transformation and martensite reorientation. The value of aR depends on the amount of reversible strain of the Ni-Ti matrix constr...

Annealed and quenched M50NiL steel specimens were plasma nitrided at 460°C for 4 h under a mixture gaseous supply of 0·05N2+0·4H2 L min−1 to investigate the effects of the substrate microstructure on the features of nitrided layer, such as microstructure, microhardness and wear resistance. The results show that the nitrided layer is mainly consisted of α′-Fe, γ′-Fe4N and α′N (nitrogen expanded martensite) phases. The surface hardness is significantly enhanced. For the quenched specimens, the nitrided depth decreases with increasing quenched temperature. Compared to the quenched specimens, the nitrided depth of the annealed specimen is thicker under the same nitriding condition. The mechanism may be the variation in the amounts of interstitial carbon atoms and substitution alloying elements in the lattice, and the compressive residual stresses in the substrate. It is also demonstrated that the wear resistance for both the annealed and the quenched specimens can be significantly improved by plasma nitriding treatment.