Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16 ; DOI: 10.3221/IGF-ESIS.07.01 12 Fig. 11 exhibits the microhardness distribution along the depth of the samples. Compared with the as-received sample, hardness of the treated sample has been increased significantly, but the hardness profile does not change much with the processing time. Since work hardening is a consequence of severe plastic-deformation process, it can be seen that after a 30 min treatment, the depth of the deformation-affected zone changes only slightly. In other words, because the intensity of the impact of balls does not change, the plastic-deformation zone remains nearly constant, although it has already been known that the surface nanolayer could continue to extend with the processing time. Thermal properties Thermal properties of shot peened surface nanocrystallized materials have also been studied in some experiments [96,97]. Surface nanocrystallized iron obtained by ultrasonic shot peening with the following process parameters were investigated for thermal properties: material Iron with a purity of 99.95 wt. %, vibration frequency of the chamber driven by ultrasonic generator 20 kHz and the shot diameter of 3 mm. The samples used in the study were treated in vacuum for 400 s at room temperatur e [97]. It was found that the thermal conductivity of the nanostructured surface layer decreases clearly compared with that of coarse-grained matrix of the sample. The conducted analysis shows that the decrease of thermal conductivity is mainly due to the decrease of the electron and phonon mean free path and to electron and phonon scattering at the grain boundaries. Small grain size with large volume fraction of interfaces within which a large amount of defects as well as high random atomic arrangement may exist, would strongly lead to electron and phonon scattering at grain boundaries. Hence, when electrons and phonons pass the interfaces, they are scattered intensely, that inevitably leads to the reduction of thermal conductivity of the microstructure [97]. In Fig. 12, it is interesting to observe that, from positions 1 to 2, the average image voltage is almost the same, and from positions 3 to 7, the voltage values increase clearly, then they become approximately stable again. The variations in average image voltage imply that, with the refinement of the microstructure, the thermal conductivity decreases clearly. (a) (b) Figure 12: (a) Schematic diagram of the scanning positions from the surface layer to the matrix on the cross-sectional surface. (b)Variation in average image voltage while scanning from the treated surface layer to the matrix as indicated in (a) [97]. In the treated layers, a large value of residual stress was induced by the ultrasonic shot peening, which leads to an important lattice distortion and a high dislocation density. These residual stresses and dislocations can act as both phonon and electron scatterers and thereby reduce the thermal conductivity of the microstructure [96]. Magnetic properties Magnetic properties, as an important physical property, have attracted many researchers in ferromagnetic nanocrystalline materials. This phenomenon largely shows dependence on the composition, microstructure, and grain size [98, 99]. Magnetic properties were measured for SMAT Fe-30 wt. % Ni alloy. The samples were first heated and water quenched in order to obtain uniform grain size. Then they were treated at a 50 Hz frequency with spherical stainless steel balls of 8 mm in diameter under vacuum at ambient temperature for different durations from 30 to 90 mins. The results indicated that the saturation magnetization (Ms) and specially coercivity (Hc) of the nanostructured surface layer increase significantly compared to the coarse grains sample prior to SMAT. Experimental and theoretical analysis attributed the increase of Ms to the change of lattice structure resulting from strain-induced martensitic transformation. Meanwhile, Hc was further increased from residual microstress and superfined grains [98]. The enhancement of material’s magnetic properties is significantly favorable for its application in several fields of engineering. Treated Layer Treated Layer Matrix Scanning from the surface of the layer to the Matrix 1,02 1,03 1,04 1,05 1,06 1,07 1,08 1,09 1 3 5 7 9 Scanning position from the treated surface layer to the matrix Average image voltage (V) Scanning positions