In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (-1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation

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a b s t r a c tCoupons of a Ni base super alloy, Inconel alloy 718 (IN718) were laser shock peened with and without an ablative layer and resulting microstructure and residual stress state were studied. Laser shock peening without a protective overlay results in both mechanical and thermal loading of the material, leading to melting and re-solidification on the surface along with deformation due to laser induced shock wave. The surface shows presence of a non-uniform recast layer which increases the roughness of the surface and also results in a tensile state of residual stresses on surface. The recast layer has areas with modified chemistry and shows presence of nano particles deposited on top of the matrix. In this study, recast layer formed in Inconel alloy 718 (IN718) as a result of laser shock peening without protective overlay was characterized and compared with surface condition of a sample peened with a protective overlay. These results are presented and discussed in relation with those of previous studies.

The effects of thermomechanical processing (TMP) with iterative cycles of 10% cold work and strain annealing, on corrosion and stress corrosion cracking (SCC) behavior of alloy 600 was studied. The associated microstructural and cracking mechanisms were elucidated using transmission (TEM) and scanning electron microscopy (SEM), coupled with precession electron diffraction (PED) and electron back scatter diffraction (EBSD) mapping. TMP resulted in increased fraction of low coincident site lattice (CSL) grain boundaries whilst decreasing the connectivity of random high angle grain boundaries. This disrupted random grain boundary network and large fraction of low CSL boundaries reduced the propensity to sensitization, i.e. carbide precipitation and Cr depletion. After TMP, alloy 600 (GBE) also showed higher intergranular corrosion resistance. Slow strain rate tests in 0.01 M Na 2 S 4 O 6 solution at room temperature show TMP lowered susceptibility to intergranular SCC. To better understand the improvements in corrosion and SCC resistance, orientation maps of regions around cracks were used to analyze the interactions between cracks and various types of grain boundaries and triple junctions (TJs). Detailed analysis showed that cracks were arrested at J1 (1-CSL) and J2 (2-CSL) type of TJs. The probability for crack arrest at special boundaries and TJs, calculated using percolative models, was found to have increased after TMP, which also explains the increase in resistance to corrosion and SCC in GBE alloy 600. A clear correlation and mechanistic understanding relating grain boundary character, sensitization, carbide precipitation and susceptibility to corrosion and stress corrosion cracking was established.