Plastic deformation was introduced to the austenitic (γ) stainless steel of SS304 by air blast shot peening, ultrasonic shot peening, and ultrasonic nanocrystalline surface modification. Various deformation structures were formed. The hardness, the deformation structure and the underlying grain refinement mechanism were investigated. In the deformed region, planar dislocation arrays and deformation twin (DT), the DT-DT intersection and ε-martensite structures, and α'-martensite were formed in the respective regions of low, medium, and high strain. The grain refinement mechanism is found to be closely related to the 1) sub-division of coarse grains by DT, shear bands and their intersection, and 2) formation of nano-sized α'-martensite due to the high plastic deformation.

The ultrasonic nanocrystalline surface modification (UNSM) was applied to disk specimens made of Cu-Zn alloy in order to investigate the UNSM effects under five various conditions on wear of deformation twinning. In this paper, ball-on-disk test was conducted, and the results of UNSM-treated specimens showed that surface layer dislocation density and multi-directional twins were abruptly increased, and the grain size was altered into nano scale. UNSM delivers force onto the workpiece surface 20,000 times per second with 1,000 to 4,000 contact counts per square millimeter. The UNSM technology creates nanocrystalline and deformation twinning on the workpiece surface. One of the main concepts of this study is that defined phenomena of the UNSM technology, and the results revealed that nanocrystalline and deformation twinning depth might be controlled by means of impact energy of UNSM technology. EBSD and TEM analyses showed that deformation layer was increased up to 268 microm, and initial twin density was 0.001 x 10(6) cm(-2) and increased up to 0.343 x 10(6) cm(-2). Wear volume loss was also decreased from 703 x 10(3) mm3 to 387 x 10(3) mm3. Wear behavior according to deformation depth was observed under three different combinations. This is related to deformation depth which was created by UNSM technology.

Surface modification of Al5052 alloy was carried out by an ultrasonic nano‐crystalline surface modification (UNSM) technology at various processing conditions. The effect of UNSM processing parameters (amplitude and ball size) on the evolution of microstructure and mechanical properties was investigated. Analyses using electron microscopy, microhardness and wear test showed an obvious grain refinement up to 200 µm in depth from the surface. The hardness of the UNSM‐treated surface increased by approximately 50%, resulting in approximately 48% increase of wear resistance. The effect of ball size in UNSM was predominant on the grain refinement. Copyright © 2012 John Wiley & Sons, Ltd.

The purpose of this study is to analyze the effect of ultrasonic nano-crystalline surface modification (UNSM) treatment on rolling contact fatigue (RCF) characteristics of bearing steels. It was found that severe plastic deformation occurred at surface by over 100 µm after UNSM treatment. The micro surface hardness was increased by 18%, and the measured compressive residual stress was as high as -700~-900MPa. The polymet RCF-2 roller type RCF test showed over 2 times longer fatigue lifetime after UNSM treatment under Hertzian contact stress of 425.2kg/mm2 and 8,000 rpm. And SEM study showed a spalling phenomenon at the samples which went through the RCF test after UNSM treatment. Samples before UNSM treatment produced surface initiated spalls and multi shear lips by progressive spalling at the end along the rolling direction, but sub-surface initiated spalls were formed without multi shear lips after UNSM treatment. The spalling occurred at once, and the size and depth of spalls were larger than those before UNSM treatment. And micro cracks were found to form within the spallings after UNSM treatment, and stress distribution at the maximum Herzian shear stress through these micro cracks is thought to improve the fatigue lifetime of bearing materials.

In this study, a Cu-Sn sintered bronze, used largely for con-rod bushing and automotive transmission, was treated by ultrasonic nanocrystalline surface modification (UNSM). Then, Vickers hardness and microstructural evolution of the treated region were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The hardness of the treated surface doubled, which is attributed to the developed of nanoscale grains, deformation twins, and high density of dislocations induced by the UNSM. Microstructural modification beneath the UNSM treated surface was typically characterized with increase of the depth: (i) nanoscale grains (top surface), (ii) intersection of deformation twins (~30 μm), (iii) high density nanoscale twin/matrix lamellae (~50 μm), (iv) interception of micro band and deformation twins (~100 μm), (v) dislocation arrays (~200 μm), (vi) low density dislocations (~300 μm) and (vii) pre-existing coarse grains and annealing twins in unaffected region (400 μm ~deeper).