Gradient Structures in Thin-Walled Metallic Tubes Produced by Continuous High Pressure Tube Shearing Process Rimma Lapovok,* Yuanshen Qi, Hoi P. Ng, Laszlo S. Toth, and Yuri Estrin A new severe plastic deformation process, the authors refer to as High Pressure Tube Shearing (HPTS), is proposed. This type of deformation processing enhances the strength of the walls of metallic tubes by producing gradient microstructures with ultrafine grained layers in near-surface regions. The thickness of the layers associated with a gradient in microstructure can be controlled by tuning the rotational and translational speeds of the process. The paper describes several examples of steel and titanium tubes processed by different variants of HPTS. The possibility of producing gradient microstructures with ultrafine grained layers at inner or outer surface of a tube, or at both surfaces is demonstrated by in-depth theoretical analysis, finite element simulations, and experimental investigation of the microstructure and texture of tube walls. 1. Introduction Thin-walled metallic tubes are used in various medical device applications, including coronary and peripheral stents, heart valves, surgical implants, trauma and reconstructive implants, drug delivery, and many others. [1] These devices require specific mechanical prop- erties, which could be uniform through the wall thickness or vary from the inner side of the tube to its outer surface. In particular, the requirements on the me- chanical strength of these medical devices are quite challenging. A promising way of meeting these demands is by producing a gradient microstructure within the tube wall. Over the past decade, a great deal of attention was paid to improving the me- chanical properties of commercial alloys by extreme grain refinement, which can be furnished by severe plastic deformation (SPD) techniques. [2,3] SPD-processed metal- lic materials generally exhibit mechanical properties superior to those achieved by conventional processing. The SPD technique, that is arguably most promising for commercial applications is ECAP-C (Equal Channel Angular Pressing  Conform), [4] which was developed to produce solid metallic rods with the grain size in a deep submicron range. This technique gives rise to enhanced strength and fatigue resistance [5,6] due to uniform ultrafine grained (UFG) structure of the processed solid rod. However, this process is not applicable for tubular products or for creating gradient microstructures within the cross-section of the billets. Among the known SPD techniques, there is a special group of processes that are based on friction-induced shear. One of the dimensions of samples or work-pieces used in such processes, namely the thickness, is much smaller than the other two dimensions. These SPD processes involve friction forces acting on large surfaces and a high hydrostatic pressure within the deformation zone. The well-known process of High Pressure Torsion (HPT) [7] and the relatively new Cone-Cone (CC) and High Pressure Tube Twisting (HPTT) methods applied to thin walled samples fall into this category of SPD techniques and are efficient tools to produce gradient microstructures across the wall thickness. [8–10] While the HPTT technique is an attractive way to increase the strength and the wear resistance of tube products, it was designed for processing of short tubes and is not applicable for long ones. A short tube is fully confined between a mandrel and a cylindrical confining die, the pressure is directly applied on the bottom and the top surfaces resulting in high hydrostatic pressure in the entire volume of the tube. [11–14] This creates large friction forces on both the inner and the outer surfaces of the Prof. R. Lapovok, Dr. Y. Qi Institute for Frontier Materials, Deakin University, Waurn Ponds, Geelong, Vic 3216, Australia E-mail: r.lapovok@deakin.edu.au Prof. Y. Estrin Department of Materials Science and Engineering, Monash University, Clayton, Vic 3800, Australia Dr. H. P. Ng NISI Bio-Medical Engineering (Dongguan) Limited, 1/F, Building No. 9, Innovative Technology Park, Songshan Lake National Hi-Tech Industrial Development Zone, Dongguan City, Guangdong Province, P.R. China Prof. L. S. Toth Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (DAMAS), Universite de Lorraine, F-57045, Ile du Saulcy, Metz, France Laboratoire d’Etude des Microstructures et de Mecanique des Materiaux (LEM3), CNRS UMR 7239, Universite de Lorraine, F-57045, Ile du Saulcy, Metz, France Dr. Y. Estrin Laboratory of Hybrid Nanostructured Materials, NUST «MISIS», Leninsky prospect 4, 119049 Moscow, Russia DOI: 10.1002/adem.201700345 Full Papers www.aem-journal.com FULL PAPER Adv. Eng. Mater. 2017, 00, 1700345 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1700345 (1 of 14)