Y. Yue

Tribological properties of plasma-nitrided AISI 4340 steel in vacuum

Tribological properties of nitrided AISI 4340 steel are evaluated using ball-on-disk tribometer in vacuum. Optical microscopy and Vickers hardness tester are used to analyse compound layer thickness and case depth. X-ray diffraction is used to observe the presence of different constituents of nitrided surface before and after the wear tests. The wear mechanisms are investigated by microscopic observations of the worn surface using scanning electron microscopy. At 200 rpm, mild adhesive wear forms the main wear mechanism under the lower load. With increasing the normal load, the dominant wear mechanism changes to mild plastic deformation and abrasive wear. At 600 rpm, the wear mechanisms vary from the mild plastic deformation to severe plastic deformation, abrasive wear and even delamination with increasing the normal load.

High-cycle fatigue behaviour and mechanism analysis of a forged TiZr-based alloy

Fatigue resistance, particularly the endurance limit, is an important design consideration in engineering applications for TiZr-based alloys. The investigated Ti–20Zr–6Al–4 V (wt-%) alloy exhibited a high fatigue endurance limit of 775 MPa. Results showed that severe local stress concentration due to extensive dislocation pile-up at α/β interfaces was responsible for the crack initiation. A transition from a tensile mode to a shear mode crack was observed during crack propagation. Many striations as well as some micro-cracks which can improve the resistance to crack propagation exist in the stable crack-propagation region. A localised deviation between the crack-growth direction was also found, and this outcome combined with micro-cracks and tear ridge may be attributed to varied crystallographic orientations between different phases.