Abstract
Alloy 720Li with a polished surface and with a 1000% coverage shot peened surface has been tensile tested at 200–650 °C and two loading rates. Shot peened specimens had lower ductilityDuctility than unpeened specimens across the temperature range, but the effect was greatest at 450 and 500 °C. This reduction in ductility resulted in reduced tensile strength (TS)Tensile Strength (TS) or fracture stress at 300-500 °C. The shot peeningShot peening effect was present at both loading rates. Shot peened specimens showed surface micro-cracking all over the gauge length of tested specimens and at the point of fracture which explains their reduced ductility. The micro-cracks started at the surface and propagated along strain bands in the material. In contrast, unpeened specimens showed no surface micro-cracks. Interrupted testing on peened surfaces at 500 °C 300 MPa/sec showed that the micro-cracks formed first at 2.3% plastic strain. The application of further strain caused new cracks to form and existing cracks to grow deeper until the specimen fractured at ~9% elongation. Further specimens were tested with different peening coverage levels of 125%, 200%, 400%, 600%, and 1000%. Peening coverage of 125% caused a large reduction in ductility and small reduction in TS relative to the unpeened state. Higher coverage caused further reductions in ductility and TSTensile Strength (TS). The reduced ductility of shot peened specimens is attributed to the low ductility of the shot peened layer which is heavily strained and work hardened.
Published Version
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