Abstract
Small amounts of prestrain at 295K (0.005<ε295<0.08) decrease the yield strength σ0.2 at 77K. When ε295=0.01, σ0.2 is ∼60MPa lower than that with no prestrain. X-ray diffraction analysis showed that the decrease of σ0.2 is due to more formation of ε martensite just at and after yield in the prestrained specimen. Large amounts of prestrain (0.1<ε295<0.454) increase σ0.2 and cause the yield plateau of the load-elongation curve. The yield plateau is attributed to the existence of a transition zone (3-8mm long, depending on prestrain) in the gauge length, where deformation occurs and a burst of α martensite transformation takes place. The transition zone propagates through the gauge length during the yield plateau. An explanation is given for the surface upheaval in a machined flange which has been cryogenically serviced.
