| Popis: |
Effect of stress-assisted aging on stress induced martensitic transformation in hot-rolled Ni-rich 50.7at. Ni%-Ti alloy has been investigated. Alloys are aged freely and under 20 MPa, 100 MPa, and 200 MPa stress at 400 o C for 90 minutes. Aging procedure affected both stress-induced and thermally induced transformation behavior. Superelasticity behavior is correlated with the multistep transformation in aged Ni-rich NiTi alloys and the aging stress level is found to be effective. Relative to the free aged alloy, the alloy aged under 20 MPa exhibited a slight and the alloy aged under 100 MPa exhibited a considerable reduction, whereas the alloy aged under 200 MPa exhibited an increase in the critical transformation stress. DSC studies have shown that the transformation is multistep for freely aged and aged under 20 MPa alloys, whereas it is single step and two-step for alloys aged under 100 MPa and 200 MPa, respectively, and this has been attributed to the effect of stress on nucleation and growth rates. As a result of the different response mechanisms to the applied stress upon loading during superelasticity testing, the recovered strain amounts varied considerably depending on the aging conditions and the test temperatures. |
