THESIS
2022
1 online resource (xii, 103 pages) : illustrations (some color)
Abstract
A surface nano-crystallization process, surface mechanical attrition treatment
(SMAT), is used to tailer the microstructure of alloys and achieve ideal properties. To
evaluate the effect of strengthening and fatigue resistance enhancement, two widely
used alloy systems, NiTi shape memory alloy (SMA) and CoCrNi medium-entropy
alloy are chosen to investigate.
Many structures like medical stents made of superelastic NiTi SMA are subject to
cyclic bending loads, where the material shows a limited fatigue life due to crack
nucleation and growth in the surface layers under local tensile stress. In this thesis, the
bending fatigue life of NiTi plates is enhanced by pre-strain warm surface mechanical
attrition treatment (pw-SMAT) where the austenite phase is directly subject to severe
plastic d...[
Read more ]
A surface nano-crystallization process, surface mechanical attrition treatment
(SMAT), is used to tailer the microstructure of alloys and achieve ideal properties. To
evaluate the effect of strengthening and fatigue resistance enhancement, two widely
used alloy systems, NiTi shape memory alloy (SMA) and CoCrNi medium-entropy
alloy are chosen to investigate.
Many structures like medical stents made of superelastic NiTi SMA are subject to
cyclic bending loads, where the material shows a limited fatigue life due to crack
nucleation and growth in the surface layers under local tensile stress. In this thesis, the
bending fatigue life of NiTi plates is enhanced by pre-strain warm surface mechanical
attrition treatment (pw-SMAT) where the austenite phase is directly subject to severe
plastic deformation and grain refinement without inducing phase transformation.
Amorphous and grain size gradient (5-100 nm) microstructures as well as a maximum
compressive residual stress of 1093 MPa are produced in the surface of the NiTi plates
via the pw-SMAT. The compressive residual stress notably reduces the surface tensile
stress from bending. The grain size gradient layers with improved hardness and
reduced hysteresis have high fatigue crack nucleation resistance, whereas the middle large-grained layer has high fatigue crack growth resistance. The combined effects of
the gradient nanostructure and the compressive residual stress substantially increase
the bending fatigue life of the NiTi plates from an original 10
3 cycles to over 1.3x10
4
cycles. The results open up a new route to improve the bending fatigue life of NiTi
plates by heterogenous nanostructures.
SMAT was used to produce a grain-size gradient layer on the surface of spark-plasma-sintered medium-entropy alloy CoCrNi. In comparison with the as-sintered
fine-grained CoCrNi, the SMAT alloy shows a 46% increase in yield strength and over
three times higher strain hardening rate (dσ/dε). The ultimate tensile strength (σ
UTS) and
the elongation (ε
f) reach 1420 MPa and 61.3%, respectively. The excellent strength-ductility
synergy should be attributed to the Hall-Petch strengthening and the
formation of high density of dislocations and nanotwins.
Post a Comment