Nattiree Chiranavanit. Assessment of mechanical properties and transformation behavior of locally-made ni-ti alloys used in orthodontics . Master's Degree(Orthodontics). Mahidol University. : Mahidol University, 2008.
Assessment of mechanical properties and transformation behavior of locally-made ni-ti alloys used in orthodontics
Abstract:
Ni-Ti alloy wires have been widely used in clinical orthodontics because of
their properties of superelasticity (SE) and shape memory effect (SME).
The purpose of this study was to assess the mechanical properties and phase
transformation of 50.7Ni-49.3 Ti (at%) alloy (NT) and 45.2Ni-49.8Ti-5.0Cu (at%)
alloy (NTC), both of which underwent different degrees of reduction (10%, 20% and
30 %) and then received heat treatment at 400°C or 600°C respectively. The Ni, Ti
and Cu (purity 99.99%) were prepared and melted by Argon atmosphere arc furnace
with a non-consumable tungsten electrode. After being sliced with wire cutting
machine, the specimens were cold-rolled with a reduction ratio ranging from 10% to
30%. In order to investigate SE and SME, heat-treatment was performed for 1 h at
400°C or 600°C respectively. The specimens were examined using Energy-Dispersive
X-ray Spectroscope (EDS), Differential Scanning Calorimeter (DSC), Universal
Testing Machine (Instron), Vickers hardness tester, Optical Microscope (OM) and Xray
Diffractometer (XRD).
The chemical compositions were Ni 47.65at%, Ti 52.01at%, Si 0.24at% for
NT, and Ni 41.94at%, Ti 50.21at%, Cu 7.56at%, Si 0.29at% for NTC. On the threepoint
bending test, superelastic load-deflection curve was seen in NTC heat-treated at
400°C. NT heat-treated at 400°C with 30% reduction produced a partially superelastic
curve. For SME, NTC heat-treated at 600°C tended to be superelastic at the oral
temperature if the Af was set slightly lower than 26°C-30°C. For tensile test, the
ultimate tensile strength increased when increasing the percentage reduction. On the
other hand, the percentage elongation decreased when increasing the percentage
reduction. Micro-harness value increased when increasing the percentage reduction.
The average grain size was typically 50-80 μm. In order to improve the superelasticity
and shape memory effect, the transformation temperature range should be lower than
the oral temperature, either by increasing cold-working over 30% reduction, or adding
more Ni/Cu.
The results showed that locally-made Ni-Ti alloys have various
transformation behaviors and mechanical properties depending on three principal
factors: chemical composition, work-hardening (% reduction) and heat-treatment
temperature. In order to fabricate Ni-Ti alloy used in orthodontics, these three factors
should be carefully monitored. This information was valuable as a baseline data for
further development of locally-made Ni-Ti alloyed used in orthodontics