Abstract
Neutron scattering in combination with scanning electron and atomic force microscopy were employed to quantitatively resolve elemental composition, nano- through meso- to metallurgical structures and surface characteristics of two commercial stainless steel orthodontic archwires—G&H and Azdent. The obtained bulk composition confirmed that both samples are made of metastable austenitic stainless steel type AISI 304. The neutron technique’s higher detection sensitivity to alloying elements facilitated the quantitative determination of the composition factor (CF), and the pitting resistance equivalent number (PREN) for predicting austenite stability and pitting-corrosion resistance, respectively. Simultaneous neutron diffraction analyses revealed that both samples contained additional martensite phase due to strain-induced martensite transformation. The unexpectedly high martensite content (46.20 vol%) in G&H was caused by combination of lower austenite stability (CF = 17.37, p = .03), excessive cold working and inadequate thermal treatment during material processing. Together, those results assist in revealing alloying recipes and processing history, and relating these with corrosion resistance and mechanical properties. The present methodology has allowed access to unprecedented length-scale (μm to sub-nm) resolution, accessing nano- through meso-scopic properties. It is envisaged that such an approach can be extended to the study and design of other metallic (bio)materials used in medical sciences, dentistry and beyond.
Highlights
Medicalwires are designed to support, retain or move bone and/or tooth positions with light and continuous force towards a pre-planned final alignment
Common features of standard Prompt Gamma Activation Alalysis (PGAA) spectra and their sources reveal the following: The peak at 511 keV is due to the electron-positron annihilation post pair-production [39]; the peak at 417 keV arises from Indium (In), commonly used in neutron spectroscopy to seal sample-holders, as well as inside the instrument’s HPGe detector [40], complemented by multiple Ge isotope peaks from the HPGe detector
From the bulk composition determined, both samples are majority-comprised of austenitic stainless steel (Fe-Cr-Ni), grade S30400/AISI 304, as per ASTM specification A240/A240M [42], known as A2 or 18/8 stainless steel by alternative specifications
Summary
Medical (arch)wires are designed to support, retain or move bone and/or tooth positions with light and continuous force towards a pre-planned final alignment. Surface characteristics are routinely investigated and used to predict properties [8,9,10], there lacks information about the fundamental composition as well as metallurgical and phase structures, which directly influence final mechanical properties [11,12,13,14,15,16]. This is especially pertinent for austenitic stainless steel alloy, due to its high susceptibility to work-hardening effect in both metastable and stable types; composition governs its stability [17]
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