L. Kovarik

Metallurgical Characterization of a New Nickel-Titanium Wire for Rotary Endodontic Instruments

INTRODUCTIONnA novel thermomechanical processing procedure has been developed that yields a superelastic (SE) nickel-titanium (NiTi) wire (M-Wire) that laboratory testing shows has improved mechanical properties compared with conventional SE austenitic NiTi wires used for manufacture of rotary instruments. The objective of this study was to determine the origin of the improved mechanical properties.nnnMETHODnSpecimens from 2 batches of M-Wire prepared under different processing conditions and from 1 batch of standard-processed SE wire for rotary instruments were examined by scanning transmission electron microscopy, temperature-modulated differential scanning calorimetry, micro-x-ray diffraction, and scanning electron microscopy with x-ray energy-dispersive spectrometric analyses.nnnRESULTSnThe processing for M-Wire yields a microstructure containing martensite, that the proportions of NiTi phases depend on processing conditions, and that the microstructure exhibits pronounced evidence of alloy strengthening.nnnCONCLUSIONSnThe presence of Ti(2)Ni precipitates in both microstructures indicates that M-Wire and the conventional SE wire for rotary instruments are titanium-rich.

High Resolution Microscopy Analysis of a New Precipitate Phase in the High-Temperature Shape Memory Alloy Ni30Pt20Ti50

Shape memory alloys constitute a group of materials that have the ability to recover deformation through heat induced phase transformation. The most prevalent NiTi alloys have found a wide use in a variety of dental, surgical and engineering applications. New types of alloys based on the NiPtTi system are promising for high temperature actuation of up to 300°C [1]. Their implementation in aeronautics could significantly improve the efficiency of devices on new generation turbine engines. A previously unidentified precipitate phase in the Ni30Pt20Ti50 alloy has been analyzed using electron diffraction and STEM HAADF imaging. The observations were performed on FEI Tecnai TF20 operated at 200kV and FEI Titan 80-300 with cs-correction on the electron probe, and operated at 300kV. The HAADF method is especially valuable for structural analysis at the atomic level in the present NiPtTi alloy. Given the atomic number of the elements in the alloy: Z(Ni)=28, Z(Pt)=78, Z(Ti)=22, it is expected that atomic columns enriched in Pt are possible to identify due to the nature of imaging process. A lower magnification view of the microstructure is shown in Fig.1(a). The microstructure consists of the precipitate phase and B19 martensite. The precipitates have a diameter of approximately ~200-400 nm and are commonly seen to contain number of internal faults. The crystallographic nature of the precipitates was analyzed on several zone axes as plotted on the stereographic projection in Fig.1(b). The diffraction pattern analysis reveals that the precipitates have a close structural connection with B2 austenitic phase; the main subset of the diffraction spots in each pattern has the required symmetry and the spacing of B2 phase. Moreover, the corresponding zones axes have the correct angular relationship. An example of the highest symmetry zone (D1), which contain a subset of diffraction spots that are fully consistent with the (110)B2 reflections from [111]B2 is shown in Fig.1(c). An example of HAADF observation from several other zone axes is shown in Fig.1(d,e,f). Images such as those from D2 and D3 zones reveal that symmetry and the periodicity of the visualized columns is fully consistent with Ni sub-lattice of B2 austenite. The additional intensity variation within these columns is detected, which suggests Pt partitioning on the Ni sublattice. Observations from zones that are perpendicular to the D1 axis reveal more details about the partitioning of Pt on the Ni sub-lattice. For example, the observation from D6 zone reveals that Pt is present in two closely spaced columns, shown in Fig.1(f). The columns appear as dumbbell motifs that can be fully mapped onto B2 lattice in the [110] projection. The other Ti and Ni sublattice sites are not clearly detected in the HAADF images due to their much weaker scattering strength. The crystallography of the new precipitates will be discussed in light of the experimental observations. It will be shown that the observations can be fully explained with Pt ordering on B2 lattice and that the precipitates internal faults represent different crystallographic variants, as shown in Fig.2. The possible role of the ordered phase in promoting desirable high temperature shape memory properties will also be described. This work was supported by NASA Glenn Research Center [2]. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S143192760909730XX 1402

The Importance of High-Resolution Scanning Transmission Electron Microscopy For Fine-Scale Dislocation Analysis

Ni-based superalloys are currently used in the hot section of turbine engines due to their retained high-temperature strength under extreme environments. Extensive, detailed work has been done on the deformation mechanisms of these materials under creep conditions [1]; however, in the area of low cycle fatigue (LCF) the same detailed characterization is somewhat lacking and the deformation is not well understood. High temperature cyclic softening [2] is observed during LCF of the polycrystalline disc superalloy R104. In general, it is accepted that planar anti-phase boundary (APB) shearing of the strengthening γ′−precipitates, followed by their dissolution, is the principal cause of cyclic softening [3]. Given the precipitate morphology and bimodal microstructure of R104, this mechanism is unlikely to be responsible for cyclic softening as the γ′ will not be sheared away after a nominal number of cycles; therefore there must exist another principal cause.