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摘要
The common way to produce parts from NiTi shape memory alloys is preparing the alloy by arc or induction melting with several re-melts to reach a sufficient homogeneity. The use of graphite crucible can cause carbon contamination. After cold and/or warm working with annealing steps, final shaping is done by drawing, cutting, and drilling. But high work hardening and pseudoelasticity make NiTi alloys difficult to machine. High wear of cutting tools is observed. The material loss is also an expense factor. Powder Metallurgical (PM) methods are promising ways to overcome these problems. Many investigations have focused on combustion synthesis (CS) also called self-propagating high temperature synthesis (SHS) or powder metallurgy for production of bulk NiTi phase [1-3]. Considerable attentions have also been attracted on the porous NiTi SMA. The porous NiTi SMA was usually used for hard tissue implants because of its porous structure, good mechanical properties and biocompatibility [4]. SHS, pre-alloy powder metallurgy, and element powder metallurgy (EPM) [5] were the three popular methods to produce porous NiTi SMA. Aim of this paper is to present a study on the preparation by sintering of NiTi porous sheets from the characterized NiTi pre-alloyed powders. A better understanding about the microstructural and functional properties, the effects of composition and sintering parameters on shape memory and superelastic behavior were investigated by dynamo-mechanical analysis (DMA). The experimental campaign has been carried out on NiTi 55.5 and 56 wt.% pre-alloyed powders according to Table I. The powder has been constrained in a graphite mould and sintered under vacuum at 1200 degrees C and 1250 degrees C for different times. Carbon and Oxygen concentrations were detected by LECO analyzers, using the combustion infrared absorption method. The samples morphology has been investigated by using an Field Emission Scanning Electron Microscope (FESEM) equipped with an Oxford x-ray microprobe (EDS) for elemental micro-analysis. Transformation Temperatures Range (TTR) were investigated using Differential Scanning Calorimetry (DSC). The analysis was carried out in accordance to the standard practice, ASTM F2004-05. For Dynamo Mechanical Analysis (DMA) a static loading cell of 25N, an heating rate of 1 degrees C/min and a frequency of 10 Hz were fixed. Common 3-points bending test and hysteresis test under different bending load conditions were performed (Table 1). C and 0 content and DSC results for fresh and sintered powders are given in Table II. For fresh NiTi 56 powder a flat peak was detected under cooling. This peak is identified as martensite. A separate, clear R-phase peak was also revealed. Since the martensitic transformation temperature is directly correlated with the Ni content of the NiTi phase, Hie less pronounced peak is considered to be an indication of a non-uniform Ni distribution within the starting powder. DSC measurements on the same powders after sieving at different grain sizes confirmed this hypothesis. After sintering a general increase of the transformation temperatures was observed. Due to diffusion processes, for the austenitic transformation the wide shoulder present on the main peak decreases, confirming that a more homogeneous distribution of Ni in the NiTi phase occurs with increasing sintering temperature. The shift of the transformation peaks to higher temperatures in general is associated to the Ni reduction within the intermetallic due to the formation of Ni-rich precipitates at the grain boundaries region.
However, Ti2Ni orcomplex oxide have been detected. These compounds have the tendency to reduce Ti content within the matrix and a decreasing of TTR might be observed. In fact, the measured increasing of Oxygen content can explain the shift of some DSC peaks to lower temperatures. The microstructures of the NiTi 56 powder sintered at 1250 degrees C for 2h and 411 are compared in Fig. 1. EDS analysis of the matrix gives a composition of near equiatomic NiTi. In addition to the NiTi phase, a secondary phase is present in the material with a Ni/Ti ratio of about 1:2. This larger dark grey phase can be attributed to Ti2Ni or Ti4Ni2Ox. These two phases show overlapping XRPD peaks which do not allow a clear distinction. By increasing the sintering time, a general reduction of the porosity was observed. From a mechanical point of view, the samples sintered at 1200 degrees C appear to be more fragile. To obtain a good mechanical stability, sintering temperature had to be elevated at 1250 degrees C, which is more than 0.95*Tmelt. The high sintering temperature indicates a low sinterability of the intermetallic NiTi phase itself. Figure 2 shows tan cycling values of specimen 3 under different loading conditions. Several peaks are evident during the phase transformation. The behavior of these peaks is, however, very complicated since it depends on experimental parameters such as frequency, cycling rate, as well as composition and thermo-mechanical history of the samples. On heating it seems that only one peak appears. At least two peaks are observed on cooling. This could be a further indication of a non-uniform Ni distribution within the starting powder. A general shift to higher temperatures is visible by increasing the testing load. The stress-strain curves and the hysteresis curves under constant bending load of 0.5, 1.0, and 1.5N on sintered sheets will also be presented. Aim of this paper was to present some interesting results obtained by using powder metallurgy technologies in order to realize NiTi shape memory or superelastic components. The preparation of Nitinol sintered-porous sheets from pre-alloyed powders was reported. Both microstructural and functional properties of the sintered samples were characterized. The summary of data suggests that the evolution of DSC charts of NiTi sintered Porous sheets may be very complex, depending on several factors, such as starting composition, impurity content, grain size distribution, homogeneity of the pre-alloyed powders. DMA results confirm the feasibility of NiTi sintered porous parts with sound shape memory properties. Further experiments have to be done in order to better understand the sintered sheets behavior under different working conditions.
- 出版日期2011-4
