Nitinol (NiTi) has become one of the most important implant materials in biomedical applications such as endovascular stents, embolic protection filters, and endodontic files. However, NiTi stents sometimes do not provide satisfactory visibility under fluoroscopy during angioplasty. A new ternary alloy, radiopaque nitinol (NiTiPt), exhibits much better radiopacity than NiTi and has great potential for future biomedical applications. This paper presents the first characterization of anisotropic stress-strain behavior in micron-sized specimens cut directly from hypotubes of this new alloy NiTiPt. Experimental results show that NiTiPt hypotube has very different anisotropic characteristics and high stress-strain nonlinearity when compared to its NiTi counterpart. Its ultimate tensile strength and strain are approximately 15-40% higher than NiTi with statistically significant difference for all three cutting angles measured. Its stress-strain behavior has several unique features including lower initial austenite elastic modulus, smaller hysteresis loop, and sharper tails during the loading-unloading cycle. The 458 behavior is particularly different from other angles, with stronger hardening effect and an approximately 25-30% reduction in ultimate strain when compared to 08.

Recently, we have proposed a theory to analyze the first-order phase transition ͑FOPT͒ in solids. In order to test the concept of the physics of dissipation during FOPT in solids, it is necessary to test the theory with different FOPT system. We study here a burst-type martensite transformation in a Fe-18.8% Mn alloy sample for this purpose. We investigate the characteristics of ␥͑fcc͒ ͑hcp͒ transformation in this alloy and measure the dependence of internal friction ͑IF͒ during ␥/ transformation in varying rate of temperature Ṫ and vibration frequency. For free oscillations, the IF was defined to be Q ␦ Ϫ1 ϭ␦/ where ␦ is the logarithmic decrement. For general ͑forced͒ oscillations, IF is usually defined to be Q w Ϫ1 ϭ(1/2)(⌬W/W), where ⌬W is the dissipation over one cycle, while W is the maximum stored energy. During our analysis, the relation between Q ␦ Ϫ1 and Q w Ϫ1 is deduced. The parameter l ͑coupling factor between phase interface and oscillating stress͒ takes a small value ͑0.015-0.035͒ during PT, but takes a large value ͑0.86͒ during static state. The parameter n ͑exponent of rate for effective PT driving force͒ takes a large value 0.33 during heating and 0.47 during cooling. The physical meaning of n and l is discussed. The methodology introduced here appears to be an effective way of studying FOPT in solids. ͓S0163-1829͑96͒02533-7͔

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Nitinol fatigue strain limit versus both strain amplitude (range 0.25-1.25%) and mean strain (1.0, 2.0, and 4.0%) was investigated using a tension-tension method. In order to apply the fatigue testing results to a nitinol stent and evaluate stent fatigue performance, the dog-bone style specimens were processed from the same raw material common to implantable stent manufacturing, i.e., similar nitinol tubing, surface quality, and electropolished surface. To simulate a physiological environment, the tension-tension fatigue tests were conducted in water at 37°C. This strain-controlled fatigue test was conducted with a run-out set at 10 6 cycles. The fatigue strain limit at 10 6 cycles as well as the mean strain effect and the effects of inclusions are discussed. Fatigue results appeared in a bi-modal pattern when the strain amplitude was at a level between too high, which made all specimens to fail, and too low, which allowed all specimens to survive.

In order to test the concept of the physics of dissipation during first-order phase transitions in solids, we measured the internal friction (Q Ϫ1) and the relative shear modulus () during a thermoelastic martensitic transformation in a NiTi alloy. We adopted two approaches: temperature variation and application of external stress. This investigation of internal friction was carried out with various vibration frequencies , temperature variation rates Ṫ , and strain variation rates. The index l ͑coupling factor between phase interface and oscillating stress͒ and index n ͑rate exponent for the effective phase transformation driving force͒ have been calculated from the experimental data for each case and the values of l and n are about the same in the two ͑doped͒ NiTi samples, irrespective of whether the phase transition is driven by a temperature variation or stress induced process. We compare the values of n and l for the NiTi samples with that of the other samples ͑VO 2 ceramics and FeMn alloys͒, reinforcing the previous physical interpretations of these indices. We believe the indices n and l are indeed fingerprints of first-order phase transitions in solids. ͓S0163-1829͑97͒00230-0͔