The Influence of the Soaking Temperature Rotary Forging and Solution Heat Treatment on the Structural and Mechanical Behavior in Ni-Rich NiTi Alloy

Abstract

The structural and thermophysical characteristics of an Ni-rich NiTi alloy rod produced on a laboratory scale was studied. The soak temperature of the solution heat-treatment steps above 850 °C taking advantage of the precipitate dissolution to provide a matrix homogenization, but it takes many hours (24 to 48) when used without thermomechanical steps. Therefore, the suitable reheating to apply between the forging process steps is very important, because the product’s structural characteristics are dependent on the thermomechanical processing history, and the time required to expose the material to high temperatures during the processing is reduced. The structural characteristics were investigated after solution heat treatment at 900 °C and 950 °C for 120 min, and these heat treatments were compared with as-forged sample structural characteristics (one hot deformation step after 800 °C for a 30 min reheat stage). The phase-transformation temperatures were analyzed through differential scanning calorimetry (DSC), and the structural characterization was performed through synchrotron radiation-based X-ray diffraction (SR-XRD) at room temperature. It was observed that the solution heat treatment at 950 °C/120 min presents a lower martensitic reversion finish temperature (Af); the matrix was fully austenitic; and it had a hardness of about 226 HV. Thus, this condition is the most suitable for the reheating stages between the hot forging-process steps to be applied to this alloy to produce materials that can display a superelasticity effect, for applications such as crack sensors or orthodontic archwires.

Microstructure and Physico-Mechanical Properties of Biocompatible Titanium Alloy Ti-39Nb-7Zr after Rotary Forging

“…Previous studies devoted to the effect of RF on the microstructure of steels and nonferrous metals have shown effective refinement of grains and second phases in highnitrogen steel [25]; improved performance characteristics of the low-carbon steels [26], titanium alpha alloy PT-7M [27], titanium pseudo-beta alloy Ti-15V-3Cr-3Al-3Sn-1Zr-1Mo [28]; and enhancement of such properties as shape memory, corrosion resistance and increased mechanical properties in biocompatible alloys Zr-2.5Nb, nitinols [29,30], and titanium β-alloys Ti-25Nb-15Zr, Ti-15Mo with metastable β-phase [31,32]. However, there is a lack of information in the literature regarding the use of RF for biocompatible alloys based on mechanically stable β-solid solution.…”