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.
Highlights
NiTi alloy is one the most-interesting shape-memory alloys (SMAs) due to its noticeable functional properties, namely, superelasticity and a shape memory effect, combined with excellent mechanical properties, high corrosion resistance, and biocompatibility [1].Functional properties are originating from a reversible phase transformation between austenite (B2 cubic structure) and B190 [1].This transformation occurs as a temperature variation (TIM, thermally induced martensite) or by applying stress (SIM, stress-induced martensite) results, and it may take place directly from austenite (parent phase with B2 cubic symmetry; space group ( Pm3m) to martensite (product phase with B190 monoclinic symmetry; space group ( P21 /m), or it may go through an intermediate R-phase (trigonal symmetry; space group ( P3)
In the current forging process, the remelted ingot was submitted to hot forging (1F sample), which consisted in previous reheating with soaking at 800 ◦ C for 30 min, in a muffle furnace before transfer to a four-hammers tool (10.41 mm diameter), which was followed by hot deformation and slow cooling to room temperature
Multiple peaks superimposed were observed for 1F sample differential scanning calorimetry (DSC) results, which were attributed to the previous deformation imposed and/or the compositional inhomogeneity of particles [18,19,20], while the heat-treated samples showed the single-phase transformation peak, on cooling as well on heating
Summary
Functional properties are originating from a reversible phase transformation between austenite (B2 cubic structure) and B190 (monoclinic martensite) [1]. This transformation occurs as a temperature variation (TIM, thermally induced martensite) or by applying stress (SIM, stress-induced martensite) results, and it may take place directly from austenite (parent phase with B2 cubic symmetry; space group ( Pm3m) to martensite (product phase with B190 monoclinic symmetry; space group ( P21 /m), or it may go through an intermediate R-phase (trigonal symmetry; space group ( P3). The superelasticity occurs by the stress-induced martensitic transformation (SIM) [1,2] These functional properties are guaranteed by the combination of the chemical and microstructural homogeneity. These characteristics are acquired through the most-suitable thermo-mechanical processing route and heat treatment [3]
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