Effects of composition on transformation temperatures and microstructure of Ni-Ti-Hf shape memory alloys

Abstract

High transformation temperature shape-memory alloys (HTSMA) usually present a martensitic transformation temperature (Ms) starting at 100 ºC. That is the case of high nickel Ni-Ti-Hf alloys. This article presents experimental results obtained from arc melting of Ni 50 Ti 50-X Hf X .at% (X = 8, 11, 14, 17 and 20 .at%) alloys. This process homogenized every composition with similar relative crystallinity. Results confirm that transformation temperatures (TT) increase with increasing the amount of Hf. A martensitic matrix is formed by two metastable phases: R and B19'. From all the alloys studied, the B19' phase presented the highest percent fraction. Gradually adding Hf 3 .at% promoted a slow increase of crystalline fraction of R phase and a slow reduction of phase (Ti, Hf) 2 Ni, located at grain boundaries. Coherent/semi-coherent interface between (Ti, Hf) 2 Ni phase and the matrix may intensify the driving force for the formation of R phase, present on X-ray diffractograms.

Investigation of shape memory effect in Ni-Ti based alloys

“…The Ti-Ni alloys not only possess a good SME, mechanical and corrosion resistance but also have high biological compatibility. Therefore, they have been widely used in industrial and biomedical fields [14][15][16][17][18][19][20][21]. Although widely used in the past few decades, Ni-Ti alloys have shown some disadvantages that prevent the expansion of their applications in practice.…”

“…Although widely used in the past few decades, Ni-Ti alloys have shown some disadvantages that prevent the expansion of their applications in practice. These are bad deformation at low temperature, high material cost, slow reaction to temperature change, small operating temperature range and poor repeatability [14][15][16]. Besides, high concentrations of Ni in the alloys can also causes allergies to the human body leading to limitations in biological applications.…”

“…Besides, high concentrations of Ni in the alloys can also causes allergies to the human body leading to limitations in biological applications. Therefore, some researchers have added other elements (Cu, Zr, Nb, Ta...) into Ni-Ti alloys and changed the manufacturing technology conditions to minimize the disadvantages of these alloys [14][15][16][19][20][21]. The addition of other elements causes the change of transformation temperature and the transformation from β to α type of martensitic phase (orthorhombic structure).…”

Structure and Properties of Coatings Made of High-Entropy Alloy with Martensitic Transformations Obtained by HVOF Method in a Protective Atmosphere

“…Most of the HEA studies are focused on the structure of alloys, microstructural and mechanical properties [1]. However, few works are devoted to TRIP-effect [2][3][4] in these alloys. Alloys with shape memory effect (SME) have high strength, unique effects of single and multiple reversible thermomechanical memory, high level of reactive stresses, recovery and damping, high corrosion resistance and cyclic durability.…”