S.K. Wu

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy

Abstract The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy have been studied by internal friction and shear modulus measurements, hardness test and TEM observation. The martensite stabilization can be induced by cold rolling at room temperature. A variety of deformed martensite structures has been observed. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. The hardness test results also support this viewpoint. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. Experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy.

Infrared brazing of TiAl intermetallic using BAg-8 braze alloy

TiAl intermetallic alloy joined by infrared brazing using BAg-8 braze alloy was investigated. The microstructural evolution of the brazed joint, shear strength and reaction kinetics across the joint was comprehensively evaluated. According to the experimental observations, silver would not react with the TiAl substrate, but copper reacted vigorously with the TiAl, forming continuous reaction layer. The consumption of copper from molten braze during infrared brazing resulted in depletion of the copper content from the braze. Therefore, chemical composition of the braze deviated from Ag-Cu eutectic into hypoeutectic with increased brazing time and/or temperature. Both AlCuTi and AlCu2Ti phase were observed at the interface between BAg-8 and TiAl substrate for the specimen brazed at 950°C. By increasing the brazing temperature and time, the growth rate of AlCuTi phase was much faster than that of AlCu2Ti phase. The maximum shear strength achieved 343 MPa for the specimen infrared brazed at 950°C for 60 s. Further increasing the brazing time resulted in excessive growth of brittle AlCuTi reaction layer, which greatly deteriorated the shear strength of the joint.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

A study of electrical resistivity, internal friction and shear modulus on an aged Ti49Ni51 alloy

Abstract The 400°C aged Ti 49 Ni 51 alloy can exhibit the transformation sequence of B2 →r premartensite R-phase →r martensite. In the early aging stage, only the premartensitic transformation is observed due to the M s point being deeply depressed by the coherent stress of Ti 11 Ni 14 precipitates. In the later aging stage, internal friction peaks associated with premartensitic and martensitic transformations are all observed on both heating and cooling. The sharp peaks associated premartensitic transformation on heating is believed to be related to the “bias” effect of the coherent stress induced by the Ti 11 Ni 14 precipitates. The serrations of internal friction appearing significantly in the temperature around −30 to −80°C are found to be caused by the stress induced accomodation of R-phase or martensite variants, and are not associated with the transformation. The Ti 11 Ni 14 precipitates can enhance the amount of martensite formed by unit of temperature or time during the martensitic transformation.

A study of B2↔B19↔B19′ two-stage martensitic transformation in a Ti50Ni40Cu10 alloy

Abstract The B2↔B19↔B19′ two-stage martensitic transformation in a Ti50Ni40Cu10 alloy has been investigated by electrical resistivity, DSC, X-ray diffraction and internal friction measurements. The shear modulus of B19 martensite has an unusually low value over a broad temperature range between the two shear modulus minima. The B2↔B19 transformation is thus proposed to proceed under the condition of deep shear modulus softening. X-ray diffraction results show that the B19↔B19′ is an incomplete transformation and that the monoclinic angle β of B19′ martensite will increasing with decreasing temperature. This indicates that the B19↔B19′ transformation has the characteristic of the continuously monoclinic distortion of B19′ martensite, which is similar to that of the continuously rhombohedral distortion of R-phase. The opposite behavior observed in electrical resistivity and DSC measurements for B2↔B19 and B19↔B19′ transformations is also discussed.

The tensile behavior of a cold-rolled and reverse-transformed equiatomic TiNi alloy

The tensile test shows that the martensite accomodation/reorientation process in the ascold-rolled equiatomic TiNi alloy is depressed due to the hindrance of deformed martensite structures and defects. If the cold-rolled equiatomic TiNi alloy is subjected to a reversed martensitic transformation (RMT) at temperature < 300°C, the strengthening effect induced by cold-rolling can significantly improve the alloys characteristic shape memory effect (SME) and pseudoelasticity (PE) by raising the critical shear stress for slip. The σyM can improve from 380 MPa of solution treated specimen to 1000 MPa of 31% cold-rolled RMT specimen. Meanwhile, for its PE characteristic, a cold-rolling RMT process can also increase significantly the stored mechanical energy and the energy storage efficiency. Experimental results show that, even in up to 20% thickness-reduced RMT TiNi alloy, the specimen still has 20% elongation at fracture. If the cold-rolled specimens are subjected to an annealing at temperature ⩾400°C, the martensite accomodation/reorientation process can recover gradually due to the nullification of deformed martensite structures and defects. Finally the recrystallization occurs at annealing temperature ⩾600°C.

The improvement of high temperature oxidation of Ti-50Al by sputtering Al film and subsequent interdiffusion treatment

Abstract The high temperature oxidation resistance of Ti–50Al can be improved by sputtering an Al film and subsequent interdiffusion treatment at 600 °C for 24 h in high vacuum. In these conditions, a TiAl 3 layer is formed on the surface, which exhibits good adhesion with Ti–50Al substrate and provides high oxidation resistance. Cyclic and isothermal oxidation tests show that the Ti–50Al with 3–5 μm Al film can dramatically reduce the oxidation at 900 °C in air, at which the parabolic oxidation rate constant K p of specimen with 5 μm Al film is only about 1/15,000 of that of bare Ti–50Al. XRD and SEM results indicate that the TiAl 3 layer can promote the formation of a protective Al 2 O 3 scale on the surface as well as react with γ-TiAl to form TiAl 2 during the oxidation. Simultaneously, layers of Al 2 O 3 /TiAl 2 /Al-enriched γ-TiAl/Ti–50Al are also formed on specimens. The TiAl 2 layer thickness will decrease gradually with increasing the oxidation time. After oxidation at 900 °C for 300 h, there is a clearly discontinuous thin layer of Ti 37 Al 53 O 10 compound observed in between Al 2 O 3 and TiAl 2 .

Oxidation behavior of equiatomic TiNi alloy in high temperature air environment

Abstract The isothermal oxidation behavior of equiatomic TiNi shape memory alloy in dry air from 550 to 1000°C is studied with thermogravimetric analysis. The alloys scale layer is further investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer. Experimental results indicate that a multi-layered scale is formed, consisting of an outer rutile layer, a porous intermediate layer of mixture of TiO 2 and Ni(Ti), and a thin inner TiNi 3 layer. The intermediate layer, furthermore, exhibits a stripe-like lamellar structure at the TiNi 3 interface. The apparent activation energy of TiNi alloy oxidation is 226 kJ/mol, and the oxidation rate follows a parabolic law. A schematic oxidation mechanism of TiNi alloy is proposed to explain the observed results.

Damping characteristics of TiNi shape memory alloys

The damping characteristics of TiNi SMAs have been systematically studied by using techniques of resonant-bar and low-frequency inverted torsion pendulum. Experimental results show that both the martensite phase (M) and R phase (R) have high damping due to the movement of twin boundaries. Because the B2 parent phase (B2) has smaller damping, it is suggested that this may come from the dynamic ordering process of lattice defects. In the transformation re-gions of B2 ↔ M, B2 ↔ R, and R ↔ M, there are maxima of the damping capacity which are attributed to two contributions. One arises from the plastic strain and twin-interface move-ment during the thermal transformation, which obeys a linear variation of peak heightsQ−1max vst att ≥ 1 °C/min. The other originates from the stress-induced transformation formed by the applied external stress which dominates atT < 1 °C/min. The elastic modulusE of martensite and the R phase is lower than that of the B2 phase, and a modulus minimum appears in the transformation region.

Martensitic transformations and the shape memory effect in Ti50Ni10Au40 and Ti50Au50 alloys

Martensitic transformations in Ti50Ni10Au40 and Ti50Au50 alloys are of the type B2 a3 B19(2H) as found in a Au-47.5%Cd alloy. The Ms temperatures of the Ti50Ni10Au40 and Ti50Au50 alloys are 440°C and 610°C, respectively. The martensite in both alloys is orthorhombic (2H) and identified as a modified B19 structure with secondary layer stacking at (a2.60, 0, c2). The morphology of the martensite is spearlike with (111)M internal twins. In the Ti50Au50 martensite, (100)M stacking faults and type II twins are also found. Both Ti50Ni10Au40 and Ti50Au50 alloys exhibit a one-way shape memory effect, but not a two-way memory. This may result from the fact that the accumulated “bias” stresses are released when the temperature is raised to above As during the “training” process.

A Study on Ternary Ti-rich TiNiZr Shape Memory Alloys

Abstract The martensitic transformation in Ti50.5-XNi49.5ZrXand Ti51.5-XNi48.5ZrX alloys (X = 0–25 at.%) was studied by using thermomechanical treatments. These alloys have a B2↔B19′ transformation sequence, and their transformation peak temperature M* can be raised to 50–450°C by different additions of Zr. Although a great many second-phase particles exist around (Ti,Zr)Ni grain boundaries, these alloys still exhibit ≥80% shape-memory recovery. Thermal cycling can depress the M* temperature more significantly in the Ti41.5Ni48.5Zr10 alloy than in the Ti40.5Ni49.5Zr10 alloy in the first ten cycles, owing to the former’s having greater hardness and more second-phase particles. Martensite stabilization can be induced by cold rolling at room temperature for Ti-rich ternary TiNiZr alloys. The strengthening effects of cold rolling and thermal cycling on Ms temperatures of these alloys were found to follow the expression Ms = T0-KΔσy, in which K values are related to the as-annealed hardness of these alloys. For the study of 400°C aging effects, the martensite stabilization appearing in the Ti26.5Ni48.5Zr15 alloy may be due to the pinning effect on the interfaces of martensite plates by the point defects.