Safaa N. Saud

Correlation of microstructural and corrosion characteristics of quaternary shape memory alloys Cu–Al–Ni–X (X=Mn or Ti)

The effects of different contents (0.4%, 0.7%, and 1.0%, mass fraction) of Mn or Ti additions on the micro structure, shape memory effect and the corrosion behaviour of Cu–Al–Ni shape memory alloys were studied by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and electrochemical and immersion tests in NaCl solution. It was observed that the microstructure, shape memory effect and corrosion characteristics are highly sensitive to the composition variations. It was found that the highest strain recovery was with 0.7% addition of Mn or Ti. This may be attributed to the presence of precipitation with a high volume fraction and the grain refinement. The electrochemical test showed that the formation of oxide layers in both Cu–Al–Ni–Mn and Cu–Al–Ni–Ti shape memory alloys (SMAs) provided good passivation which enhanced the corrosion resistance of the alloys. Immersion test showed that in Cu–Al–Ni–Mn– SMAs, pitting corrosion occurred through feebleness in the oxide layer. A corrosion product adjacent to the pits was rich in Al/Mn oxide and depleted in Cu while inside of the pit it was rich in Cu. In Cu–Al–Ni–Ti– SMAs, localized corrosion occurred on the surface of the specimens and dealuminization attack was also observed in the matrix.

Corrosion and bioactivity performance of graphene oxide coating on Ti-Nb shape memory alloys in simulated body fluid

In the present work, the microstructure, corrosion, and bioactivity of graphene oxide (GO) coating on the laser-modified and -unmodified surfaces of TiNb shape memory alloys (SMAs) were investigated. The surface morphology and chemical composition was examined using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The surface modification was carried out via a femtosecond laser with the aim to increase the surface roughness, and thus increase the adhesion property. FE-SEM analysis of the laser-treated Ti-30at.% Nb revealed the increase in surface roughness and oxygen/nitrogen containing groups on the Ti-30at.% Nb surface after being surface modified via a femtosecond laser. Furthermore, the thickness of GO was increased from 35μm to 45μm after the surface was modified. Potentiodynamic polarisation and electrochemical impedance spectroscopy studies revealed that both the GO and laser/GO-coated samples exhibited higher corrosion resistance than that of the uncoated TiNb SMA sample. However, the laser/GO-coated sample presented the highest corrosion resistance in SBF at 37°C. In addition, during soaking in the simulated body fluid (SBF), both the GO and laser/GO coating improved the formation of apatite layer. Based on the bioactivity results, the GO coating exhibited a remarkable antibacterial activity against gram-negative bacteria compared with the uncoated. In conclusion, the present results indicate that Ti-30at.% Nb SMAs may be promising alternatives to NiTi for certain biomedical applications.

Structure-Property Relationship of Cu-Al-Ni-Fe Shape Memory Alloys in Different Quenching Media

This paper presents the effects of heat treatments using various quenching media on the phase transformation parameters and microstructure parameters. The effects of different quenching methods, step-quenched and up-quenched, in various media were evaluated by using differential scanning calorimetry, field emission electron microscopy, energy-dispersive spectrometry, atomic force microscopy, x-ray diffraction, and Vicker’s hardness. The variations of the structure and properties of Cu-Al-Ni-Fe shape memory alloys were linked to the variations of morphology, type, and stabilization of the obtained phase. From the DSC results, the use of ice water as a quenching medium produced the highest transformation temperatures, while a brine solution-quenching medium resulted in the highest change of the entropy and enthalpy. Additionally, it was found that the best grain refinement was observed through the use of an oil-quenching medium, due to its high cooling rate.

Influence of addition of carbon nanotubes on structure–properties of Cu–Al–Ni shape memory alloys

Abstract The influence of the addition of 0·2 wt-% carbon nanotubes on the microstructure and mechanical properties of Cu based shape memory alloy with a composition of Cu–11·5 wt-%Al–4·5 wt-%Ni were investigated using optical microscopy, X-ray diffraction, scanning electron microscopy and a universal testing machine. The experimental results show that the carbon nanotubes’ (CNTs’) addition has an effective influence on the martensite–austenite transformation temperatures, type, behaviour and morphology. Furthermore, the addition of CNTs can control the grain size and thus the tensile strength, elongation, fracture stress–strain, yield strength and shape memory effect can be enhanced. Remarkably, the shape recovery ratio reached ∼68% of the original shape; this can be attributed to the precipitation and formation of the CNTs into the microstructure, effectively on the grain boundaries, which can help to reduce/relief the concentration of internal stress.

Effects of Quenching Media on Phase Transformation Characteristics and Hardness of Cu-Al-Ni-Co Shape Memory Alloys

This paper presents the investigation on the effects of various thermal treatments and quenching media on the phase transformation behaviour of Cu-Al-Ni-Co shape memory alloys (SMAs). The transformation temperatures were determined using a differential scanning calorimeter. The variation of cooling rates had a consequential effect on the phase transformation characteristics of the Cu-Al-Ni-Co SMAs. Nevertheless, the transformation temperature peaks were varied in terms of location as well as heat flow. The results indicated that there was an improvement in transformation temperatures whenever ice water was used as quenching medium. It was also observed that the forward transformation temperatures were higher than the reverse transformation. It was verified that the required heat for the transformation of martensite into austenite was more than the transformation of austenite into martensite. Moreover, thermodynamic parameters, such as enthalpy and entropy, tended to decrease and increase as a result of the changes in the cooling rates of each medium. To clarify the variations of the structures and properties of Cu-Al-Ni-Co SMA quenched samples, x-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, and Vickers hardness were used.

Effect of Ta Additions on the Microstructure, Damping, and Shape Memory Behaviour of Prealloyed Cu-Al-Ni Shape Memory Alloys

The influence of Ta additions on the microstructure and properties of Cu-Al-Ni shape memory alloys was investigated in this paper. The addition of Ta significantly affects the green and porosity densities; the minimum percentage of porosity was observed with the modified prealloyed Cu-Al-Ni-2.0 wt.% Ta. The phase transformation temperatures were shifted towards the highest values after Ta was added. Based on the damping capacity results, the alloy of Cu-Al-Ni-3.0 wt.% Ta has very high internal friction with the maximum equivalent internal friction value twice as high as that of the prealloyed Cu-Al-Ni SMA. Moreover, the prealloyed Cu-Al-Ni SMAs with the addition of 2.0 wt.% Ta exhibited the highest shape recovery ratio in the first cycle (i.e., 100% recovery), and when the number of cycles is increased, this ratio tends to decrease. On the other hand, the modified alloys with 1.0 and 3.0 wt.% Ta implied a linear increment in the shape recovery ratio with increasing number of cycles. Polarization tests in NaCl solution showed that the corrosion resistance of Cu-Al-Ni-Ta SMA improved with escalating Ta concentration as shown by lower corrosion current densities, higher corrosion potential, and formation of stable passive film.

Microwave sintering effects on the microstructure and mechanical properties of Ti−51at%Ni shape memory alloys

Ti−51at%Ni shape memory alloys (SMAs) were successfully produced via a powder metallurgy and microwave sintering technique. The influence of sintering parameters on porosity reduction, microstructure, phase transformation temperatures, and mechanical properties were investigated by optical microscopy, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), compression tests, and microhardness tests. Varying the microwave temperature and holding time was found to strongly affect the density of porosity, presence of precipitates, transformation temperatures, and mechanical properties. The lowest density and smallest pore size were observed in the Ti−51at%Ni samples sintered at 900°C for 5 min or at 900°C for 30 min. The predominant martensite phases of β2 and β19′ were observed in the microstructure of Ti−51at%Ni, and their existence varied in accordance with the sintering temperature and the holding time. In the DSC thermograms, multi-transformation peaks were observed during heating, whereas a single peak was observed during cooling; these peaks correspond to the presence of the β2, R, and β19′ phases. The maximum strength and strain among the Ti−51at%Ni SMAs were 1376 MPa and 29%, respectively, for the sample sintered at 900°C for 30 min because of this sample’s minimal porosity.

The Interface Morphology of Thixo-Joined Dissimilar Steels

In many circumstances, a high quality joint of dissimilar metals can be hardly achieved by conventional welding process. In this study, a new process of joining semi-solid AISI D2 tool steel and AISI 304 stainless steel using a partial remelting method is proposed. The differences of two dissimilar steels at the test temperature on microstructural developments across the interface joint area were investigated. After Thixo-Joining, Interfacial microstructures were examined by optical microscope and scanning electron microscope (SEM) equipped with EDS (energy dispersive spectroscopy) in order to verify the interface welded area during the joining process while X-ray phase analysis was performed to identify the phase evolution and the type of carbides. The experimental results showed that at 1300°C a full penetration welded joint can also be clearly seen, whereby the base metal (D2 tool steel) is connected to the insert metal (304 stainless steel) along the bonding boundary to achieve what appears to be a perfect joining at the interfaces of both metals.

Influence of Tin Additions on the Phase-Transformation Characteristics of Mechanical Alloyed Cu-Al-Ni Shape-Memory Alloy

The influence of the addition of Sn to Cu-Al-Ni alloy as a fourth element with different percentages of 0.5, 1.0, and 1.5 wt pct on the microstructure, phase-transformation temperatures, mechanical properties, and corrosion behaviors was investigated. The modified and unmodified alloys were fabricated by mechanical alloying followed by microwave sintering. The sintered and homogenized alloys of Cu-Al-Ni-xSn shape-memory alloys had a refined particle structure with an average particle size of 40 to 50 µm associated with an improvement in the mechanical properties and corrosion resistance. With the addition of Sn, the porosity density tends to decrease, which can also lead to improvements in the properties of the modified alloys. The minimum porosity percentage was observed in the Cu-Al-Ni-1.0 wt pct Sn alloy, which resulted in enhancing the ductility, strain recovery, and corrosion resistance. Further increasing the Sn addition to 1.5 wt pct, the strength of the alloy increased because the highest volume fraction of precipitates was formed. Regarding the corrosion behavior, addition of Sn up to 1 wt pct increased the corrosion resistance of the base SMA from 2.97 to 19.20 kΩ cm2 because of formation of a protective film that contains hydrated tin oxyhydroxide, aluminum dihydroxychloride, and copper chloride on the alloy. However, further addition of Sn reduced the corrosion resistance.

Effect of Microwave Sintering Time and Homogenization Treatment on Biomedical Ti-51%Ni SMAs

Objective: The influence of microwave sintering time and Homogenization Treatment (HT) on the microstructure, density, phase composition, mechanical properties and phase transformation temperatures of biocompatible Ti-51%Ni Shape Memory Alloys (SMAs) are determined. Methods/Statistical Analysis: These alloys are fabricated at 900˚C for two different sintering times such as 5 min. and 30 min. Findings: The Field Emission Scanning Electron Microscopy (FESEM) micrographs show microstructure of needle-like morphology except for the sample which sintered at 900˚C for 30 min. without HT. Applications/Improvements: Sample synthesized at 900˚C for 30 min. without HT revealed the highest performance in terms of maximum compressive strength (1376 MPa) at 29% strain, Austenite finish temperature (Af) of 27˚C and Martensite finish temperature (Mf) of 47˚C at 18% porosity. They showed the Af temperature very close to the human body temperature, thus prospective for biomedical applications. During heating, the Differential Scanning Calorimeter (DSC) baseline shows multi-endothermic peaks, while during the cooling process there is only one exothermic peak.