Hailin Bai

Phase evolution upon ion mixing and solid‐state reaction and thermodynamic interpretation in the Ni‐Nb system

The amorphization of Ni‐Nb multilayered alloy films by xenon ion irradiation at room temperature and by high‐temperature solid‐state reaction was studied. The composition range favoring amorphization was carefully determined to be 20–85 at. % Ni by energy‐dispersive spectroscopy attached to the transmission electron microscope. A new metastable crystalline phase (MX) of hexagonal structure was formed in Ni75Nb25 and Ni70Nb30 multilayered films. Interestingly, in the Ni75Nb25 multilayered films, with increasing mixing dose an amorphous phase was first formed and then the MX‐phase was observed, while in the Ni70Nb30 multilayered films the MX phase was formed at relatively low doses and turned amorphous upon further mixing or 400 °C annealing for 2 h. Besides, annealing of the as‐deposited Ni70Nb30 multilayered films at 300 °C for half an hour also resulted in the formation of the MX phase. The thermal stability of the ion‐mixed amorphous alloys was also studied by subsequent annealing. To give semiquantitat...

Femtosecond laser-induced phase transformations in amorphous Cu77Ni6Sn10P7 alloy

In this study, the femtosecond laser-induced crystallization of CuNiSnP amorphous ribbons was investigated by utilizing an amplified Ti:sapphire laser system. X-ray diffraction and scanning electronic microscope were applied to examine the phase and morphology changes of the amorphous ribbons. Micromachining without crystallization, surface patterning, and selective crystallization were successfully achieved by changing laser parameters. Obvious crystallization occurred under the condition that the laser fluence was smaller than the ablation threshold, indicating that the structural evolution of the material depends strongly on the laser parameters. Back cooling method was used to inhibit heat accumulation; a reversible transformation between the disordered amorphous and crystalline phases can be achieved by using this method.

Sintering bonding process with Ag nanoparticle paste and joint properties in high temperature environment

Ag nanoparticle paste is prepared based on the polyol method and subsequent concentration by centrifuging. The sintering bonding process using Ag nanoparticle paste at different bonding pressures is studied. The joint strengths are increased as the bonding pressure increases from 0 MPa to 7.5 MPa. This is due to the fact that the higher assistant bonding pressure is beneficial to the growth of neck size between the adjacent particles and forms denser sintered Ag layers. The joint strength bonded under 10 MPa is lower than that bonded under 7.5 MPa, which may be due to the residue of organic component in the sintered Ag layer. The joint properties bonded with Ag nanoparticle paste in high temperature environment are evaluated by heat treatments at temperatures ranges of 200-350°C for 50 hours. The results show that the mechanical properties of joint with Ag nanoparticle paste are better than the joint with Pb95Sn5 solder after storage at high temperatures.

Free energy diagram and spontaneous vitrification of an ion mixed metastable hexagonal phase in the NiMo system

Abstract In the NiMo system, a metastable crystalline (MX) phase was formed by ion mixing and identified to have a hexagonal structure with a stoichiometry around Ni75Mo25. The MX phase transformed later into an amorphous state upon room temperature aging for 5 weeks. The observed spontaneous vitrification of the MX phase can be explained by our calculated free energy diagram, including free energy curves of the solid solution, the amorphous phase, the equilibrium compound and, for the first time, the ion mixed phase. The enthalpy of the MX phase was computed by considering the identified structural characteristics and the ferromagnetic effect contributed by nickel atoms.

Thermally induced transition from metastable Ni-Nb hexagonal phase to amorphous and its thermodynamic interpretation

Abstract In the Ni-Nb system, the free energy curves were calculated for the solid solution, amorphous phase, equilibrium compound and, for the first time, a metastable crystalline (MX) phase, which was formed by ion mixing and identified to be a hexagonal structure with a stoichiometry around Ni75Nb25. In calculating the enthalpy of the MX phase, the structural characteristics and ferromagnetic effect were considered. The constructed Ni-Nb free energy diagram can explain an unusual observation, i.e. the thermally induced vitrification of the MX phase, as well as the glass-forming ability and evolution of ion-mixed amorphous alloys on annealing.

Microstructure of diffusion-brazing repaired IN738LC superalloy with uneven surface defect gap width

ABSTRACT Slots with uneven width were cut by femtosecond laser in small plates of IN738LC superalloy to imitate service cracks. The ‘cracks’ were repaired by diffusion brazing using BNi-1a or a mixed filler alloy at 1100°C. The joint region was composed of isothermal solidification zone (ISZ), diffusion affected zone (DAZ) and precipitate zone (PZ). The compositions were different between the upper and lower ISZ due to the variation of gap width. The sample, repaired with two kinds of filler metals, had similar DAZ microstructure. PZ of mixed filler alloy bonded sample had a similar microstructure with that of BNi-1a, but less borides. The maximum gap sizes of complete isothermal solidification were almost the same for different filler alloys, followed a square root relationship with time. However, PZ of BNi-1a bonded was larger, resulting from a more base metal dissolution. The relationship between the PZ, ISZ and crack width is discussed.

Large-scale synthesis of Ag nanoparticles by polyol process for low temperature bonding application

Lead is a toxic element which usually brings harm to natural resources and human health. The use of Pb-containing solder alloys are forbidden to be employed in electronic packaging, which led to the great efforts to the investigation and implementation of lead-free bonding materials. The novel low temperature sintering bonding using metal nanoparticles is a promising technique for lead free bonding method used in electronic packaging industry. In this article, a large scale chemical reduction method to synthesize silver nanoparticles (Ag NPs) for this low temperature bonding application was put forward. The obtained Ag NPs were confirmed by XRD to be crystalline silver with a face-centered cubic (fee) structure. We observed monodisperse spherical Ag NPs with a diameter range of 35 ± 15 nm. The influence of reaction parameters on the resulting particle size and its distribution is investigated. Robust joints were formed using the Ag NPs as the bonding materials at the low bonding temperature. The shear strengths of joints reach about 49.93 MPa at low temperature of 250 °C. The results show that the Ag NPs synthesized by this large scale chemical reduction method are suitable to be used as low temperature lead bonding material in electronic packaging industry.

A study on the low temperature sintering-bonding through in-situ formation of Ag nanoparticles using Ag 2 O microparticles

Based on the rapid sintering characteristic of nanoparticles at a relatively low temperature, a novel bonding process using Ag metallo-organic nanoparticle paste has been proposed. This paste is considered as an alternative for lead-rich high temperature solder. However, further decrease of bonding temperature and pressure is required for industrial application. Additionally, the cost of Ag nanoparticle preparation is relatively high. In this paper, Ag2O powders were mixed with triethylene glycol (TEG) as a reducing agent to form a paste, which was used to bond the Ag-coated Cu bulks. The TEG shew its reducibility to Ag2O at temperature as low as 110 °C. It could be inferred that when the paste was heated above 110°C, in-situ formation of Ag nanoparticles would occur in the sintering process. With the increasing of sintering time and temperature, more and more Ag nanoparticles would be formed and sintered. The effects of sintering-bonding temperature and external bonding pressure on joint strength and microstructures were studied. The shear strength increased with sintering temperature and reached up to about 22MPa at sintering temperature of 250°C under pressure of 2MPa. In addition, the strength first increased with pressure and then began to decrease under 5MPa. Moreover, the microstructures of the joints were observed and analyzed by FE-SEM.

Diffusion brazing repair of IN738 superalloy with crack-like defect: microstructure and tensile properties at high temperatures

ABSTRACT In this study, tapered slots with an opening width about 200 μm were artificially fabricated in IN738LC superalloy to imitate service cracks. The ‘cracks’ were repaired by diffusion brazing with a Ni–Cr–Co–Al–Ta–B filler alloy at 1150°C and then heat treated at 1180°C (HT-A) and 1190°C (HT-B), respectively. A joint with uniform microstructure, chemical composition and hardness was obtained using HT-B. The mechanisms of the borides evolution during homogenisation were discussed. The tensile strength of the HT-B joint tested at 20, 600 and 800°C reached up to approximately 96, 90 and 87% of the base metal, respectively. The fracture modes of the joints tensile tested at various temperatures were discussed.

Construction of free energy diagram and phase formation by ion mixing in the NiNb system

Abstract In the NiNb system, eight amorphous alloys were formed by 200 keV xenon ion mixing at room temperature of multilayered films with various compositions covering a range of 20–85 at.% of Ni. In addition, a new metastable crystalline (MX) phase was observed and identified to be of hexagonal structure with a stoichiometry around 75 at.% of Ni. The MX phase transformed into an amorphous state upon thermal annealing at 400°C for two hours. The free energy diagram of the system was constructed by calculating the free energy curves of the solid solution, the amorphous phase, the equilibrium compound and the MX phase. In calculation of the MX phase the identified structural characteristics and the ferromagnetic effect from Ni atoms were considered. The calculated diagram can explain the amorphization range upon ion mixing as well as the unexpected phase transition from the MX to the amorphous phase upon heating.