Chaoli Ma

Formations of amorphous and quasicrystal phases in Ti–Zr–Ni–Cu alloys

Abstract The structure of melt-spun Ti45Zr35Ni20−xCux (x=0, 1, 3, 5 and 7) alloys and a cast Ti45Zr35Ni17Cu3 bulk alloy have been studied. The melt-spun alloys have different structures depending on the thickness of the melt-spun ribbons. When the thickness is 86±5 μm, icosahedral quasicrystal (I) and Laves (L) phases are formed for x=0 and 7, and a single I phase is formed for x=1, 3 and 5. When the thickness decreases to 20±2 μm, I and amorphous phases are formed for x=0 and 1, and a single amorphous phase is formed for x=3, 5 and 7. The thickness of 15±1 μm for x=0 and 1 also causes the formation of a single amorphous phase. The cast alloy also has different structures depending on the size of the bulk samples. The structure of the cast rod sample with a diameter (D) of 1 mm consists mainly of I and amorphous phases. The I phase is homogenously distributed and its particle size is less than 5 μm. I and C14 L phases are formed in the cast rod sample with D of 2 mm. The particle size of the I phase is less than 10 μm. The volume fraction (Vf) of the I phase is about 67% for D of 1 mm rod and 55% for D of 2 mm rod. Three phases of I phase, C14 L and β-(Ti,Zr) are formed in the cast rod sample with D of 3 mm. EDS measurement indicates that the I phase has an approximate composition of Ti49Zr36Ni13Cu2.

Environmental embrittlement and grain boundary segregation of boron and carbon in Ni3(Si, Ti) alloys

Abstract The effect of the addition of small amounts of boron and carbon on the environmental embrittlement of Ni 3 (Si, Ti) alloys was investigated by the room temperature tensile test. It is shown that the addition of boron to Ni 3 (Si, Ti) alloys completely suppresses the embrittlement in air and distilled water over the whole range of strain rates, and slightly suppresses the embrittlement in H 2 gas. The addition of carbon to Ni 3 (Si, Ti) alloys completely suppresses the embrittlement in air and distilled water at high strain rates, moderately suppresses the embrittlement at low strain rates and slightly suppresses the embrittlement in H 2 gas over the whole range of strain rates. The distinction between the embrittlement in air (or distilled water) and H 2 gas is due to the different decomposition kinetics into hydrogen atoms for H 2 O and H 2 molecules. The beneficial action of dopant atoms on the environmental embrittlement is attributed to boron (and carbon) segregation to grain boundaries, where they compete for site occupation and/or diffusion with hydrogen atoms, resulting in the alteration of the grain boundary to transgranular fracture mode.

Suppression of environmental embrittlement of Ni3(Si,Ti) alloys by shot peening

It has been observed that most ordered intermetallics are very susceptible to embrittlement due to hydrogen released from moisture in an air or from an aqueous solution at ambient temperatures. A recent observation by the present authors showed that tensile elongation of predeformed and then hydrogen-charged Ni{sub 3}(Si,Ti) polycrystals increases with increasing pre-plastic deformation although total amount of absorbed hydrogen content increases. It was suggested from this observation that hydrogen within lattice or trapped to dislocations is ineffective of causing the embrittlement, and conversely effective of suppressing the embrittlement of the Ni{sub 3}(Si,Ti) alloys through reducing hydrogen content enriched to the grain boundaries. Thus, this result indicates that existence of the dislocations is beneficial to reduce the hydrogen embrittlement. Therefore, it is expected that the dislocations existing in the surface layer scavenge the hydrogen atoms injected from the environment, and thereby is effective of suppressing the environmental embrittlement. In this work, the effect of shot-peening on the environmental embrittlement of the Ni{sub 3}(Si,Ti) alloys is investigated by tensile test in different atmosphere at room temperature.

The effect of pre-deformation on environmental embrittlement of Ni3(Si,Ti) alloys

Abstract The effect of pre-deformation at liquid nitrogen temperature on embrittlement of Ni 3 (Si,Ti) alloys due to hydrogen released from air is investigated, comparing the results in the specimens which were re-deformed in air and in vacuum at room temperature. As the pre-deformation increases, tensile elongation of the Ni 3 (Si,Ti) specimen re-deformed in air increased. A total amount of tensile elongation (i. e. elongation by pre-deformation + elongation by re-deformation in air) reached a value of the specimen which was deformed simply in vacuum at room temperature. Correspondingly, fracture mode changed from inter-granular to transgranular fracture. This result indicates that deformation microstructure introduced by the pre-deformation has the effect of reducing the embrittlement due to hydrogen decomposed from moisture in air, through scavenging hydrogen atoms to crystal defects such as dislocations or vacancies.

The influence of chromium addition on the environmental embrittlement of Ni3(Si,Ti) alloys at ambient temperatures

The environmental embrittlement in ordered intermetallics as well as ordinary materials is caused via some microscopic processes involving the decomposition, absorption, permeation and condensation of hydrogen atoms in the associated region, and finally by the bond breaking of the materials. Therefore, it is expected that the substitutional solutes are also capable of influencing the environmental embrittlement of Ni{sub 3}(Si,Ti) alloys by affecting a microscopic process differently than boron and carbon do. In this study, it is shown that the embrittlement of Ni{sub 3}(Si,Ti) alloys in air is suppressed by the addition of chromium at a level of one atomic percent. The embrittlement/ductilization behavior and the associated alloying effect are characterized by the tensile tests at room temperature in several environmental media and strain rates, and the fractographic observation.

Structural characterization of Cu–Ti-based bulk metallic glass by advanced electron microscopy

The atomistic structure of a high-strength Cu42.5Ti41.5Ni7.5Zr2.5Hf5Si1 bulk metallic glass prepared by copper-mould casting has been characterized by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). High-resolution TEM and high-angle annular dark-field observations revealed that crystalline nanoparticles with sizes less than 10 nm are embedded in an amorphous matrix. It was confirmed by energy-dispersive X-ray spectroscopy in combination with STEM that these nanoparticles possess higher copper content than the amorphous matrix. Nanobeam electron diffraction experiments indicated that their crystalline structure is basically face-centred cubic with ordered atomic arrangements.

The effect of trapped hydrogen on mechanical behavior of Ni3(Si,Ti) intermetallic compound

Abstract The effect of residual hydrogen on mechanical properties and fracture behavior of Ni 3 (Si,Ti) single crystals and polycrystals is investigated by tensile tests, using materials with a low level ( 3 (Si,Ti) single crystals are primarily insensitive to the hydrogen content. The tensile elongation of the Ni 3 (Si,Ti) polycrystals are markedly reduced by a high level of hydrogen content, and associated fractography shows a mainly brittle intergranular fracture pattern. However, boron-doped Ni 3 (Si,Ti) polycrystals are not embrittled by a high level of hydrogen content. The tensile elongation of pre-deformed and then hydrogen-charged Ni 3 (Si,Ti) polycrystals increases with increasing pre-deformation although absorbed hydrogen content increases. It is suggested that residual hydrogen at ∼2 mass ppm is enough to embrittle the Ni 3 (Si,Ti) alloys when the hydrogen is trapped at grain boundaries, but, ineffective when the hydrogen is distributed within the grain interior or trapped at dislocations. Also, it is suggested that boron competes for site occupation with the hydrogen, and/or directly enhances grain boundary cohesion, thereby resulting in the effect of suppressing hydrogen embrittlement.

Suppression of moisture-induced embrittlement of Ni3(Si, Ti) alloys by a surface nickel alloying layer

The effect of a surface nickel alloying layer on moisture-induced embrittlement of Ni3(Si, Ti) alloys has been investigated by tensile tests at room temperature, using nickel-deposited materials. Undeposited Ni3(Si, Ti) alloy became remarkably embrittled in air. However, nickel-deposited Ni3(Si, Ti) alloy showed a high elongation value, indicating the suppression of embrittlement caused by hydrogen decomposed from moisture in the air. When the surface nickel alloying layer consists of f c c(γ) solid solution with a high nickel concentration and good adhesion to the substrate, improvement of tensile elongation is the greatest. The results have been discussed from the chemical and structural viewpoints of the surface nickel alloying layer.