Hyun-Goo Kim

Formation and thermal stability of nanocrystalline CuTiNi prepared by mechanical alloying

Abstract The formation and thermal stability of nanocrystalline Cu70Ti20Ni10 powders by mechanical alloying have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The composition (at.%) of the sample has been examined by energy dispersive X-ray analysis (EDX) attached to SEM. After milling, the material was found to have a disordered f.c.c. structure, with nanocrystalline grain sizes around 4.7 nm determined from the f.c.c. (111) diffraction peak. The DSC signal decreases monotonically with isothermal transformation owing to grain growth. The crystal grains do not grow by heating until the intermetallic TiCu3 and NiTi phases nucleate at 450°C.

Glass transition behaviour of Zr- and Ti-based binary amorphous alloys

Abstract A study has been made to investigate the effects of free-volume fraction, steady state viscous flow temperature and viscosity on the glass transition and viscous flow behaviour of Cu x Zr 100− x ( x = 40, 50 and 60), Ni x Zr 100− x ( x = 40 and 50) and Cu x Ti 100− x ( x = 40 and 50) amorphous alloys. It was observed that the displacement at T x increases in the order Cu 40 Zr 60 > Ni 40 Zr 60 > Cu 40 Ti 60 . We found that this order can be explained by the free-volume fraction and maximum displacement rate at T vs . The relationship between viscosity and free-volume fraction below T x is consistent with Doolittles empirical formula for the CuZr, CuTi and NiZr systems. It is presumed that the kinetics of free-volume production and annihilation are involved in the thermal stability and glass transition temperature.

Formation of nanocrystalline Cu70Ti20Ni10 powders by mechanical alloying

Abstract The formation and thermal stability of nanocrystalline Cu 70 Ti 20 Ni 10 powders by mechanical alloying has been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). After milling, the material has a disordered fcc structure, with nanocrystalline grain sizes around 4.7 nm determined from the fcc (111) diffraction peak. The DSC signal monotonically decreases with isothermal transformation owing to grain growth. The crystal grains do not grow by heating until the intermetallic TiCu 3 and NiTi phases nucleate at 450 °C.

Metastable bcc Ni produced by rod milling and chemical leaching

Abstract Nanocrystalline Al 60 Ni 40 and Ni have been obtained by rod milling Al and Ni powder mixtures and chemical leaching Al atoms from the rod-milled Al 60 Ni 40 , respectively. The rod-milled alloy powders retained their bcc structure after being treated at room temperature and at 85 °C with a 25–30 wt.% KOH solution. The leached powders are very active and easily explode when they come into contact with air. The leached powders were transformed to a ferromagnetic fcc phase at high temperature. On cooling of the specimen from 600 °C, spontaneous magnetization M sharply increased at about 350 °C, indicating that the bcc phase was transformed to an fcc phase. It has been confirmed that the leaching temperature and annealing temperature and KOH concentration have a considerable effect on structural and magnetic properties.

Structural relaxation and crystallization of amorphous Ti60Cu30Ni10 alloy

Abstract Structural relaxation and crystallization have been followed in a Ti 60 Cu 30 Ni 10 metallic glass by a combination of experimental techniques. At a frequency of 0.1 Hz, the internal friction shows two distinct peaks at 662.5 ± 1.6 and 722.2 ± 1.7 K, and the positions of the peaks correspond approximately to the peak temperatures observed in a differential scanning calorimetry measurement. The origin of the internal friction peak for Ti 60 Cu 30 Ni 10 metallic glass is related to the amorphous-crystalline transition. The onset temperature for viscous flow occurs at a temperature close to that at which the internal friction starts to increase. It is thought that structural relaxation in the low-temperature range is strongly related to the viscous-flow-induced internal friction for this alloy system.

Viscous-flow behavior of amorphous Co69Fe4.5Nb1.5Si10B15 alloys

Abstract The viscous-flow behavior of amorphous Co 69 Fe 4.5 Nb 1.5 Si 10 B 15 alloys has been investigated. Thermomechanical analysis experiments showed effective viscosity decreasing monotonically with increasing temperature until steady-state viscous-flow temperature ( T vs ), where the effective viscosity of the ribbon sample showed a minimum but increased after passing through that point. Annealing-induced changes of the free-volume fraction decreased with increasing temperature. We found that the effective linear expansion coefficient and the free-volume fraction are sensitive to annealing, and the secondary working point in vicinity of the T vs was confirmed by experiment. We confirmed that relative displacement under tensile stress depends strongly upon the state of amorphicity of the sample. We think that applied stress level and the values of the T vs temperature at each stress level are very useful information for work with consolidated powders.

Formation of a non-equilibrium nanocrystalline phase by chemical leaching of rod-milled Al0.6(Fe50Cu50) alloy

We report the formation of a non-equilibrium nanocrystalline phase by chemical leaching of Al from rod-milled alloy powders. X-ray diffractometry, transmission electron microscopy, differential scanning calorimetry, scanning electron microscopy, and vibrating sample magnetometry were used to characterize the as-milled and leached specimens. After 500 h of milling, only the broad peaks of a nano bcc crystalline phase were detected in the XRD patterns. At the same time, the crystallite size, saturation magnetization and coercivity reached values of 8.8 nm, 8.42 emu/g and 142.7 Oe, respectively. The Al content of the specimens was reduced to 7.43 wt.% after leaching at room temperature in a 25% KOH (w/w) solution. Despite this, the nanocrystalline bcc phase was retained. After annealing at 350 and 600°C for 1 h, this phase transformed into a bcc α-Fe and an fcc Cu-rich phase, accompanied by changes in magnetic properties. On cooling the specimen from 850°C, the magnetization sharply increased at about 745 and 540°C, indicating that the fcc phase was ferromagnetic.

Influences of the Eu Concentration and the Milling Time on Photoluminescence Properties of Y2O3-H3BO3:Eu3+ Powders Prepared by Mechanical Alloying

Y2O3–H3BO3:Eu3+ powders are synthesized using a mechanical alloying method, and their photoluminescence (PL) properties are investigated through luminescence spectrophotometry. For samples milled for 300 min, some Y2O3 peaks ([222], [440], and [622]) and amorphous formations are observed. The 300-min-milled mixture annealed at 800°C for 1 h with Eu = 8 mol% has the strongest PL intensity at every temperature increase of 100°C (increasing from 700 to 1200°C in 100°C increments). PL peaks of the powder mixture, as excited by a xenon discharge lamp (20 kW) at 240 nm, are detected at approximately 592 nm (orange light, 5Do → 7F1), 613 nm, 628 nm (red light, 5Do → 7F2), and 650 nm. The PL intensity of powder mixtures milled for 120 min is generally lower than that of powder mixtures milled for 300 min under the same conditions. PL peaks due to YBO3 and Y2O3 are observed for 300-min-milled Y2O3–H3BO3 with Eu = 8 mol% after annealing at 800°C for 1 h.

Electrochemical Corrosion Behaviors of the Amorphous Zr65Al8Ni5Cu22 Alloy

AbstractThis study was undertake n to m easure the e le ctroche m ical corrosion of am orphous Z r 65 Al 8 Ni 15 Cu 12 (a%)t. alol y ribbo n unde r various conditions, including 0.4 m M HCl solution, and for various value s of the pH and the im m e rsion tim e . The corrosion pote ntials(E co rr ) for the am orphous Z r 65 Al 8 Ni 15 Cu 12 alloy in 4.0 m M HC l edcrease d hwt i incre asign te m eatupe rr;the corrosion current density(I co rr ) incre ase d with incre asing te m pe rature in g e neral. The polarization re sistance (R p ) wa s inverse ly proportional to the corrosion rate . While pH=7, 9, 11 was not as sensitive as pH=3, 5, pH=3 was m ore sensitive for am orphous Z r 65 Al 8 Ni 15 Cu 12 alloy than othe r pHs spe cially. The chang e of m ass in the 70 m M H 2 SO 4 solutoi n w tih im m e rsion tim e was the g re ate st in the first 100 h.Key wods r : Am orphous Z r