New-Jin Ho

Properties of electron beam welded SAF 2205 duplex stainless steel

Abstract SAF 2205 duplex stainless steel was welded by the electron-beam technique. Optical microscopy, scanning electron microscopy (SEM), tensile, impact and potentiodynamic tests were used to examine the microstructure, mechanical and corrosion properties of the weld materials. Metallographic observations showed that the microstructure of the weld metal was characterized by large ferrite grains with intra- and inter-granular austenite. Furthermore, they also revealed that the grain growth in the heat affected zone (HAZ) was restricted when welding with the electron beam technique. The impact tests showed that the weld materials exhibited lower impact strength than that of the parent alloy, the degradation in impact properties of the weld materials being caused by the formation of a small amount of the austenite phase within the weld metal. Potentiodynamic measurements indicated that the weld metal exhibited a lower passive current density than that of the parent alloy when exposed to 1M H 2 SO 4 acid solution.

Microstructural and mechanical characterization of laser-beam welding of a 8090 Al-Li thin sheet

In extension to a previous study on electron-beam welding (EBW) under vacuum on a 8090 thin sheet, the current paper reports the parallel results of laser-beam welding (LBW) of the same material. Autogenous “bead-on-plate” laser-beam welding was performed by a 3 kW CO2 LBW machine. The power of the input laser beam, the specimen moving speed, and the focusing condition was varied from 700 to 1300 W, 1500 to 9000 mm min−1 and 1 to 3 mm below the specimen top surface, respectively. The protection atmosphere and plasma jet were achieved by blowing either Ar or N2 gas. The effects of using different gases were evaluated in terms of weld-line appearance, fusion-zone dimension, solute evaporation, microhardness, post-weld tensile properties, as well as porosity distribution. In comparing with the EBW results, LBW on the 8090 alloy was characterized with a higher fusion-zone depth/width ratio, cooling rate and porosity amount, and a lower solute loss and post-weld tensile strain. The primary formation mechanism for porosity was thought to be related to the collapsed key-holes during LBW under Ar or N2 and the hydride-induced gas pores during EBW under vacuum.

The study of fatigue in polycrystalline copper under various strain amplitude at stage I: crack initiation and propagation

Abstract During the past two decades, several studies on the crack initiation have shown that the crack initiation at high plastic strain amplitude is mainly started at the grain boundaries(GB), and at low plastic strain amplitude crack initiation is mainly started at the persistent slip band(PSB). However, the GB is still considered to be the preferred locations for crack initiation. It has also been pointed out that crack initiation started at GB results from the interactions between grain boundaries and persistent slip bands (PSB–GB), but only few studies has been devoted to finding the starting places for the following crack propagation, which may start at either the PSB or the GB. Moreover, most of the observations on the PSB–GB are done by scanning electron microscopy (SEM) — yet, observation by transmission electron microscopy (TEM) is seldom reported. In this paper, pure copper square specimens are employed in low cycle fatigue testing under strain-controlled condition. Optical microscopy (OM), SEM and TEM are used to investigate the initiation, propagation and the accompanied dislocation structures of the cracks. The results reveal that the mechanisms of crack initiation are the same as those in earlier studies. The dislocation structures, for crack initiated at the PSB, are ladder-like dislocation wall; and for those initiated at the GB, are dislocation cell. With regard to the crack propagation, crack initiated at the GB is the majority — no matter the crack was initiated at the PSB or the GB at the beginning.

Laser surface alloying of ferritic Fe-40Cr alloy with ruthenium

Abstract An investigation is made of the laser surface alloying of an experimental ferritic Fe-40Cr alloy with a rutherium powder coating using a continuous wave CO 2 laser. Themicrostructure and corrosion behaviour of the laser surface alloying layer and the Fe-40Cr bulk allyo were examined by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, microhardness, potentiodynamic and corrosion potential measurements. The results of the microstructural examination showed that fine cellular dendrites with a ruthenium content as high as 51.84wt.% were produced by laser surface alloying. The extended solubility of ruthenium in the cellular dendrites resulted in a dramatic increase of the hardness in the laser alloying layer. Potentiodynamic and open circuit corrosion potential measurements indicated that the ruthenium-containing surface layer spontaneously passivated in hydrochloric and sulphuric acid solutions, whereas the bulk Fe—40Cr alloy remained in the active state when exposed in these reducing acid solutions.

Some aspects of the dislocation microstructures in fatigued FeCrAl alloys

Abstract Fe25Cr4Al and Fe25Cr2Al (wt.%) alloys were cyclically deformed in air at various total strain amplitudes. Four different structures were observed, in order of decreasing strain: ordinary dislocation cell structures in both alloys at about 1% total strain and above; and “maze” or “labyrinth” structures at intermediate strains, again for both alloys; however, at the lowest strain rates, a typical loop patch structure was found in the 2% aluminium alloy but a precursor to the maze (labyrinth) structure was found in the 4% alloy. That precursor structure seems to be the same as that observed by Mori et al. It would thus appear that the difference between the two alloys lies in the bypassing of the loop-patch structure in the 4% alloy, with, instead, direct construction of dipolar or multipolar walls. Thus the softening observed in the 2% alloy is due to the formation of the loop patches.

Microstructures of laser-treated Al2O3-ZrO2-CeO2 composites

Abstract This work uses room temperature X-ray diffraction and electron microscopy to investigate the effect of CO 2 laser treatments (800–2000 W, 10–15 s) on phase transformations and microstructural modification of sintered (1600 °C, 4 h) disks, A x Z y C z (where A, Z and C denote Al 2 O 3 , ZrO 2 and CeO 2 respectively, and the subscripts represent molar ratios). Laser treatments under these conditions caused melting and more or less sublimation of all the samples; subsequent solidification and condension (predominant for CeO 2 -rich composition) resulted in different microstructures between the samples. Dendritic and cellular domain structures due both to eutectic growth of α-Al 2 O 3 and ZrO 2 -CeO 2 solid solution were found in specimens A 30 C 63 C 7 and A 70 Z 27 C 3 respectively. In the CeO 2 -rich specimen A 40 Z 12 C 48 a drastic effect of condensation caused dendritic clusters of CeO 2 cubes (fluorite structure with minor amount of ZrO 2 and Al 2 O 3 in solid solution) which overlaid on an Al 2 O 3 -rich matrix, predominantly CeAlO 3 . Condensation through a rapidly solidifying liquid caused incorporation of Al 2 O 3 in the fluorite structure and hence deviation from the octahedral shape predicted by the periodic bond chain model. Solute (Al 2 O 3 ) trapping also suppressed the martensitic transformation of tetragonal (t-) to monoclinic (m-) ZrO 2 as manifested by the dendritic t-ZrO 2 cooled from the Al 2 O 3 -ZrO 2 eutectic. Laser treatment increased the Ce 2+ Ce 4+ ratio and hence darkened the samples.

Enhanced retention characteristic of NiSi2/SiNx compound nanocrystal memory

The NiSi2/SiNx compound nanocrystals (CNCs) were fabricated to integrate the compound tunnel barrier into nanocrystal memory, with the inclusion of nitride traps. The analysis of high resolution transmission electron microscopy and x-ray photoelectron spectroscopy reveal that the nanocrystal is mainly composed of NiSi2 and silicon nitride with small size of 4–5 nm and high density of ∼1×1012 cm−2. The charge storage characteristics of the memory capacitor based on NiSi2/SiNx CNCs were investigated by capacitance-voltage measurement and the enhanced retention characteristics, which remain 71.7% (∼1.9 V) in 104 s, are clarified to be due to the compound tunnel barrier and traps in nitride.

Above room-temperature ferromagnetism of Mn delta-doped GaN nanorods

One-dimensional nitride based diluted magnetic semiconductors were grown by plasma-assisted molecular beam epitaxy. Delta-doping technique was adopted to dope GaN nanorods with Mn. The structural and magnetic properties were investigated. The GaMnN nanorods with a single crystalline structure and with Ga sites substituted by Mn atoms were verified by high-resolution x-ray diffraction and Raman scattering, respectively. Secondary phases were not observed by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. In addition, the magnetic hysteresis curves show that the Mn delta-doped GaN nanorods are ferromagnetic above room temperature. The magnetization with magnetic field perpendicular to GaN c-axis saturates easier than the one with field parallel to GaN c-axis.

The model of crack propagation in polycrystalline copper at various propagating rates

Abstract In this work, the fracture surface of the crack paths in specimens of pure copper have been studied by secondary electron imaging and the dislocation structures in front of crack tip were observed by backscattered electron imaging. It was observed that the propagation of the cracks is mainly transgranular, but the behavior models change in accordance with the variation of dislocation structures located at the front of the crack tip. With a 10 −6 mm/cycle rate of cracking, the dislocation structures are in full development with a vast range of area. Therefore, a model for the propagation of cracks is transgranular along the sides of grain boundaries (GB). However, with a propagation rate of 10 −7 mm/cycle, the dislocation structures evolve incompletely, and the range of cell or ladder-like walls of persistent slip band is limited to a small area. The sequence of crack propagation is initiation, propagation in strain localization in front of the crack tip, and then coalescence with the crack tip. The crack propagation is transgranular, but the side of the GB is still the preferred path. A model is proposed to explain these behaviors. The major factors of the model are the evolution of the dislocation structures at the crack tips, and the interactions between the grain boundaries and persistent slip bands.

Microstructural characteristics and creep rupture behavior of electron beam and laser welded AISI 316L stainless steel

Abstract AISI 316L stainless steel was welded by the electron beam (EB) and laser techniques. Microstructural characteristics, hardness profile, creep rupture properties and creep damage of the welds were investigated. Fully austenitic microstructure was obtained in the two welds. The solidification structure of the welds consisted of the cellular and equiaxed dendrites. The creep rupture lives of the two welds were almost the same, and they were reduced by a factor of about two compared to the base metal. Moreover, the rupture elongation of the welds was lower than that of the base metal. Creep damage was observed in the “parting” region of the welds and in the heat-affected zone (HAZ), respectively. Final creep fracture occurred in the “parting” region of the welds.