K. Oh-ishi

Broadening the applications of the atom probe technique by ultraviolet femtosecond laser.

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.

A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze

As-cast NiAl bronze (NAB) was subjected to friction stir processing (FSP). Orientation imaging microscopy (OIM) methods were used to obtain microtexture data in the stir zone (SZ) and along its periphery. At selected SZ locations, orientation data were obtained by convergent beam electron diffraction (CBED) methods in transmission electron microscopy (TEM). Random α phase textures were apparent in the SZ. The α grains tended to be equiaxed, exhibited annealing twins, and were refined to 1 to 2 µm at the edge of the SZ. The population of subgrain boundaries in α phase grains was highest near the plate surface in contact with the tool and decreased with depth in the SZ, reflecting deformation by the tool shoulder after the passage of the tool pin. Distinct shear texture components were apparent in the thermomechanically affected zone (TMAZ) outside of and along the periphery of the SZ. A texture gradient from the TMAZ into the SZ was apparent and was steeper on the advancing side and under the SZ center than on the retreating side. The apparent shear plane tended to align with the local interface between the SZ and TMAZ, while the shear direction tended to align with the FSP traversing direction. In this material, the SZ-TMAZ interface is a distinct boundary between recrystallized and deformed regions and the α-phase grain refinement reflects dynamic recrystallization and, in locations near the SZ-TMAZ interface, particle-stimulated nucleation (PSN) at undissolved Fe3Al particles.

Precipitation of nanosized nitrides in plasma nitrided Fe–M (M = Al, Cr, Ti, V) alloys

Abstract Nanosized alloy nitrides or alloying element (M)–Nitrogen(N) cluster formed in plasma nitriding of Fe–M binary alloys were investigated by means of high resolution TEM. Specimen surface of the Fe–Cr alloys was hardened by the formation of disk shaped CrN of NaCl structure, which is ∼2 nm thick and 10 nm in diameter. In nitrided Fe–Al specimen, two kinds of AlN nitrides of metastable NaCl type and stable wurtzite type were formed near the specimen surface. Both of these were much larger than CrN. Contrasting to these specimens, in nitrided Fe–Ti and Fe–V alloys, high density M–N clusters of few nanometres diameter, many of which were monolayered lying on {001}α, were formed, leading to larger hardness increase than that in Fe–Cr and Fe–Al alloys. The formation of M–N clusters in nitriding is explained by the thermodynamical analysis of phase separation between M/N poor and M/N rich bcc phases.

Microalloying Effect on the Precipitation Processes of Mg-Ca Alloys

The Mg-Ca binary alloys in the Mg-Mg2Ca two-phase region show little precipitation hardening by aging. However, the Mg-Ca alloys microalloyed with Al and Zn result in notable age hardening because of the formation of metastable, internally ordered, plate-like Guinier–Preston (GP) zones on the basal plane. To enhance the age-hardening response, we explored microalloying elements that can alter the habit plane from basal to prismatic. We found that an indium addition causes the homogeneous precipitation of thin plates on prismatic planes, resulting in a pronounced age-hardening response. Based on transmission electron microscopy and atom probe analysis results, we discuss the structure of the GP zones and the possible origin of the habit plane alternation.

Strengthening Mechanisms in NiAl Bronze: Hot Deformation by Rolling and Friction-Stir Processing

Microstructures produced by isothermal hot rolling of a NiAl bronze material were evaluated by quantitative microscopy methods and parameters describing the contributions of precipitate dispersions, grain size, solute content, and dislocation density to the yield strengths of the individual constituents of microstructure were determined. Models for the strengths of the individual constituents were combined to predict the temperature dependence of the yield strength as a function of hot rolling temperature, and the prediction was found to be in good agreement with measured yield strengths. The models were applied to microstructures in a stir zone produced by multipass friction-stir processing (FSP) and, again, found to predict measured yield strengths with high accuracy. Such models may aid in assessing the role of microstructure gradients produced during FSP and other processes.

Effect of laser power and specimen temperature on atom probe analyses of magnesium alloys

The influence of laser power, wave length, and specimen temperature on laser assisted atom probe analyses for Mg alloys was investigated. Higher laser power and lower specimen temperature led to improved mass and spatial resolutions. Background noise and mass resolutions were degraded with lower laser power and higher specimen temperature. By adjusting the conditions for laser assisted atom probe analyses, atom probe results with atomic layer resolutions were obtained from all the Mg alloys so far investigated. Laser assisted atom probe investigations revealed detailed chemical information on Guinier-Preston zones in Mg alloys.

Laser-assisted Atom Probe Analysis of Bulk Insulating Ceramics

A Yb:KGW femtosecond laser (400 fs) with 3rd and 4th harmonic generators (343 and 258 nm) was adopted to a locally built 3DAP instrument to assist field evaporation from ceramics tips that were bonded on tungsten wire. Using this setting, we have demonstrated that quantitative atom probe tomography is possible from Y2O3-ZrO2-MgAl2O4, (Ce,Y)O2, Li(Co,Ni,Mg,Al)O2 bulk ceramics, which are all insulators.

Effect of Mn Addition on Creep Property in Mg-2Al-2Ca Systems

Planar Al2Ca phase forms on the basal plane in the as-cast AX22 and AXM220 alloys. In the AXM220 alloy, Mn is detected in the Al2Ca phase. The number density of the Al2Ca phase does not alter, irrespective of the creep deformation and Mn addition. Fine and planar precipitates appear during the creep deformation. The number density increases by the addition of Mn. The size of the precipitate is reduced by the Mn addition. The Mn addition can improve the creep property.

Quantitative Atom Probe Tomographys of Magnesium Alloys

Research interest in age hardenable magnesium alloys has been recently revived because of increasing demand of high strength wrought magnesium alloys for weight reduction of transportation vehicles. The age-hardening response of Mg alloys can be promoted by the trace additions of ternary elements and the two-step aging (e.g. [1]), therefore the demand of the quantitative analysis of alloying elements and their clusters is increasing. In the Mg-Zn and Mg-Zn-Al systems, the formation of Guinier-Preston (G.P.) zones was confirmed, but their detailed chemistry has not been known. Atom probe tomography is an ideal experimental technique to detect solute clusters in alloys, but surprisingly little work has been done on magnesium alloys. This is because the field ion microscopy of magnesium alloys has been thought to be difficult, because of the low evaporation field and the difficulty of oxide-free specimen preparation. We succeeded 3DAP analysis of Mg-RE-Zn alloy [2,3] using voltage-pulse atom probe in 2002, and since then the technique has been applied to Mg-Ca-Zn[4], Mg-Gd-Zn[5,6], Mg-Zn-Al[7], Mg-Zn-Ag-Ca[8] with voltage pulse or laser assisted 3DAP. In this work, we discuss the quantitative analysis of various magnesium alloys using voltage pulse and laser pulse 3D atom probes.

Erratum to: Plasma Nitriding Behavior of Fe-C-M (M = Al, Cr, Mn, Si) Ternary Martensitic Steels

YUSAKU TOMIO, formerly Graduate Student with the Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japan, is now Researcher with the Amagasaki Research and Development Center, Nippon Steel & Sumitomo Metal Corporation, Amagasaki 660-0891, Japan. SHIGEKI KITSUYA, formerly with the Graduate School of Engineering, Tohoku University, is now Researcher with the Steel Research Laboratory, JFE Steel Corporation, Kurashiki 712-8511, Japan. KEIICHIRO OH-ISHI, formerly Post Doctoral Research Associate with the National Institute for Materials Science, Tsukuba 305-0047, Japan, is now with the Nagaoka University of Technology, Nagaoka 940-2188, Japan. KAZUHIRO HONO, NIMS Fellow, Director of the Magnetic Materials Unit, with National Institute for Materials Science, Tsukuba 305-0047, Japan. GORO MIYAMOTO, Associate Professor and TADASHI FURUHARA Professor, Deputy Director, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: furuhara@imr.tohoku.ac.jp The online version of the original article can be found under doi: 10.1007/s11661-013-1991-5. Article published online November 19, 2013