Wei-Chun Cheng

The role carbon plays in the martensitic phase transformation of an Fe–Mn–Al alloy

Abstract We observed direct evidence that 18R martensite is induced by carbon atoms in the BCC grains of an Fe–27.0wt.%Mn–5.3wt.%Al–0.1wt.%C alloy via high-temperature quenching. A single BCC phase structure formed 18R martensite in the present study. The lowest carbon content found for the formation of 18R martensite is 0.035 wt.% in Fe–Mn–Al alloys.

Observing the massive transformation in an Fe–Mn–Al alloy

Abstract Slabs with a composition of Fe–27.6 wt.% Mn–5.3 wt.% Al–0.11 wt.% C were studied in several heat treatments. They were heated at 1573 K for 30 min in air, and then quenched into cold brine or room temperature water. The austenite phase was observed as it spread through the external area, surrounding the ferrite phase within the internal part of the specimen. According to the measurement in EPMA, there are no differences in the overall concentrations of Fe, Mn and Al between the exterior austenite and the interior ferrite phase. In the glow discharge measurement, there is almost no carbon left in either the austenite or ferrite phase due to significant decarburization during the high temperature heat treatment. After the slabs were heated at high temperatures and then furnace-cooled, almost all the ferrite transformed into austenite, a result viewed by both X-ray and light microscope studies. Judging from the phases existing in the specimens with high-temperature quenching versus those with furnace cooling, we concluded that the stable high-temperature phase at 1573 K is ferrite and the stable phase at low temperature is austenite. These results are quite consistent with the phase diagrams showing the Fe–Mn–Al ternary systems. The fact that there were no changes in the overall Fe–Mn–Al concentrations between the exterior austenite phase and the interior ferrite phase is characteristic of massive transformation. Therefore, the phase transformation for changing ferrite to austenite during high-temperature quenching is massive transformation.

Magnetic properties of Fe thin films on Ag submicrometer islands

Face-centered-cubic Ag(100) submicrometer islands on an RCA-cleaned Si(100) substrate were fabricated by molecular beam epitaxy; a 500 A Fe thin film was then grown onto Ag films at 100 °C. We experimentally demonstrate that the magnetic behavior of Fe films is strongly dependent on the thickness and morphology of the Ag underlayer. The Ag film, in order to reduce the surface free energy, forms isolated three-dimensional square islands with {111} sidewall on the Si(100) substrate. The average height, grain size and surface roughness of these Ag islands were tuned by varying the deposition thickness of the Ag film. The in-plane easy axis of the Fe film was rotated 45° while the thickness of the Ag underlayer reaches 100 A and the Ag rough surface provides a source of domain wall pinning.

Magnetic properties of ultrathin cobalt films grown on Ge(111) and Si(111) substrates

Magnetic properties of cobalt films grown on Ge(111) and Si(111) substrates were investigated. On a Ge(111) substrate, cobalt films possess an in-plane anisotropy. On a Si(111) substrate, ultrathin cobalt films (3–9 monolayers), which were deposited at 300 K, exhibited a canted easy axis. The different orientations of the easy axis of magnetizations are attributed to the different roughness of the film/substrate interfaces. Interaction between Co layers and the substrates produces intermixed compound layers that are nonmagnetic. The thickness of this compound layer is 3.8 monolayers for Co/Ge(111), and 2.1 monolayers for Co/Si(111). A series of annealing experiments showed that the onset temperature of the disappearance of the magnetism for Co films is lower on Ge than that on Si substrate. For cobalt film, this onset temperature is 300 K on Ge(111) and 575 K on Si(111).

Magnetostrictive and structural properties of FeCoGa films

Fe81−xCoxGa19 (with x ranging from 0 to 19 at. % Co) films were made by the dc magnetron sputtering method. We have studied the structural (phases, texturing, and grain size D), magnetic (saturation magnetostriction λS and coercivity HC), mechanical (Young’s modulus Ef and hardness Hf), and electrical (electrical resistivity ρ) properties of these films. The main results are as follows: (i) all the films are (110) textured; (ii) the bct phase (with twinned grains) coexists with the bcc phase only in the case of x≤3 at. % Co; (iii) λS increases steadily from 42 to 86 ppm, as x increases from 0 to 19 at. % Co; and (iv) ρ reaches the saturation limit, about 200 μΩ cm, when 19≥x≥15 at. % Co. In conclusion, we report that the Fe62Co19Ga19 film has the optimal magnetic, mechanical, and electrical properties among all the FeCoGa films measured.Fe81−xCoxGa19 (with x ranging from 0 to 19 at. % Co) films were made by the dc magnetron sputtering method. We have studied the structural (phases, texturing, and grain size D), magnetic (saturation magnetostriction λS and coercivity HC), mechanical (Young’s modulus Ef and hardness Hf), and electrical (electrical resistivity ρ) properties of these films. The main results are as follows: (i) all the films are (110) textured; (ii) the bct phase (with twinned grains) coexists with the bcc phase only in the case of x≤3 at. % Co; (iii) λS increases steadily from 42 to 86 ppm, as x increases from 0 to 19 at. % Co; and (iv) ρ reaches the saturation limit, about 200 μΩ cm, when 19≥x≥15 at. % Co. In conclusion, we report that the Fe62Co19Ga19 film has the optimal magnetic, mechanical, and electrical properties among all the FeCoGa films measured.

The fracture behaviors in an Fe /Mn /Al alloy during quenching processes

Abstract The Fe–Mn–Al alloy with a composition of Fe–23.0 wt.% Mn–7.4 wt.% Al–0.03 wt.% C is single BCC phase at 1373 K, and is dual phase with BCC and FCC phase at 1323 K. When the alloy is quenched from 1373 K into cold brine, the effect of thermal stress results in intragranular fracture of the BCC phase. While the Fe–Mn–Al alloy is quenched from the BCC+FCC dual phase region at 1323 K into cold brine, the FCC phase undergoes plastic deformation in lowering the tensile thermal stress of BCC grains, and this result minimizes the fracture behavior of BCC grains. However, when the grain size exceeds a critical value, and the maximum plastic deformation of the FCC grains fails to effectively reduce the effect of thermal stress, quenching from 1323 K leads to cracking in this type of dual phase structure. According to our analysis based on the theory of the approximate thermal stress, we can be certain that the maximum tensile stress of the specimen occurs at the quenching moment and at a location near the center of the specimen surface. This theoretical analysis is consistent with experimental results. In a dual phase structure containing both brittle and ductile phases with fine grain sizes, if the minor ductile grains can effectively link the major brittle grains, it is not likely such materials will experience fracture behavior during quenching.

Magnetic properties of ultrathin Co/Ge(111) and Co/Ge(100) films

The orientation of the magnetization and the occurrence of interfacial ferromagnetic inactive layers for ultrathin Co films grown on Ge(111) and Ge(100) surfaces have been studied using the in situ surface magneto-optic Kerr effect. On a Ge(111) substrate, cobalt films (⩽28 monolayers) with in-plane easy axis of magnetization have been observed; however, on a Ge(100) substrate, ultrathin Co films (14–16 monolayers) with canted out-of-plane easy axis of magnetization were measured. The ferromagnetic inactive layers were formed due to the intermixing of Co and Ge and lowering the Curie temperature by reducing Co film thickness. The Co–Ge compound inactive layers were 3.8 monolayers thick for Co films grown on Ge(111) and 6.2 monolayers thick for Co films deposited on Ge(100). This is attributed to the difference of the density of surface atoms on Ge(111) and Ge(100).

Step edge growth of Co nanoislands on Cu(111) surface

Step edge growth of Co nanoislands on Cu(111) surface have been investigated by scanning tunneling microscopy (STM). The cobalt atoms cluster at the upper step edges and form bilayer islands of 2nm in diameter (about nine Co atoms in width) initially during the initial stage of Co deposition. This result is in accordance with the total energy calculations within density functional theory. Besides, the size and amount of nanoislands increase with increasing coverage. The average number of Co atoms contained in one island increases with a rate of 375 atoms per monolayer (ML). The statistics data on the STM images indicate that the cobalt nanoislands preferentially grow at the upper step edge during the first stage of Co deposition, then toward terrace, and finally, the growth rate of islands in edge is almost the same as that in terrace for Co thickness above 0.78–1.42 ML.

Structure and magnetic properties of Co grown on yttria-stabilized cubic zirconia substrates

Cobalt films have been grown on yttria-stabilized cubic zirconia (YSZ) (100) and (110) substrates by molecular beam epitaxy. Both hcp(0001) and fcc(111) twin structures and fcc(110) films have been successfully fabricated on the YSZ(100) and (110), respectively. For the Co on YSZ(100) case, the Co films possess either hcp(0001) or fcc(111) crystals with in-plane 30° rotation. For the Co on YSZ(110) case, the structural relationship is YSZ(110)[100] ‖ Co(110)[1–10]. All the films display an isotropiclike magnetic anisotropy with the coercivity increasing abruptly above its martensitic transition temperature. The coercivity decreases with increasing the thickness of Co films from 100 A to 500 A; and increases as the deposited temperature above 500 °C. Co films grown on YSZ(100) are in favor of the layer by layer growth, and Co films grown on YSZ(110) are in favor of the island growth.