Mechanisms of hardening, wear and corrosion improvement of 316 L stainless steel by low energy high current pulsed electron beam surface treatment

The mechanisms of corrosion and wear improvements by low energy high current pulsed electron beam (LEHCPEB) have been investigated for an AISI 316 L steel. Selective purification followed by epitaxial growth occurred in the top surface melted layer (2-3 μm thick) that was softened by tensile stresses and, to a much lower extent, by lower efficiency of MnS precipitation hardening. Electrochemical impedance spectroscopy and potentiodynamic polarization analyses used to model the corrosion behavior revealed that, while craters initiated at MnS inclusions initially served as pitting sites, the resistance was increased by 3 orders of magnitude after sufficient number of pulses by the formation of a homogeneous covering layer. The wear resistance was effectively improved by sub-surface (over 100 μm) work hardening associated with the combine effect of the quasi-static thermal stress and the thermal stress waves. The overall results demonstrate the potential of the LEHCPEB technique for improving concomitantly the corrosion and wear performances of metallic materials.     

In situ studies of nonlinear d-spacing changes in titanium and steel alloys

We traced element-partitioning behavior by measuring d-spacing changes in situ during phase transformations of titanium and steel alloys using a hybrid in situ observation system. For titanium alloy, nonlinear changes are clearly evident during heating and cooling and are related to element partitioning and microstructural evolution. For low-carbon-low-alloy steel, nonlinear changes are evident during cooling and are related to bainite and carbon-enriched austenite formation.     

Calcium zinc manganites, Ca4Mn7Zn3O21 − δ (0.5 < δ < 2.5) with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties

Phase-pure calcium zinc manganite, Ca₄Mn₇Zn₃O_21 − δ has been synthesized by the solid-state reaction between CaMnO₃, ZnO and Mn₃O₄ at ∼ 1300 °C. Conversion of Ca₄Mn₇Zn₃O_21 − δ to Ca₄Mn₇Zn₃O_19 ± δ occurs at 1330-1350 °C which is influenced by the oxygen nonstoichiometry associated with increasing Mn³⁺/Mn⁴⁺ ratio. The Ca₄Mn₇Zn₃O_21 − δ phase is rhombohedral (space group = R3¯m) and has the β″-alumina (BA)-type structure consisting dominantly of Mn⁴⁺ (3d³) ions. The Ca₄Mn₇Zn₃O_19 ± δ is hexagonal (space group = P6₃/mmc) having the magnetoplumbite (MP)-type structure containing dominantly Mn³⁺ (3d⁴) ions. The electrical transport properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ are indicative of the activated-type small polaron hopping conductivity below 366 and 392 K respectively. The anisotropic charge transport is influenced only in a limited way by the mixed-valency of Mn prevailing in the manganite spinel blocks. Highly nonlinear current-voltage (I-V) curves with nonlinearity coefficient, α, up to 64 was achieved at the turn-on field strength E_t ∼ 110-160 V/mm. Magnetic properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ have been investigated which showed the antiferromagnetic-insulating (AFI) behavior with T_N = 103 and 80 K and with an AF-ordered effective magnetic moment, μ_eff = 3.54 and 4.28 μ_B/Mn respectively. The Ca₄Mn₇Zn₃O_19 ± δ phase showed the ferrimagnetic interaction below T_N (at ∼ 20 K).     

Micromechanical characterization of bulk composite prepared by sintering of mechanically alloyed aluminum-316 stainless steel (35 wt%) powder blend

Present study concerns deformation behavior of the ball milled and consolidated powder blend comprising 316 stainless steel and elemental Al (65 wt%), studied by the micro- and nano-indentation techniques. With an aim to examine the strain hardening behavior of the consolidated samples, nano-indentation measurements have been carried out by applying variable load at multiple spots and cyclic load at the single spot. Similar experiments have been carried out for the bulk 316-stainless steel plate to compare the results with those obtained from the consolidated samples. The consolidated samples exhibited much higher hardness values than the plate along with high elastic recovery and appreciable work hardening.     

Structural and magnetic properties of LaFe0.5Cr0.5O3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe_0.5Cr_0.5O₃ have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ_B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T_N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.     

The effect of bond coat on mechanical properties of plasma-sprayed Al2O3 and Al2O3-13 wt% TiO2 coatings on AISI 316L stainless steel

In this study, Al₂O₃ and Al₂O₃-13 wt% TiO₂ were plasma sprayed onto AISI 316L stainless-steel substrate with and without Ni-5 wt% Al as bond coat layer. The coated specimens were characterized by optical microscopy, metallography and X-ray diffraction (XRD). Bonding strength of coatings were evaluated in accordance with the ASTM C-633 method. It was observed that the dominant phase was Al₂O₃ for both coatings. It was also found that the hardness of coating with bond coat was higher than that of coating without bond coat. Metallographic studies revealed that coating with bond coating has three different regions, which are the ceramic layer (Al₂O₃ or Al₂O₃-13 wt% TiO₂), the bond coating, and matrix, which is not affected by coating. The coating performed by plasma-spray process without bond coating has two zones, the gray one indicating the ceramic layer and the white one characterizing the matrix. No delamination or spalling was observed in coatings. However, there are some pinholes in coating layer, but they are very rare. The bonding strength of coatings with bond coat was higher than that of coating without bond coat. The strength of adhesion and cohesion was determined by means of a planemeter. It was seen that percentage of cohesion strength was higher than that of adhesion strength.     

Crystal structures and properties of BiMn1−xAlxO3 with x = 0.03 and 0.1

Crystal structures of BiMn_0.97Al_0.03O₃ (I) at 300 and 470 K and BiMn_0.9Al_0.1O₃ (II) at 90 and 300 K were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed at 300 K in I and associated with orbital order, disappeared at 470 K completely for one site and partially for the second site. The Mn/Al-O distances were very close to each other in I at 470 K and in II at 90 and 300 K indicating that orbital order did not appear in II even at 90 K. Magnetic properties of I and II were investigated with specific heat, high-temperature dc magnetic susceptibility, and ac magnetic susceptibility using different driving ac and applied dc magnetic fields and different ac magnetic field frequencies. The anomaly on the specific heat associated with a magnetic transition was strongly suppressed in II compared with that of I and BiMnO₃.     

Structural study by X-ray diffraction and transmission electron microscopy of the misfit compound (SbS1−xSex)1.16(Nb1.036S2)2

In the Sb-Nb-S-Se system, a new misfit layer compound (MSL) has been synthesized and its structure was determined by combining single crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It presents a composite crystal structure formed by (SbS_1−xSe_x) slabs stacking alternately with double NbS₂ layers and both can be treated as separate monoclinic subsystems. The (SbS_1−xSe_x) slabs comprise a distorted, two-atom-thick layer with NaCl-type structure formed by an array of {SbX₅} square pyramids joined by edges (X: S, Se); the NbS₂ layers consist of {NbS₆} trigonal prisms linked through edge-sharing to form sheets, just as in the 2H-NbS₂ structure type. Both sublattices have the same lattice parameters a = 5.7672(19) Å, c = 17.618(6) Å and β = 96.18(3)°, with incommensurability occurring along the b direction: b₁ = 3.3442(13) Å for the NbS₂ subsystem and b₂ = 2.8755(13) Å for the (SbS_1−xSe_x) subsystem. The occurrence of diffuse scattering intensity streaked along c^* indicates that the (SbS_1−xSe_x) subsystem is subjected to extended defects along the stacking direction.     

Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

Double perovskites La₃Co₂NbO₉ and La₃Co₂TaO₉ have been prepared by both solid state and sol-gel synthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data. Rietveld refinements show that the crystal structure is monoclinic (P2₁/n), with different degrees of ordering of B′ and B″ cations, with octahedra tilted according to the Glazer notation a⁻b⁻c⁺. Occupancy refinements show that the solid state materials are more B-site ordered than the sol-gel ones. Magnetization measurements show that these perovskites show two magnetic contributions, one with spontaneous magnetization and other with linear behaviour with the magnetic field associated to antiferromagnetic correlations. In the samples synthesized by solid state the spontaneous magnetization is more important than those synthesized by the sol-gel and present T_C of 62 K for Nb and 72 K for Ta. On the other hand, materials prepared by sol-gel have T_C 20 K for Nb and 40 K for Ta, respectively and major presence of the antiferromagnetic contribution. The competition between these magnetic behaviours is interpreted, by a microscopic point of view, as to be due to the different degrees of Co²⁺ ions disorder on the B site of the double perovskite structure. This disorder affects the ratio between the antiferromagnetic Co²⁺-O-Co²⁺ and the ferromagnetic Co²⁺-O-M⁵⁺-O-Co²⁺ couplings proposed for the system.     

Synthesis under high-oxygen pressure, magnetic and structural characterization from neutron powder diffraction data of YGa1−xMn1+xO5 (x = 0.23): A comparison with YMn2O5

A new material of nominal stoichiometry YGaMnO₅ has been prepared in polycrystalline form from citrate precursors followed by thermal treatments under high-oxygen pressure. This compound has been characterized from neutron powder diffraction (NPD) data and magnetic measurements. For comparison, the parent compound YMn₂O₅ has also been synthesized and its crystal structure refined by NPD data. The new oxide has an actual stoichiometry YGa_1−xMn_1+xO₅ (x = 0.23), determined by NPD, showing an important cationic disorder between both metal sites; it is orthorhombic, Pbam (SG), and its crystal structure contains chains of Mn⁴⁺O₆ edge-sharing octahedra, linked together by Ga³⁺O₅ pyramids and YO₈ units. With respect to YMn₂O₅, containing axially elongated MnO₅ pyramids due to the Jahn-Teller effect of Mn³⁺ cations, the GaO₅ pyramidal units in YGa_0.77Mn_1.23O₅ are substantially flattened. This compound has a paramagnetic behaviour with two weak anomalies at about 50 K and 350 K. The magnetic structures, studied at 1.4 K and 100 K show a ferromagnetic coupling along the chains of MnO₆ octahedra.