On the Prospects of Using Nanoindentation and Wear Test to Study the Mechanical Behavior of Fe-Based Metallic Glass Coating Reinforced by B4C Nanoparticles

In this study, Fe-based metallic glass was served as the matrix in which various ratios of hard B₄C nanoparticles as reinforcing agents were prepared using a high-energy mechanical milling. The feedstock nanocomposite powders were transferred to the coatings using a high-velocity oxygen fuel process. The results showed that the microstructure of the nanocomposite coating was divided into two regions, namely a full amorphous phase region and homogeneous dispersion of B₄C nanoparticles with a scale of 10 to 50 nm in a residual amorphous matrix. As the B₄C content is increased, the hardness of the composite coatings is increased too, but the fracture toughness begins to be decreased at the B₄C content higher than 20 vol pct. The optimal mechanical properties are obtained with 15 vol pct B₄C due to the suitable content and uniform distribution of nanoparticles. The addition of 15 vol pct B₄C to the Fe-based metallic glass matrix reduced the friction coefficient from 0.49 to 0.28. The average specific wear rate of the nanocomposite coating (0.48 × 10⁻⁵ mm³ Nm⁻¹) was much less than that for the single-phase amorphous coating (1.23 × 10⁻⁵ mm³Nm⁻¹). Consequently, the changes in wear resistance between both coatings were attributed to the changes in the brittle to ductile transition by adding B₄C reinforcing nanoparticles.

     

Mechanism and Kinetics of Thermal Decomposition of MgCl<Subscript>2</Subscript> × 6H<Subscript>2</Subscript>O

The reaction mechanism and kinetic behavior of thermal decomposition of MgCl₂ × 6H₂O were studied by thermal gravimetric analysis. The results showed that the thermal decomposition process of MgCl₂ × 6H₂O could be divided into six stages. In the first two stages, four crystalline waters were lost. The dehydration and hydrolysis coexisted during the third and fourth stages. The fifth stage corresponded to the evaporation of 0.3 crystalline waters, and one molecular hydrogen chloride was eliminated in the last stage. The kinetic analysis of the thermal decomposition process was performed using the Doyle, Coats–Redfern, and Malek methods. The results suggested that the mechanisms of six stages were two-dimensional phase boundary mechanism, three-dimensional phase boundary mechanism, nucleation and nuclei growth mechanism (Avrami–Erofeev equation n = 3), two-dimensional phase boundary mechanism, three-dimensional diffusion mechanism (cylinder and G-B equation), and nucleation and nuclei growth mechanism (Avrami–Erofeev equation n = 1), respectively. The apparent active energies of six stages were 66.8 kJ × mol⁻¹, 138.0 kJ × mol⁻¹, 77.2 kJ × mol⁻¹, 135.6 kJ × mol⁻¹, 77.4 kJ × mol⁻¹, and 92.2 kJ × mol⁻¹, respectively. The frequency factors were 3.6 × 10⁹ s⁻¹, 8.8 × 10¹⁷ s⁻¹, 4.6 × 10⁹ s⁻¹, 3.0 × 10¹⁴ s⁻¹, 78.6 s⁻¹, and 1.2 × 10³ s⁻¹, respectively.     

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.     

Microstructural Evolution and Mechanical Properties of Nanointermetallic Phase Dispersed Al<Subscript>65</Subscript>Cu<Subscript>20</Subscript>Ti<Subscript>15</Subscript> Amorphous Matrix Composite Synthesized by Mechanical Alloying and Hot Isostatic Pressing

The structure and mechanical properties of nanocrystalline intermetallic phase dispersed amorphous matrix composite prepared by hot isostatic pressing (HIP) of mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous powder in the temperature range 573 K to 873 K (300 °C to 600 °C) with 1.2 GPa pressure were studied. Phase identification by X-ray diffraction (XRD) and microstructural investigation by transmission electron microscopy confirmed that sintering in this temperature range led to partial crystallization of the amorphous powder. The microstructures of the consolidated composites were found to have nanocrystalline intermetallic precipitates of Al₅CuTi₂, Al₃Ti, AlCu, Al₂Cu, and Al₄Cu₉ dispersed in amorphous matrix. An optimum combination of density (3.73 Mg/m³), hardness (8.96 GPa), compressive strength (1650 MPa), shear strength (850 MPa), and Young’s modulus (182 GPa) were obtained in the composite hot isostatically pressed (“hipped”) at 773 K (500 °C). Furthermore, these results were compared with those from earlier studies based on conventional sintering (CCS), high pressure sintering (HPS), and pulse plasma sintering (PPS). HIP appears to be the most preferred process for achieving an optimum combination of density and mechanical properties in amorphous-nanocrystalline intermetallic composites at temperatures ≤773 K (500 °C), while HPS is most suited for bulk amorphous alloys. Both density and volume fraction of intermetallic dispersoids were found to influence the mechanical properties of the composites.     

Effect of Strain Rate on Evolution of the Deformation Microstructure and Texture in Polycrystalline Copper and Nickel

The evolution of crystallographic texture in polycrystalline copper and nickel has been studied. The deformation texture evolution in these two materials over seven orders of magnitude of strain rate from 3 × 10⁻⁴ to ~2.0 × 10⁺³ s⁻¹ show little dependence on the stacking fault energy (SFE) and the amount of deformation. Higher strain rate deformation in nickel leads to weaker á 101 ñ \left\langle {101} \right\rangle texture because of extensive microband formation and grain fragmentation. This behavior, in turn, causes less plastic spin and hence retards texture evolution. Copper maintains the stable end á 101 ñ \left\langle {101} \right\rangle component over large strain rates (from 3 × 10⁻⁴ to 10⁺² s⁻¹) because of its higher strain-hardening rate that resists formation of deformation heterogeneities. At higher strain rates of the order of 2 × 10⁺³ s⁻¹, the adiabatic temperature rise assists in continuous dynamic recrystallization that leads to an increase in the volume fraction of the á 101 ñ \left\langle {101} \right\rangle component. Thus, strain-hardening behavior plays a significant role in the texture evolution of face-centered cubic materials. In addition, factors governing the onset of restoration mechanisms like purity and melting point govern texture evolution at high strain rates. SFE may play a secondary role by governing the propensity of cross slip that in turn helps in the activation of restoration processes.     

Roentgenographic Determination of Residual Stresses in Carbonitrided Layers

The aim of the current investigation is to examine the influence of carbonitriding in low-temperature plasma on forming macroresidual stresses in the surface layer of the materials. Particular modes of ion carbonitriding are considered in which layers of different depth and different surface microhardness are obtained. The residual stresses in the α-Fe in carbonitride layers are determined by the method of “sin²Ψ.” The results show that at different modes of ion carbonitriding, residual macrostresses are obtained that have different values and depend on the qualitative characteristics of the formed carbonitrided layers.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

Fatigue damage accumulation in nickel modified by ion beam surface microalloying

The formation, distribution, and surface morphology of persistent slip band (PSB) structures have been studied in polycrystalline nickel with ion beam microalloyed Ni-Al surface layers. Both supersaturated solid solutions of Al in Ni andγ-γ′ dual-phase structures were formed on surfaces of low cycle fatigue specimens by ion beam mixing vapor-deposited Ni and Al layers, using 1 × 10¹⁶ ions/cm² of Kr₊ at 0.5 MeV. When cycled to saturation at constant plastic strain ranges between 6 × 10^-5 and 7 × 10^-3, ion beam modified specimens showed a retardation in the appearance of PSB's at the surface, and a reduction in their numbers and intensities at cyclic saturation. Slip bands that eventually emerged at ion beam modified surfaces displayed morphological features which differed sharply from the PSB notch-peak topographies usually found in fatigued fcc metals. Further, it was found that subsurface strain localization occurred in the presence of the modified surface layer, generating PSB's in the bulk which extended to the underside of the layer, but did not penetrate it. This behavior may be understood in terms of the resistance of the surface layer to plastic deformation, and the localized stresses produced in the surface film as a result of subsurface strain localization. Formerly with The University of Michigan at Ann Arbor     

Fatigue crack growth behavior of 2124/SiC/10p functionally graded materials

Powder metallurgy processing involving cold pressing and hot extrusion has been used to fabricate bulk functionally graded materials (FGMs) based on the 2124/SiC/10p composite system. Two forms of single-core bulk FGMs with circular cross section were fabricated. One form (designated 10SiC-2124) had a central core of unreinforced Al-2124 alloy that was surrounded by a 2124/SiC/10p reinforced surface layer: the other (designated 2124-10SiC) had a composite core and an alloy surface layer. These forms enabled the effect of the radial graded core on fatigue to be investigated with fatigue crack propagation from either (1) a ductile core to a more brittle region or (2) a brittle core to a ductile region of the FGM. The fatigue crack growth rate was measured using a constant applied stress intensity factor range (δK=7 MPa ) technique. Two main fatigue crack growth rates were distinguished corresponding to growth in the core and in the surface layer. The results show that FGMs may exhibit good fatigue crack propagation resistance. For example, when the crack propagated from the brittle core to the tough surface layer, the average fatigue crack growth rate in the Al-2124 core (3.9×10⁻⁶ mm/cycle) was significantly lower than for the Al-2124 alloy (1.5×10⁻⁵ mm/cycle) at a similar δK value (7 MPa ), due to the highly tortuous crack path in the 2124/SiC/10p brittle layer. The 2124/SiC/10p brittle layer had a lower fatigue crack growth rate (6.6×10⁻⁶ mm/cycle) than the 2124/SiC/10p conventional composite (7.5×10⁻⁶ mm/cycle) because of the compressive residual stresses in the surface layer. Thus, FGMs could be more acceptable for critical applications than their conventional composite counterparts.     

Magnetic property recovery in Nd-Fe-B bonded magnet wastes with chemical reaction and physical dissolution

The main difficulty for the recovery of Nd-Fe-B bonded magnet wastes is how to completely remove the epoxy resins. In this study, chemical reaction and physical dissolution were combined to remove the epoxy resins by adding ammonia-water and mixed organic solvents. Ammonia-water can react with the epoxy functional group of epoxy resin to generate polyols. Mixed organic solvents of alcohol, dimethyl formamide (DMF), and tetrahydrofuran (THF) can dissolve the generated polyols and residual epoxy resins. Under the optimum processing conditions, the epoxy resins in the waste magnetic powders are substantially removed. The oxygen and carbon contents in the recycled magnetic powder are reduced from 13500 × 10⁻⁶ to 1600 × 10⁻⁶ and from 19500 × 10⁻⁶ to 2100 × 10⁻⁶ with the reduction ratio of 88.1% and 89.2%, respectively. The recycled magnetic powder presents improved magnetic properties with M_s of 1.306 × 10⁻¹ A∙m²/g, M_r of 0.926 × 10⁻¹ A∙m²/g, H_cj of 1.170 T, and (BH)_max of 125.732 kJ/m³, which reach 99.8%, 99.4%, 95.9%, and 96.9% of the original magnetic powders, respectively.     

Atomic-Scale Study of Plastic-Yield Criterion in Nanocrystalline Cu at High Strain Rates

Large-scale molecular dynamics (MD) simulations are used to understand the macroscopic yield behavior of nanocrystalline Cu with an average grain size of 6 nm at high strain rates. The MD simulations at strain rates varying from 10⁹ s⁻¹ to 8 × 10⁹ s⁻¹ suggest an asymmetry in the flow stress values in tension and compression, with the nanocrystalline metal being stronger in compression than in tension. The tension-compression strength asymmetry is very small at 10⁹ s⁻¹, but increases with increasing strain rate. The calculated yield stresses and flow stresses under combined biaxial loading conditions (X-Y) gives a locus of points that can be described with a traditional ellipse. An asymmetry parameter is introduced that allows for the incorporation of the small tension-compression asymmetry. The biaxial yield surface (X-Y) is calculated for different values of stress in the Z direction, the superposition of which gives a full three-dimensional (3-D) yield surface. The 3-D yield surface shows a cylinder that is symmetric around the hydrostatic axis. These results suggest that a von Mises-type yield criterion can be used to understand the macroscopic deformation behavior of nanocrystalline Cu with a grain size in the inverse Hall–Petch regime at high strain rates.     

Structures at Glassy,Supercooled Liquid,and Liquid States in La-Based Bulk Metallic Glasses

Local atomic structures at glassy, supercooled liquid, and liquid states for La-based bulk metallic glasses (BMGs) have been investigated by in-situ high-temperature X-ray diffraction. It is found that the coordination number of about 15.1 ± 0.1 for the La₆₂Al₁₄Cu_11.7Ag_2.3Ni₅Co₅ alloy does not depend on temperature up to liquid temperature, while it decreases slightly with temperature for the La₆₂Al₁₄Cu₂₄ and La₆₂Al₁₄Cu₂₀Ag₄ alloys. The S(q) data recorded at the supercooled liquid region can be well described by the Debye theory. For the three alloys, the volume expansion coefficient and the slopes of radii variation for the first to third nearest neighboring coordination shells show differences at glassy-to-supercooled liquid transition, while no obvious changes were detected at supercooled liquid-to-liquid transition for them. The linear expansion coefficient value (β = 1.6 ± 0.1 × 10^–5 K^–1) below the glass transition temperature deduced from S(q) data is consistent with that detected by the dilatometer (β = 1.25 × 10^–5 K^–1) for the La₆₂Al₁₄Cu_11.7Ag_2.3Ni₅Co₅ BMG.     

X-Ray Diffraction Analysis of Residual Stress in Thin Polycrystalline Anatase Films and Elastic Anisotropy of Anatase

The importance of residual stress in anatase thin films for their photo-induced hydrophilicity was proved recently. Detailed X-ray diffraction (XRD) studies of residual stresses in titanium dioxide films are presented here. Measurements including multiple hkl reflections on several series of these films revealed the presence of tensile stresses in the films that were obtained by crystallization from amorphous state. Significant anisotropy of the strain was also found and compared with that of anatase, resulting from its theoretically calculated single-crystal elastic constants. The XRD data support the experimental evidence of the hypothesis that the [00l] axis is the elastically soft anatase direction, whereas the directions in the [h00] × [hk0] plane are elastically stiff. This is in agreement with the anisotropy predicted by single-crystal elastic constants that are obtained from ab-initio calculations. Residual stress analysis for materials with tetragonal symmetry is described and the theory is used to analyze the data. The anisotropy is very different from that for the rutile phase, and the experimental results agree well with the values calculated for anatase. A simplified method of XRD residual stress analysis in thin anatase films by total pattern fitting (TPF) is also presented. Tensile stresses are formed during the crystallization process and increase rapidly with reduced film thickness. They inhibit crystallization, which is then very slow in the thinnest films.