Crystallographic Analysis of Fatigue Crack Initiation Behavior in Coarse-Grained Magnesium Alloy Under Tension-Tension Loading Cycles

Plane bending fatigue tests are conducted to investigate fatigue crack initiation mechanisms in coarse-grained magnesium alloy, AZ31, under the stress ratios R = ?1 and 0.1. The initial crystallographic structures are analyzed by an electron backscatter diffraction method. The slip or twin operation during fatigue tests is identified from the line angle analyses based on Euler angles of the grains. Under the stress ratio R = ?1, relatively thick tension twin bands are formed in coarse grains. Subsequently, compression twin or secondary pyramidal slip operates within the tension twin band, resulting in the fatigue crack initiation. On the other hand, under R = 0.1 with tension-tension loading cycles, twin bands are formed on the specimen surface, but the angles of those bands do not correspond to tension twins. Misorientation analyses of c-axes in the matrix grain and twin band reveal that double twins are activated. Under R = 0.1, fatigue crack initiates along the double twin boundaries. The different manners of fatigue crack initiation at R = ?1 and 0.1 are related to the asymmetricity of twining under tension and compression loadings. The fatigue strengths under different stress ratios cannot be estimated by the modified Goodman diagram due to the effect of stress ratio on crack initiation mechanisms.     

Comparison of the Fatigue Crack Propagation Behavior of Two Different Forms of PMMA Using Two-Stage Zone Model

The fatigue crack propagation behavior of two different forms of PMMA was studied using two-stage zone model. First, the fatigue crack length and fatigue crack propagation velocities of different specimens were obtained experimentally. Then the effect of material forms and specimen types on the fatigue crack propagation velocities was analyzed. Finally, the data scatter of da/dN-ΔK curves in different forms and different types of specimens was analyzed. The results show that the expressions of fatigue crack propagation velocities of middle crack tension (MT) specimens and compact tension (CT) specimens in the same form PMMA are similar. And the scatter of MT specimens is larger than CT specimens in two forms of PMMA.     

The efficiency of bridges in a crack

Bridges are cross connections in a crack that contract its faces behind the crack tip. They appear because of the inhomogeneity of the material or differences in the kinematics of fracture. We obtained a general solution to the nonlinear singular integral equation that relates the crack opening h(x) to the stress caused by bridges p(x) and determined the stress-intensity factor F(B) (degree of unloading of the crack tip due to the presence of bridges) depending on the dimensionless stiffness of bridges B. As the measure of the efficiency of bridges, we suggest the ratio of the stress for crack start in a matrix without bridges σ_m to that in the presence of bridges σ_∞. When bridges with a strength σ_u occupy a fraction f of the fracture area, their efficiency Y(ζ, B) depends on the degree of reinforcement ζ = fσ_u/σ_m and on their stiffness B. The Y(ζ, B) dependences have been obtained in an explicit form for the limiting cases of brittle and fully ductile bridges.     

Investigation into Corrosion Pit-to-Fatigue Crack Transition in 7075-T6 Aluminum Alloy

Transition of corrosion pit to crack under fatigue condition was investigated in high-strength 7075-T6 aluminum alloy. The pit was formed at the edge of a hole in a specimen. Specimen was subjected to a constant stress during the pit formation. Two types of corrosion pit were considered: corner-pit and through-pit. Two sizes were tested for each pit type. Also, the baseline data of cycles to initiate a 250-µm-long crack were established when the corrosion pit was created without any applied stress on the specimen, i.e., S _appl = 0. The cycles to initiate a 250-µm-long crack initially decreased with increasing S _appl relative to the baseline value and then increased with increasing S _appl such that this increase was significant with higher value of S _appl. The transition between this increase and decrease occurred when the S _appl was greater or less than a value which caused the onset of plastic deformation at the root of the pit, respectively. Microstructural analysis showed that this decrease in cycles to initiate the crack was due to microcracks at the pit front which developed at the lower level of S _appl, and the increase was due to plastic deformation at the higher levels of S _appl.     

Effect of Compressive Mode I on the Mixed Mode I/II Fatigue Crack Growth Rate of 42CrMo4

Subsurface cracks in mechanical contact loading components are subjected to mixed mode I/II, so it is necessary to evaluate the fatigue behavior of materials under mixed mode loading. For this purpose, fatigue crack propagation tests are performed with compact tension shear specimens for several stress intensity factor (SIF) ratios of mode I and mode II. The effect of compressive mode I loading on mixed mode I/II crack growth rate and fracture surface is investigated. Tests are carried out for the pure mode I, pure mode II, and two different mixed mode loading angles. On the basis of the experimental results, mixed mode crack growth rate parameters are proposed according to Tanaka and Richard with Paris’ law. Results show neither Richard’s nor Tanaka’s equivalent SIFs are very useful because these SIFs depend strongly on the loading angle, but Richard’s equivalent SIF formula is more suitable than Tanaka’s formula. The compressive mode I causes the crack closure, and the friction force between the crack surfaces resists against the crack growth. In compressive loading with 45° angle, da/dN increases as K _eq decreases.     

Comprehensive analysis of fractures,microstructure, and physical and mechanical properties for the evaluation of the crack resistance of medium-carbon Cr–Ni–Mo steel

The crack resistance of the 38CrNi3MoV (34NiCrMoV14—5. 35NiCrMoV12—5) tempered steel (at various tempering temperatures) has been estimated based on a comprehensive study of the steel fractures, microstructure, physical, and mechanical properties. Stress-intensity factor K _1C at the apex of the crack is growing continuously with an increase in the tempering temperature from 200 to 620°C. This indicates that K _1C is a structural-sensitive parameter, which depends on the steel microstructure and submicrostructure, the fracture mechanism that occurs under these structural conditions, the internal stress level, and the existence of microdefects and microcracks. The linear correlation dependence is found between the coefficient K _1C and the transverse velocity V _transv. The obtained results reveal that the acoustic method can be used to quickly and efficiently estimate the crack resistance of the thermostrengthened steel without the recourse to labor-consuming mechanical tests and computations of the K _1C value.     

The Microstructure and Gamma Prime Distributions in Inertia Friction Welded Joint of P/M Superalloy FGH96

A gamma prime (γ′) precipitation (~35% in volume)-hardened powder metallurgy (P/M) superalloy FGH96 was welded using inertia friction welding (IFW). The microstructure and γ′ distributions in the joints in two conditions, hot isostatic pressed state and solution-treated and aged state, were characterized. The recrystallization of grains, the dissolution and re-precipitation of γ′ in the joints were discussed in terms of the temperature evolutions which were calculated by finite element model analysis. Regardless of the initial states, fully recrystallized fine grain structure formed at welded zone. Meanwhile, very fine γ′ precipitations were re-precipitated at the welded zone. These recrystallized grain structure and fine re-precipitated γ′ resulted in increasing hardness of IFW joint while making the hardness dependent on the microstructure and γ′ precipitation.     

Assessment of Fatigue Resistance of Aluminide Layers on MAR 247 Nickel Super Alloy with Full-Field Optical Strain Measurements

This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.     

Anisotropy of the electric transport properties of decagonal Al–Cu–Co(Fe) quasicrystals

The method of growth from a melt solution was used to obtain iron-alloyed (0.08 at %) Al–Cu–Co single crystals with a decagonal symmetry. The temperature dependences of the electrical resistivity in magnetic fields of 0–18 T were measured using samples oriented in the periodic direction (ρ_p(T)) and in the quasi-periodic plane (ρ_q(T)). A strong anisotropy of the resistivity was observed; the ρ_p(T) curve is linear, whereas the ρ_q(T) curve is approximated well by a second-order polynomial. A strong anisotropy of the magnetoresistance was also observed; a positive magnetoresistance Δρ/ρ ~ 10^–3 for the current flowing in the quasiperiodic plane; and a weak (close to zero) negative magnetoresistance for the current flowing along the periodic direction.     

Determination of Johnson–Cook Plasticity Model Parameters for Inconel718

In order to simulate foreign object damage (FOD) phenomenon in aircraft high-pressure compressor blades made of a nickel-based super-alloy, Johnson–Cook (J–C) plasticity model was used. For prediction of material’s plastic behavior at temperature of 400 °C (working temperature of the blades) in the range of strain rates associated with the FOD phenomenon (in order of 10⁶ s^?1), material parameters of A, B, C, n and m for the J–C plasticity model had to be determined experimentally. Parameters of A, B and n with values of 1108, 699 MPa and 0.5189, respectively, were obtained from quasi-static tensile tests. Moreover, m was determined to be 1.2861, also through quasi-static tensile tests with a strain rate of 1 s^?1 at three temperatures of 475, 550 and 625 °C. However, in order to determine C, firstly a steel ball was impacted on the surface of a flat specimen made of a precipitation-hardening alloy, and then, the impact site was 3D scanned to obtain the induced crater profile. Finally, the impact test (ballistic) was simulated using Abaqus, and a C value of 0.0085 was determined by comparing the actual crater profile with the one obtained from the simulation through a trial-and-error approach.     

Simulation and Experiment on Surface Morphology and Mechanical Properties Response in Nano-Indentation of 6H-SiC

The nano-indentation test for 6H-SiC is carried out with a Berkovich indenter. The indentation surface morphology is analyzed by SEM, which show that when the maximum load P _max is 8 mN, there is only plastic deformation and no cracks on the surface of workpiece after unloading process, and when P _max is 10 mN, there is the initiation of crack occurring on the surface of workpiece after unloading process. Based on the strain hardening model, the three-dimensional finite element method of nano-indentation for 6H-SiC is carried out. Simulation results show that in the unloading process the maximum stress and the maximum strain occur in the contact area between the workpiece with the indenter edges, which is consistent with the experimental results. When propagate to the surface from the subsurface, the cracks are subjected to the type I stress and the type II stress due to elastic recovery. After propagating to surface of workpiece, the cracks propagate along a fixed direction because the proportion of type I stress is much larger than that of type II stress. The influence of the cleavage plane on the propagation direction of cracks is obvious. The cracks propagate more easily when the indenter edges are along cleavage plane. The indentation depth and residual depth increase with the increase of P _max. While, the elastic recovery rate gradually decreases and tends to be stable with the increase of P _max. When P _max is <10 mN, the micro-hardness and the elastic modulus increase linearly with the increase of P _max. When P _max exceeds 10 mN, the micro-hardness decreases with the increase of P _max and then gradually tends to be stable, and the elastic modulus increases by power function with the increase of P _max and then gradually tends to be stable.     

Determination of Triaxial Residual Stress in Plasma-Sprayed Hydroxyapatite (HAp) Deposited on Titanium Substrate by X-ray Diffraction

Measurement of residual stress in plasma-sprayed coating is of key importance to optimize their microstructure and mechanical properties. In this study, the x-ray diffraction analysis was carried out using the sin²ψ method to evaluate the residual stress distribution of hydroxyapatite (HAp) coatings produced on titanium substrate by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS). The sin²ψ method measured strains at different tilt ψ and rotating φ angles around the specimen surface normal. A non-uniform and inhomogeneous stress distribution was present in the both coatings. The measured strain ε_ψφ is plotted versus sin²ψ, showing a nonlinear (elliptical) behavior, which indicates the presence of inhomogeneous triaxial stress distributions within coating, due to the crystalline anisotropy, inhomogeneous cooling rate or solidification of the molten particles. The normal stress values within both HAp coatings produced were found to be tensile in nature, but the values of tensile stresses are significantly higher in APS coating than those values obtained for VPS coating.     

Dynamic recrystallization behavior and constitutive modeling of as-cast 30Cr2Ni4MoV steel based on flow curves

The compression deformation of 30Cr2Ni4MoV steel at different temperatures and strain rates is carried out on Gleeble 1500 thermal mechanical simulation tester. Based on the experimental flow curves, the strain hardening rate curves (θ = dσ/dε versus σ) are derived, from which the characteristic stresses and strains are identified. Meanwhile, the dependences of the characteristic stresses and strains on Zener-Hollomon parameter are determined and the results show that the value of the critical stress of dynamic recrystallization is close to the value of the steady stress. With the aid of the experimental flow curves, the Avrami equation is employed to describe the kinetics of dynamic recrystallization. The time exponent (n) is expressed as a power law function of Zener-Hollomon parameter and the Avrami constant (k) is determined as a function of half of the time for the complete dynamic recrystallization (t ₅₀). Furthermore, a constitutive model is presented based on the rule of mixtures when the dynamic recrystallization occurs. Validation of the constitutive model is implemented and the simulated results agree well with the experimental results.     

Symmetry analysis of the magnetic structures of alloys of the quasi-binary system Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>2</Subscript>

To estimate the reliability of the literature data on the magnetic structures of quasi-binary alloys Fe _x Mn_{1 ? x} Sn₂ obtained without using methods of symmetry analysis, we calculated the basic functions of the irreducible representations of the space group D _4h ¹⁸ (I4/mcm) with the stars of wave vectors determined from an analysis of previously published models of these structures. A comparison of these models with the results of calculations has been performed. A conclusion is made that the models of magnetic structures examined are in agreement with the results of the symmetry analysis performed in this work.     

Short-range atomic order in Fe<Subscript>2</Subscript>B powders

The possibility of application of a local atomic displacement (LAD) model for describing the mechanism of formation of a low-field part (H _hf ≤ 23.6 T) of a hyperfine-field distribution P(H) in a Fe₂B alloy at early stages of grain refinement has been investigated. It has been found that the appearance of Mössbauer contributions, which are not typical of the tetragonal C16 structure, in the P(H) function of the Fe₂B alloy is induced by a decrease in the relative interatomic distances Δr _Fe-B/r _Fe-B to ≤0.18 for some configurations of iron and boron atoms in the unit cell.     

Hard and Corrosion-Resistant Ni-<Emphasis Type="Italic">x</Emphasis>Al-<Emphasis Type="Italic">y</Emphasis>Ti Nanocomposite Coatings for CNT/Al Composites

The ternary Ni-xAl-yTi (x, y in wt.%) nanocomposite protective coatings were electroplated on carbon nanotubes (CNT)/Al composite by the co-deposition of Al and Ti particles at several current densities. The dependences of microstructure, microhardness, residual stress, and corrosion resistance of the ternary nanocomposite coating on current density were investigated. The results showed that the embedded Al and Ti particles caused the crystallite refinement and the decrease of [200] fiber texture of nanocrystalline Ni matrix when current density decreased. The microstructure evolution endowed the ternary nanocomposite coating with high microhardness and corrosion resistance. The surface residual stress of the ternary nanocomposite coating increased with decreasing current density.     

Magnetostructural investigation of ball-milled cobalt-copper alloy

In this work we investigate a metastable inhomogeneous Co-Cu alloy produced by mechanical alloying. We use both conventional structural method (X-ray diffraction) and magnetic measurements of M(T) and M(H) dependences to obtain additional information about the process of mechanical alloying.     

Specific Features of the Frequency Variations in Magnetic Losses in Fe–3%Si {110} Crystals in Rotating Magnetic Fields

The frequency dependence of the total and eddy-current magnetic losses P_tot and P_edd for rotary magnetization reversal has been studied on an Fe–3%Si single crystal with a thickness of d = 0.10 mm within a range of frequencies f = 20–200 Hz and induction amplitudes B_m = 0.25–1.90 T. Some new specific features in the behavior of magnetic losses have been revealed. In particular, the induction that corresponds to the maximal magnetic losses in the curve P_tot = P(B_m) has been established to monotonically decrease with increasing frequency of magnetic field rotation. The specific features of the frequency variation of magnetic losses were discussed based on the observed dynamics for the domain structure of the specimen.     

Structure,magnetic and magnetocaloric properties of nonstoichiometric TbCo<Subscript>2</Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compounds

The structure and magnetic and magnetocaloric properties of new nonstoichiometric TbCo₂Ni _x compounds (0 ≤ x ≤ 0.4) have been studied. The alloys with х ≤ 0.1 have been shown to be single-phase with the MgCu₂-type structure; in alloys with х > 0.1, an additional phase with a PuNi₃-type structure has been formed. It has been found that the concentration dependences of the Curie temperature and magnetic moment of the 3d-metal sublattice have a maximum at x = 0.025. The magnetocaloric effect magnitude for the TbCo2Nix compounds has been estimated using the results of magnetic and heat-capacity measurements.     

The Microstructural Evolution and Special Flow Behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr During Isothermal Compression at a Low Strain Rate

The microstructural evolution and special flow behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr during isothermal compression at a strain rate of 0.0001 s^?1 were investigated. The dislocation climbs in elongated α grains resulted in the formation of low-angle boundaries that transform into high-angle boundaries with greater deformation, and the elongated α grains subsequently separated into homogenous globular α grains with the penetration of the β phase. The simultaneous occurrence of discontinuous dynamic recrystallization and continuous dynamic recrystallization in the primary β grains resulted in a trimode grain distribution. The β grains surrounded by dislocations presented an equilateral-hexagonal morphology, which suggests that grain boundary sliding through dislocation climbs was the main deformation mechanism. The true stress–strain curves for 1073 and 1113 K abnormally intersect at a strain of ~0.35, related to the α → β phase transformation and distinct growth of the β grain size.