Molecular dynamic modelling of fatigue crack growth in aluminium using LEFM boundary conditions

A molecular dynamic (MD) model of a crack in pure aluminium has been developed with isotropic Linear Elastic Fracture Mechanics (LEFMs) boundary displacements that simulates the fatigue crack growth process. The model consists of a cylindrical region filled with atoms around a crack tip and subject to boundary displacements that change due to cyclic loading. A sinusoidal load that produced a $K_{\max }=1.0\text{MPa}\sqrt{\text{m}}$ was applied to produce fatigue crack growth using three different atomic potentials for aluminium at T = 20 K, and a range of different K_min. Each run consisted of the application of fifteen or more loading cycles. In some cases, the crack tip was seen to advance in each cycle typical of fatigue, however, growth was smooth and continuous during the entire cycle with contraction occurring during the unloading phase of the cycle. The model contained 3 × 10⁶ atoms and had a diameter and width of 20 nm. This width was just large enough for fragments of sessile dislocations to form and couple with the glissile dislocations emitted from the crack tip, resulting in work hardening about the crack tip. The model was oriented for cracking on the {1 1 0} plane in the 〈1 0 0〉 direction. Crack advance was observed to be due to a combination of dislocation emission and atomic separation.     

Microstructure,texture evolution and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn biomedical beta titanium alloy

Ti-32.5 Nb-6.8 Zr-2.7 Sn(TNZS,wt%) alloy was produced by using vacuum arc melting method,followed by solution treatment and cold rolling with the area reductions of 50% and 90%.The effects of cold rolling on the microstructure,texture evolution and mechanical properties of the experimental alloy were investigated by optical microscopy,X-ray diffraction,transmission electron microscopy and universal material testing machine.The results showed that the grains of the alloy were elongated along rolling direction and stress-induced α' martensite was not detected in the deformed samples.The plastic deformation mechanisms of the alloy were related to {112} 111 type deformation twinning and dislocation slipping.Meanwhile,the transition from γ-fiber texture to α-fiber texture took place during cold rolling and a dominant {001} 110_(α-fiber) texture was obtained after 90% cold deformation.With the increase of cold deformation degree,the strength increased owing to the increase of microstrain,dislocation density and grain refinement,and the elastic modulus decreased owing to the increase of dislocation density as well as an enhanced intensity of {001} 110_(α-fiber)texture and a weakened intensity of {111} 112_(γ-fiber)texture.The 90% cold rolled alloy exhibited a great potential to become a new candidate for biomedical applications,since it possesses low elastic modulus(47.1 GPa),moderate strength(883 MPa) and high elastic admissible strain(1.87%),which are superior than those of Ti-6 Al-4 V alloy.     

Diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couples

The diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couple method were investigated. Results demonstrate that Al_2Ca is the only phase existing in the diffusion reaction layers.The volume fraction of Al_2Ca in diffusion reaction layers increases linearly with temperature. The standard enthalpy of formation for intermetallic compounds was rationalized on the basis of the Miedema model. Al-Ca intermetallic compounds were preferable to form in the Mg-Al-Ca ternary system under the same conditions. Over the range of 350–400?C, the structure of Al_2Ca is more stable than that of Al_4Ca, Al_(14)Ca_(13) and Al_3Ca_8. The growth constants of the layer Ⅰ, layer Ⅱ and entire diffusion reaction layers were determined. The activation energies for the growth of the layer Ⅰ, layer Ⅱ and entire diffusion reaction layers were(80.74 ± 3.01) k J/mol,(93.45 ± 2.12) k J/mol and(83.52 ± 1.50) k J/mol, respectively.In layer Ⅰ and Ⅱ, Al has higher integrated interdiffusion coefficients D~(Int, layer)ithan Ca. The average effective interdiffusion coefficients D_(Al)~(eff) values are higher than D_(Ca)~(eff) in the layer Ⅰ and Ⅱ.     

Variation in fatigue properties of cast A359-SiC composites under total strain controlled conditions: Effects of porosity and inclusions

The effects of casting defects, such as inclusions and porosity on fatigue properties of permanent mold cast bars of A359-20 vol% SiC particle composites cast and heat treated under identical conditions, were studied under total strain control conditions. The fatigue fracture surfaces and the near fracture regions were observed using optical stereo-microscope and scanning electron microscope (SEM) to identify the effects of microstructure and casting defects on fatigue properties of pairs of specimens tested under the same total strain that showed large scatter in fatigue life. It was found that the largest porosity size on the fracture surfaces of the fatigue specimens measured was around 700 μm, whereas the average dendrite arm spacing was 60–100 μm and the size of SiC particle clusters in the near fracture region was around 100 μm. Fracture surfaces of the composite bars showed that fatigue cracks frequently initiated from porosity present near the surface of the samples. Test specimens containing larger porosity size were observed to have lower fatigue life. However, some specimens with larger amounts of inclusions had a lower fatigue life than those with larger porosity size within the range observed in this study. The size and amount of inclusions, and the size and shape of the porosity near the surface seems to have the greatest influence in decreasing fatigue life. Fatigue strength, and fatigue ductility constants for permanent mold cast A359-20 vol% SiC composite were determined from the strain controlled fatigue data. They were as follows: fatigue strength exponent (b) is −0.1145, fatigue strength coefficient $({\sigma }_{\text{f}}^{\prime })$ is 558 MPa, fatigue ductility exponent (c) is −0.6066 and fatigue ductility coefficient (${\epsilon }_{\text{f}}^{\prime })$ is 0.0108.     

An investigation into the room temperature mechanical properties of nanocrystalline austenitic stainless steels

The present work has been conducted to evaluate the mechanical properties of nanostructured 316L and 301 austenitic stainless steels. The nanocrystalline structures were produced through martensite treatment which includes cold rolling followed by annealing treatment. The effect of equivalent rolling strain and annealing parameters on the room temperature mechanical behavior of the experimental alloys have been studied using the shear punch testing technique. The standard uniaxial tension tests were also carried out to adapt the related correlation factors. The microstructures and the volume fraction of phases were characterized by transmission electron microscopy and feritscopy methods, respectively. The results indicate that the strength of nanocrystalline specimens remarkably increases, but the ductility in comparison to the coarse-grained one slightly decreases. In addition the strength of nanocrystalline specimens has been increased by decreasing the annealing temperature and increasing the equivalent rolling strain. The analysis of the load–displacement data has also disclosed that the universal correlation of linear type (UTS = mτ_max) between shear punch test data and the tensile strength is somehow unreliable for the nanocrystalline materials. The results suggest that the actual relation between the maximum shear strength and ultimate tensile strength follows a second order equation of type $\text{UTS}=a{\tau }_{\mathit{max}}^2-b{\tau }_{\mathit{max}}+c$.     

Study on hot deformation behavior and microstructure evolution of cast-extruded AZ31B magnesium alloy and nanocomposite using processing map

The hot deformation behavior and microstructural evolution of cast-extruded AZ31B magnesium alloy and nanocomposite have been studied using processing-maps. Compression tests were conducted in the temperature range of 250–400 °C and strain rate range of 0.01–1.0 s⁻¹. The three-dimensional (3D) processing maps developed in this work, describe the variations of the efficiency of power dissipation and flow instability domains in the strain rate ($\dot{\epsilon }$) and temperature (T) space. The deformation mechanisms namely dynamic recrystallization (DRX), dynamic recovery (DRY) and instability regions were identified using processing maps. The deformation mechanisms were also correlated with transmission electron microscopy (TEM) and optical microscopy (OM). The optimal region for hot working has been observed at a strain rate ($\dot{\epsilon }$) of 0.01 s⁻¹ and the temperature (T) of 400 °C for both magnesium alloy and nanocomposite. Few instability regimes have been identified in this study at higher strain rate ($\dot{\epsilon }$) and temperature (T). The stability domains have been identified in the lower strain rate regimes.