Combined Effect Of Microstructure Research Articles

Suppressing Side‐Scattering on Laser‐Written Bragg Gratings for Back‐Reflection Engineering in Fibers

AbstractLaser direct writing (LDW) is versatile in structuring fibers with micro‐sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser‐induced refractive index modified (RIM) points. However, the side‐scattering is a lesser‐explored property in laser‐structured fibers. In this work, a concise physical model is established to understand the side‐scattering as a combined effect of microstructure and geometry of RIM points. Based on a single‐pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross‐section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra‐low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record‐breaking κ/α of 2083. Using this FBG as the high‐reflective mirror of a home‐made bismuth‐doped fiber laser, narrow‐band lasing with a high optical signal‐to‐noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back‐ and side‐scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all‐fiber networks.

Multiscale residual stress and mechanical behavior analysis in machining of Ti-6Al-4V alloy with advanced microscopic characterization

Machining of polycrystalline metal materials results in metallurgical modifications in the machined surface, and it can introduce multiscale residual stresses (macroscale and microscale residual stress), which may be detrimental to the mechanical properties thereby interfering with the service performance of machined parts. However, most of the current studies only focus on the macroscale residual stress, the microscale residual stresses have always been neglected since the limitations of the commonly used techniques for residual stress measurement in machined surfaces. In this study, the recently developed Focused Ion Beam-digital image correlation (FIB-DIC) ring core method was employed to evaluate the time-resolved strain relaxation and multiscale residual stress of the machined surface under different cutting conditions. The quantitative evaluation of residual stress and microstructural effects on hardness of the machined surface was conducted by combining with FIB incremental milling and nanoindentation technique. It has been observed that although the global tensile strain relaxation was captured in the ring-core region, local microscale compressive strain relaxation also occurred. The hardness of the machined surface was determined by the combined effects of microstructure and residual stress induced by machining. Specifically, the grain refinement and compressive residual stress contributed to hardening, while tensile residual stress resulted in softening of the machined surface. Furthermore, it was also observed the hardening/softening effects were enhanced with increasing intensity of tension and compression.

Coordinated effect of microstructure and defect on fatigue accumulation in dual-phase Ti-6Al-4V: Quantitative characterization

Fatigue accumulation in dual-phase Ti-6Al-4V is strongly associated with the combined effects of microstructure and defects in itself, which is experimentally challenging to be characterized and quantified. Based on crystal plasticity finite element method (CPFEM), the present work aims to address the coordinated effect of microstructure and defect on fatigue behavior in Ti-6Al-4V, highlighting the influence of morphology and orientation of defects, in conjunction with the designated idealized microstructure. In light of these considerations, a modelling strategy is proposed to focus on defect morphology and orientation in polycrystalline RVE models. Firstly, the geometric roundness parameter is adopted to characterize the diverse defect morphology and is quantitatively correlated with the characteristic of fatigue accumulation in Ti-6Al-4V. Secondly, a liner relationship with respect to corner angle and plastic deformation is formulated to characterize the influence of defect orientation, reflecting the microstructural heterogeneity in dual-phase Ti-6Al-4V. The proposed modelling strategy overcomes the experimental limitation on characterizing the complex effect of microstructure and defect, holding the potential of revealing fatigue mechanism around defects in metal alloys.

Study on the effects and mechanisms of induction heat treatment cycles on toughness of high frequency welded pipe welds

The low toughness of the high frequency welded pipe welds seriously affects the performance of the welded pipe. Two induction heat treatment cycles of quenching + tempering (Q&T) and similar normalizing (SN) were put forward in this paper to study the toughness of the high frequency welded pipe welds. The effect of heating rate on the phase transition temperatures (Ac1, Ac3) of the raw material of the high frequency welded pipe was studied and the reasonable ranges for heat treatment parameters were determined. It was found that the austenite transformation temperatures increase with the increase of heating rate. For the heating rate of 200 ℃/s., Ac1 and Ac3 were determined as 769 ℃ and 920 ℃, respectively, and then the specific heat treatment parameters for Q&T and SN were determined. The mechanical properties of the base metal, as-welded, Q&T and SN treated welds were obtained by tensile and impact tests. The optimum heat treatment parameters corresponding to Q&T and SN respectively were determined based on the principle of a good matching of strength and toughness of the welds. Compared with those of the as-welded welds, the tensile strength of the optimum SN treated welds decreased from 843 MPa to 827 MPa, while the absorbed energy of the Charpy V-notched specimen at room temperature increased from 20 J to 42 J, which is larger than that of the base metal. The tensile strength of the optimum Q&T treated welds increased from 843 MPa to 861 MPa, and the absorbed energy of the Charpy V-notched specimen at room temperature increased from 20 J to 36 J, which is slightly lower than that of the base metal. The reasons for improving the toughness of the welds were also analyzed from the aspects of microstructure, texture, inclusion, grain and grain boundary. It is found that the toughness of the high frequency welded welds is affected by the combined effects of microstructure, the density of cleavage planes oriented for easy fracture, the grain number and size, the fraction of high-angle grain boundaries. The toughness of the welds is good when the density of cleavage planes oriented within 10° to the fracture surface is low, the total grain number, the fraction of fine grains, and the fraction of high-angle grain boundaries are high.

Experimental study and modelling of the effect of microstructure on friction and wear mechanisms of low alloy steel

Few models are focused on the combined effects of microstructure and roughness on the tribological behavior of materials. Hardness is the material property mainly used in the tribological models which are usually at a macroscopic scale. For a dual-phase steel, experimental and predicted values of friction coefficients and specific wear resistances are compared. The investigated models are declined into two pressure distribution modes between the phases. Friction tests are performed between steel pins composed of a ferrite-martensite dual-phase microstructure against abrasive papers with various abrasive particle sizes ranging from 15 µm to 200 µm. By using heat treatments on a low alloy steel, dual-phase microstructures with various martensite volume fractions, ranging from 45% to 100%, are generated.As martensite volume fraction increases, the experimental and predicted results show that the specific wear resistance increases whereas the friction coefficient decreases. Furthermore, the latter evolutions depend on roughness. For a predominance of abrasive wear mechanisms generated by coarse abrasive particles (~200 µm), the experimental tribological parameters tend to follow the predicted ones associated to the mode characterized by equal pressures between the phases. Then, as the abrasive particle size decreases, abrasive wear mechanisms reduce whereas adhesive wear mechanisms increase and the experimental tribological parameters tend to follow the predicted ones associated to the mode characterized by equal wear rate between the phases.

Effect of MnS inclusion and crystallographic texture on anisotropy in Charpy impact toughness of low carbon ferritic steel

The combined effect of microstructure and crystallographic texture on the anisotropy in Charpy impact toughness has been studied in two hot-rolled low-carbon steel plates having different sulphur contents: 0.03wt% S in high-sulphur, HS, steel and 0.01wt% S in low-sulphur, LS, steel. Sub-size Charpy impact specimens were prepared from the rolled plates along 0°, 30°, 60° and 90° to the rolling direction and tested over the temperature range of +40°C to −196°C. Microstructure, inclusions and crystallographic texture have been characterized on the plane parallel to the fracture plane of each sample. The variation in upper shelf energy (USE) was more severe (∼69%) in HS steel due to the presence of coarse and elongated MnS inclusions. Crystallographic texture, especially, higher fraction of alpha- to gamma-fibre texture and stronger cube texture resulted in stronger variation in ductile to brittle transition temperature (DBTT) in LS steel (by ∼26°C). Increase in the fraction of {001} planes of the crystals on the fracture plane of the sample increased the DBTT by helping the cleavage crack propagation. On the other hand, increase in the fraction of {110} planes of the crystals on the shear planes (inclined at 45° to fracture plane) increased the USE by promoting the plastic deformation.

Effective conductivity of spheroidal particle composite with imperfect interfaces: Complete solutions for periodic and random micro structures

The rigorous analytical approach to the conductivity homogenization problem for spheroidal particle composite with imperfect interfaces has been developed. The complete multipole series solution for a single inclusion with imperfect interface constitutes mathematical background of the work and provides a basic building block for the Maxwell and Rayleigh homogenization schemes. The low- and highly conducting spheroidal interface are considered in a unified way. The developed theory enables an analysis of composites of periodic and random micro structure with imperfect interfaces. Numerical study shows quite a significant combined effect of micro structure and imperfect interfaces on the macroscopic conductivity of composite. The obtained accurate solution serves as a benchmark for the known approximate theories. Their error due to neglecting the interaction effects and simplified treating the imperfect interfaces is estimated.

The effect of microstructure and metal-oxide barriers on carrier transport in top-down processed, low density nanograined n-type PbTe

The combined effects of microstructure and interfacial metal-oxide barriers at grain boundaries on carrier transport in top-down processed, nanogranular n-type bulk PbTe have been investigated. It is found that the porous nature of the microstructure leads to significantly reduced thermal conductivity. Naturally occurring metal-oxide potential barriers at the PbTe grain interface result in notably reduced Hall carrier concentration and significantly enhanced resistivity with non-degenerate temperature dependence; they also incorporate additional carrier scattering along with acoustic phonons in the material system. Further, our investigation shows that unlike Hall carrier concentration and resistivity, thermopower is not an inherent property of the bulk granular solid but of its constituent grains.

Numerical study of mesoscale surface roughening in aluminum polycrystals under tension

A 3D numerical analysis of mesoscale surface roughening in a polycrystalline aluminum alloy under uniaxial tension has been performed. Three-dimensional microstructural models with equiaxial and extended grains inherent in cold rolling have been developed and used in calculations. Crystallographic anisotropy is not described in an explicit form by means of, say, a well-known crystal plasticity model. Instead, the elastic-plastic properties of individual grains are varied within 10% and in so doing the mechanical anisotropy due to the grain shape is accounted for. A combined effect of microstructure and loading and boundary conditions is investigated. Pronounced surface roughness of equiaxial grain structure and of a textured material loaded at the right angle to the rolling direction is observed in comparison with extended grain structure loaded along the rolling direction where the surface relief is fairly smooth. Computational results suggest that grain structure is responsible for the formation of a free surface relief during uniaxial loading. Microscale normal and shear stresses associated with the direction perpendicular to the free surface develop in the bulk of the material and, acting from the inside, give rise to surface ridges and valleys.

Thermal fatigue behaviour of hot-work tool steels: heat check nucleation and growth

Cyclic thermal loading causes thermal gradients near the loaded surfaces, which are responsible for the development of cyclic strains and mechanical stresses in the sample that subsequently cause accumulated damage. The stress state changes during thermal fatigue loading if plastic strains occur. Most relevant are the formation of tensile stresses, which favour crack nucleation and crack growth, but thermal softening and cyclic softening also play an important role. The combined effect of microstructure, mechanical and physical properties on the thermal fatigue behaviour was studied by testing hot-work tool steels using a testing facility based on a pulsed laser beam. The results indicate that purely martensitic microstructures have a better thermal fatigue resistance than mixed martenitic/bainitic microstructures. High strength seems to be especially beneficial in cases with lower fatigue loads owing to thermal cycling. Higher thermal conductivity plays a beneficial role regarding the thermal fatigue resistance.