Fine Homogeneous Microstructure Research Articles

Surface, microstructural and mechanical characterization of contemporary implant abutment screws

ObjectivesThe aim of the present work was to evaluate six commercially available abutment screws by characterising roughness parameters, microstructure and mechanical properties. MethodsSix abutment screws from each implant system, were used. The surface roughness parameters (Sa, Sq, Ssk, Sku, Spk, Sk and Svk) were identified by an optical interferometric profiler. Microstructural observations and crystallographic analysis were performed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDX) device for elemental analysis and an X-ray diffractometer (XRD), respectively. The Martens Hardness (HM), Indentation Modulus (EIT), elastic index (ηIT) and Vickers hardness (HV) of all specimens were determined by instrumented indentation testing (IIT). The results were analyzed by one-way ANOVA and Tukey multiple-comparison tests (a=0.05). ResultsEDX and XRD showed the abutment screws to be mixed α- and β-phase titanium alloys. Microstructural analysis revealed a fine homogeneous microstructure without porosity, consisting of fine dispersoid rods of β-phase embedded in a continuous α-phase matrix. Statistically significant differences were found among the mechanical properies and surface roughness parameters apart from Sq, Spk and Svk. ConclusionsThe tested abutment screws showed significant differences in the probed properties, and, thus, differences in their clinical behaviour are anticipated.

Multi-scale modeling and simulation for multi-pass processing of Ta-2.5 W alloy

Multi-pass deformation and annealing process is commonly utilized to prepare billets with homogeneous fine microstructure. Characterizing the deformation and microstructural evolution is not easy due to the complex process. This study proposed a multi-scale modeling and simulation method for the multi-pass processing and it was validated by experiments. Firstly, an integrate multi-scale model involving strain hardening and static softening equations was developed based on multi-pass cold deformation and annealing tests of Ta-2.5 W alloy. Of note, the model captures both deformation behavior and static softening and the model equations of each pass were integrated into a unified manner. Then, multi-scale simulation including finite element (FE) and cellular automatic (CA) methods was coupled with the developed model for static recrystallization (SRX) analysis. FE analysis was firstly performed to predict the distributions of strain, dislocation density as well as SRX. Simultaneously, the predicted data was used for generation of the initial conditions for mesoscopic model. Finally, a CA method was applied for further analysis on recrystallization nucleation and grain growth. The modeling and simulation method is examined by multi-pass cold deformation and annealing test and a reasonable consistency is obtained between experimental and predicted results.

Microstructure, texture and mechanical properties of the rolled high modulus Mg-Y-Zn-Al-Li alloy

In our previous work, we designed a new as-cast Mg–Y–Zn–Al–Li alloy with a high elastic modulus of 52.9 GPa. The alloy was further subjected to hot deformation under different rolling reductions (40% and 80%) in this work. The microstructure evolution, texture and mechanical properties of Mg–Y–Zn–Al–Li alloy compared with Mg–Y–Zn alloy were systematically investigated. The result showed that the Mg–Y–Zn–Al–Li alloy with a rolling reduction of 40% displayed a more homogeneous fine microstructure and a weakened basal texture, accompanied by the formation of Al–Li and nano-precipitates Al0.9Li34.3Mg64.5Zn0.3 phase. Compared with the Mg–Y–Zn alloy, the strength and ductility of the alloy also were obviously enhanced while possessing a weak mechanical anisotropy. Grain refinement and second phase strengthening (fine Al–Li phase and nano-precipitates Al0.9Li34.3Mg64.5Zn0.3) mainly contributed to the enhanced strength. The texture weakening, low dislocation density and non-basal slip commonly guaranteed the high ductility, especially the weakened texture gave rise to the low anisotropy. When the rolling reduction increased to 80%, the volume fraction of recrystallized grains in Mg–Y–Zn–Al–Li alloy increased significantly to73.3%. As a result, the symmetry of the texture distribution around the ND was appreciably improved. In addition, the lower dislocation density and more non-basal slip in the Mg–Y–Zn–Al–Li alloy with the rolling reduction of 80% further enhanced the ductility. Simultaneously, the anisotropic degree of mechanical was effectively minimized, which was mainly attributed to the weak symmetric texture caused by fine DRXed grains with random misorientation and non-basal pyramidal <c+a> slip.

Dense MgB2 Ceramics by Ultrahigh Pressure Field-Assisted Sintering

Magnesium diboride (MgB2) ceramics, due to their impressive transition temperature of 39 K for superconductivity, have been widely investigated. The possibility to obtain highly dense MgB2 ceramics with fine microstructure and grain boundaries acting as pinning sites by novel high-pressure-assisted spark plasma sintering (HP-SPS) is reported in this article. HP-SPS was employed to reach 100% density in MgB2 ceramics, and high pressure was utilized in the consolidation of MgB2. An increase in pressure helped in stabilizing the MgB2 phase above thermal decomposition, thus avoiding the formation of non-superconducting phases such as MgO and MgB4. Pressure allowed strengthening of the covalent bond (condensation effect) to increase the thermal stability of MgB2. HP-SPS yielded high mechanical hardness in MgB2 (1488 HV). For better electrical connectivity, which leads to large magnetic moments in high density samples were obtained with the beneficial effect of high applied pressure (1.7–5 GPa) at high temperature (&gt;1000 °C). The combination of the SPS process and high pressure ensured retention of the homogeneous fine microstructure required to obtain high current density and high hardness.

Electrochemical Corrosion Resistance of Ni and Co Bonded Near-Nano and Nanostructured Cemented Carbides

The advantages of nanostructured cemented carbides are a uniform, homogenous microstructure and superior, high uniform mechanical properties, which makes them the best choice for wear-resistant applications. Wear-resistant applications in the chemical and petroleum industry, besides mechanical properties, require corrosion resistance of the parts. Co as a binder is not an optimal solution due to selective dissolution in an acidic environment. Thus, the development of cemented carbides with alternative binders to increase the corrosion resistance but still retaining mechanical properties is of common interest. Starting mixtures with WC powder, grain growth inhibitors GGIs; VC and Cr3C2, and an identical binder amount of 11-wt.% were prepared. GGIs were added to retain the size of the starting WC powder in the sintered samples. The parameters of the powder metallurgy process were adapted, and samples have been successfully consolidated. A very fine homogeneous microstructure with relatively uniform grain-size distribution and without microstructural defects in the form of carbide agglomerates and abnormal grain growth was achieved for both Ni-bonded and Co-bonded samples. Achieved mechanical properties, Vickers hardness, and Palmqvist toughness, of Ni-bonded near-nanostructured cemented carbides are slightly lower but still comparable to Co-bonded nanostructured cemented carbides. Two samples of each grade were researched by different electrochemical direct current corrosion techniques. The open circuit potential Ecorr, the linear polarisation resistance (LPR), the Tafel extrapolation method, and the electrochemical impedance spectroscopy (EIS) at room temperature in the solution of 3.5% NaCl. From the carried research, it was found that chemical composition of the binder significantly influenced the electrochemical corrosion resistance. Better corrosion resistance was observed for Ni-bonded samples compared to Co-bonded samples. The corrosion rate of Ni-bonded cemented carbides is approximately four times lower compared to Co-bonded cemented carbides.

Phase field simulation of dendrite growth in gas atomized binary Al–Ni droplets

Catalytic powders with fine microstructures can be produced from a rapidly solidified gas atomized Al–Ni alloy. The solidification microstructure of the droplets is closely linked with heat flow conditions. Thus, the heat transfer conditions between the gas and droplet are essential to microstructural evolution. In this study, a phase field model for simulating single and multiple dendrite growth in a binary Al–Ni alloy was constructed and the microstructural evolutions occurring during a gas atomization process were evaluated. Temporal variations in the heat transfer coefficient and the boundary heat flux were taken into account. The results revealed that the heat transfer coefficient of the atomized droplet in flight is correlated with the droplet size and the relative velocity between the droplet and the atomizing gas. During the simulation, the competition between boundary heat flux extraction and latent heat release from phase transition causes a recalescence process in thermal history, thereby affecting the gradient temperature distribution and, consequently, the dendrite morphology. Dendrite growth under the effects of the heat transfer coefficient is restrained continuously because of the decreasing amount of extraction. The computational results confirmed the fine homogeneous microstructure and low microsegregation levels of gas atomized powders.

Strong ZrC ceramics at high temperatures with the addition of W

AbstractThe effect of W addition on densification, microstructure, and mechanical properties of ZrC ceramics was investigated. W reacted with carbon in ZrC to form WC, which resulted in the formation of ZrC1‐x at 1300‐1700°C, while WC was further dissolved in ZrC to form a (Zr1‐yWy)C1‐x solid solution at 1800‐2000°C. The relative density of ZrC with 5 mol% W (ZW5, 96.8%) was markedly higher than that of pure ZrC (Z0, 94.8%). ZW5 exhibited a fine homogeneous microstructure with a grain size (2.6 ± 0.5 μm) much smaller than that of Z0 (10.9 ± 3.0 μm), while excess W addition (10 mol%) in ZrC adversely affected the densification and the microstructure. The flexure strength of Z0 was 446 ± 46 MPa at room temperature, which almost linearly decreased to 281 ± 10 MPa at 1800°C in a high‐purity flowing argon atmosphere. The flexure strength of ZW5 was 512 ± 40 MPa at room temperature, and had no degradation even up to 1800°C. The fine and homogeneous microstructure of ZW5 and the removal of oxygen impurity from the grain boundaries promoted the enhancement of high‐temperature mechanical properties.

Effects of pre-recovery on the recrystallization microstructure and texture of high-purity tantalum

The effects of recovery on the subsequent recrystallization of tantalum were investigated via two-step annealing at two temperatures. Transmission electron microscopy, electron back-scattered diffraction, and X-ray diffraction were employed to determine the respective microstructures and textures after various annealing regimes. The results show that many large grains with {111} orientations are developed when heating at 1573 K without pre-recovery, while pre-recovery can introduce a homogeneous fine microstructure and weaken the texture. This difference can be attributed to the recovery-induced change in nucleation mechanisms. Elongated grains stretch along grain boundaries or locate in the interior of deformed grains, mainly due to the heterogeneous distribution of stored energy. Dislocations characterized by submicron bands in the deformed state evolve into sub-grains during pre-recovery and grow continuously, indicating a homogeneous distribution of dislocation or stored energy after pre-recovery, which can significantly influence subsequent nucleation and grain growth.

Sinterability of Low-Cost 3Y-ZrO2 Powder and Mechanical Properties of the Sintered Body

This study investigated the effects of grain size and phase constitution on the mechanical properties of 3Y-ZrO₂ by varying the sintering conditions. The raw powder prepared by a low-cost wet milling using the coarse solid oxide powders was sintered by both pressureless sintering and hot-pressing, respectively. As increasing holding time at 1450℃ for pressureless sintering, it promoted the microstructural coarsening of matrix grains and the phase transformation to tetragonal phase, whereas the bimodal microstructure embedded with abnormal cubic-ZrO₂ grains was observed regardless of sintering time. On the other hand, the specimens hot-pressed at 1300℃ for 2 h reached ~ 97% of relative density with homogeneous fine microstructure and mixed phase constitution. It was found that the proportion of untransformed monoclinic zirconia had the most adverse effect on the biaxial strength compared to the impacts of grain size and density. The pressureless sintering of the low-cost powder for prolonged sintering time to 8 h led to a decent combination of mechanical properties (H V = 13.2 ㎬, K IC = 8.16 ㎫·m 1/2 , σ = 981 ㎫).

Effects of multi-pass FSP on the β phase (Mg17Al12) distribution and mechanical properties of AZ91C magnesium alloy

Purpose: The principle and advantages of friction stir processing (FSP) for the production of a highly formable Mg alloy, and some convincing experimental results are reported in this paper. The aim is to understand the relationship between the microstructure characteristics and the mechanical properties behaviour of the number of FSP passes AZ91C alloy. Design/methodology/approach: FSP is a solid state processing technique which involves plunging and traversing a square pin profiles FSP tool through the material. In this study, a fine-grained multi-phase AZ91C magnesium alloy was produced by applying FSP on as-cast AZ91C alloy. Findings: FSP achieved grain refinement and homogenization of the as-cast microstructure in Mg alloy AZ91C. FSP produced a fine homogeneous microstructure having a grain size of 6 μm throughout the plate. Also tensile properties of the specimen produced in one, two and three passes were investigated by standard tensile test. Results show that FSP improved the tensile characteristics of the as cast AZ91C alloy significantly. As the number of passes increased, higher UTS and TE were achieved due to finer grains and more dissolution of β phase (Mg17Al12). Originality/value: The present study shows that FSP is an efficient production method for a large-scale plate of a highly formable Mg alloy.

Novel casting TiAl alloy with fine microstructure and excellent performance assisted by ultrasonic melt treatment

A novel casting TiAl alloy with fine homogeneous microstructure and excellent performances is obtained assisted by the ultrasonic melt treatment. The coarse columnar crystal is modified into non-dendritic structure and the large grain is refined from 3098μm to 88μm; the micro-segregation, micro-porosities and precipitates are eliminated as well. After ultrasonic melt treatment for 90s, the highest compressive strength is improved up to 1705MPa, about 12% higher than the “quasi-oriented” TiAl alloy.

Phase evolution, microstructure and mechanical properties of equi-atomic substituted TiZrHfNiCu and TiZrHfNiCuM (M = Co, Nb) high-entropy alloys

In this study, alloys with composition of equi-atomic substituted TiZrHfNiCu, TiZrHfNiCuCo, and TiZrHfNiCuNb high-entropy alloys (HEAs) were produced by suction casting method. The effects of addition elements on phase composition, microstructure and mechanical behaviors of the HEA were studied. The suction casted Ti20Zr20Hf20Ni20Cu20 HEA exhibits single C14 Laves phase (MgZn2-type) with fine homogeneous microstructure. When Co or Nb elements are added, morphologies are slightly modulated toward well-developed dendritic microstructure, phase constitutions are significantly changed from single Laves phase to mixed multi-phases as well as mechanical properties are also altered with increased plasticity and high strength. It is believed that modulated mechanical properties are mainly ascribed to the change of phase constitution and crystalline structure, together with the microstructural characteristics. This clearly reveals that the selection and addition of supplementary elements based on the formation rule for HEAs play an important role on the evolution of phase, microstructural morphology and mechanical properties of Ti20Zr20Hf20Ni20Cu20 HEA.

Achieving superplasticity in a Bi-Sn alloy processed by high-pressure torsion

A Bi-Sn alloy was processed by high-pressure torsion (HPT) at room temperature for 10 turns to achieve a homogeneous fine microstructure. Microhardness observations showed that the refined microstructure experienced self-annealing during storage at room temperature. Following HPT processing and storage at room temperature for 35 days, a sample had excellent superplastic properties when testing at an initial tensile strain rate of 1.0 × 10-4 s-1. The enhanced superplastic flow behavior of the fine-grained Bi-Sn alloy was attributed to the homogeneously distributed Bi-Sn and Sn-Sn boundaries after HPT processing since these are preferred sites for grain boundary sliding.

Optimization of a Closed Die Forging Process to Manufacture a Gear Wheel by the Use of a Response Surface Model

Heavy-duty components used in the automotive industry, in wind turbines and in many other industrial applications are often produced using hot forging processes. Nowadays the design of hot forging processes aims for the optimization of process efficiency on the one hand and final mechanical product properties on the other hand. Excellent mechanical properties needed for hot-forged components e.g. high load capacity and high fatigue resistance depend on a fine homogeneous microstructure distribution across the final product’s cross-section. Efficiency in hot forging can be optimized by increasing the temperature during processing, which allows for lower forging loads and lower die stresses, thus improving die life in terms of mechanical fatigue. To guarantee for a fine homogenous microstructure across the cross section of the forged good, dynamic recrystallization (DRX) has to be initiated during deformation and Grain Growth (GG) has to be avoided during dwell times and cooling. Due to the high computational costs of finite element simulations an optimization aiming for lowest possible forging loads and finest possible grain sizes is very time-consuming. In this paper a Response Surface Model (RSM) of the forging process is introduced, which allows for much faster evaluation of the outcome of forging simulations, albeit by interpolation of simulation results, and thus allows for optimization. The information required to create the RSM is obtained by Design Of Experiments (DOE) techniques using an FE-model of the forging process which was calibrated earlier. The process variables considered include the initial temperature of the billet and the die kinematics. Subsequently, an optimization algorithm is combined with the RSM to find the design variables giving minimum possible loads during deformation and finest possible grain sizes in the forged product. The RSMs results are validated by the use of the existing FE-model.

Laser synthesised TaC for improving copper tribological property

To improve the wear resistance of copper through in situ synthesis of ceramic particulate reinforcement, laser surface modification on copper was performed using a powder mixture of Ni based alloy with (Ta2O5+C) doping. The microstructure, microhardness, wear resistance and bulk electrical conductivity of the integral material were investigated. The results indicated that the modified layer had a homogeneous fine microstructure containing TaC particles and Cr3C2 acicular phases dispersed uniformly in the matrix of γ (Ni,Fe) and (Cu,Ni,Fe) solid solutions. The modified layer exhibited higher hardness and better wear resistance than the Ni based alloy modified layer because of the presence and the uniform distribution of in situ synthesised TaC particles. However, the addition of TaC particles slightly decreased the electrical conductivity of the integral bulk.

Microstructure and Wear Resistance of ZrC Reinforced Metal Matrix Composite Coating by Gas Tungsten Arc Cladding

The in situ synthesized ZrC particulate reinforced metal matrix composite coating was fabricated on AISI1020 steel by gas tungsten arc (GTA) cladding powder mixture of Nickel, Zirconium and Graphite. The microstructure and wear resistance of the composite coating were investigated. It is shown that the coating is bonded metallurgically to the substrate and has a homogeneous fine microstructure containing both approximate cubic ZrC particle uniformly dispersed in matrix of (Fe,Ni) solid solution. Compared to a substruate, the hardness of the composite coating was increased by a factor of 6, could achieve a Vicker microhardness of HV0.21100. And the wear resistance in a block on ring test against YG8 was increased by a factor of 10. This is attributed to the presence of in situ synthesized ZrC particles and their well distribution in the coating.

Die and Process Design for Hot Forging of a Gear Wheel – A Case Study

Gearing components are an example for widely used machining parts in engines. Nowadays the development and optimization of materials and process chains are driven towards a concurrent improvement of final product properties and production efficiency. Excellent mechanical properties needed for gearing components e.g. high load capacity and high fatigue resistance depend on a fine homogeneous microstructure in the final product. Efficiency in gear manufacturing can be optimized by increasing the temperature during processing, which allows for lower forging loads and lower die stresses, thus improving die life in terms of mechanical fatigue. Additionally, increasing the temperature during case hardening reduces the process duration significantly. Hence process efficiency also increases. To meet the need of a fine homogenous microstructure, dynamic recrystallization has to be initiated during hot forging and grain growth has to be avoided during dwell times and case hardening. This grain size control can be achieved by applying micro-alloying concepts. Recently, an Nb-Ti-based alloying concept for case hardening steels was introduced, which increases fine grain stability and therefore potentially allows for higher forging and case hardening temperatures, leading to improved process efficiency [1]. In this paper a 25MoCr4-Nb-Ti steel grade is characterized in terms of flow resistance and microstructure evolution by hot compression tests and annealing experiments. The processing limits of this material in terms of abnormal grain growth are determined and a JMAK-based microstructure model considering these limits is presented and implemented in the FE-Software DEFORM 3dTM. The model is used in a case study to design a laboratory scale forging process for lowest possible die stresses and finest possible grain sizes. Experimentally measured grain sizes and forging loads from forgings at the laboratory scale are used to evaluate the process design. It is shown that considering microstructure evolution in process design is absolutely necessary to jointly optimize for process efficiency and final properties. The application of the Nb-Ti-based micro-alloying concepts allows for lower die stresses and thus seems to reduce mechanical fatigue of the dies compared to conventional case-hardening steels. [1] S. Konovalov et. al.: Testcase gearing component. In: G. J. Schmitz, U. Prahl (Ed.): Integrative Computational Materials Engineering, Wiley-VCH Verlag GmbH &amp; Co. KGaA, 2012, ISBN 978-3-527-33081-2

Synthesis and characterization of lanthanum aluminate powders via a polymer complexing plus combustion route

Lanthanum aluminate powders were prepared by a simple polymer complexing plus combustion method using PVA or PEG as complexing agent and fuel. The influence of different polymers on phase purity, powder morphology and sintering performance were investigated. Trace amount impurity La2O3 exists in the PEG powder, but it could be eliminated after high temperature sintering. The pure phase LaAlO3 can be easily obtained in PVA powders calcined at 950°C even severe aggregation always exists. PEG shows advantages over PVA in terms of the densification and microstructure control during sintering process. The high relative density of 97.0% and homogeneous fine microstructure with grain size<3μm can be obtained in the PEG-derived sample sintered at 1600°C for 5h. To obtain better quality LaAlO3 powders through combustion route, PEG is preferred over PVA.

Spark plasma sintering of TiCN nanopowders in non-linear heating and loading regimes

Consolidation of commercially available nanostructured titanium carbonitride (TiCN) powder has been performed by Spark Plasma Sintering (SPS) in the temperature range from 1300 to 1600°C. The effect of non-linear heating and loading regimes on consolidation of high melting point nanocomposites has been investigated. SPS consolidated TiCN material has demonstrated near fully dense and fine homogeneous microstructure with average grains size about 150nm. Nanohardness and fracture toughness of the TiCN nanocomposite have been measured as 33±0.9GPa and 3.2MPam1/2 respectively.

Microstructure and wear resistance of TaC reinforced Ni-based coating by laser cladding

The in situ synthesized TaC particulate reinforced Ni-based composite coating was fabricated on a mild steel by laser cladding of powder mixture of Ni60 alloy powder with (Ta 2O 5 + C)-doping. The microstructure and wear resistance of the TaC/Ni60 composite coating were investigated. It is shown that the coating is bonded metallurgically to the substrate and has a homogeneous fine microstructure containing both approximate cubic TaC particle and acicular chromium carbide uniformly dispersed in the dual phase matrix of γ(Ni) solid solution and eutectic of Cr 3C 2, Fe 2B with γ(Ni). Compared to a Ni60 coating, the hardness of the TaC/Ni60 composite coating was enhanced by a factor of 1.38, could achieve a Vicker microhardness of Hv 0.31100. And the wear rate in a block on ring test against hardened steel was reduced by a factor of five. This is attributed to the presence of in situ synthesized TaC particles and their well distribution in the coating.