Stochastic failure of isotropic,brittle materials with uniform porosity

Porous materials present serious technological constraints on all applications, such as battery electrodes, solid oxide fuel cells, synthetic bone grafts, filters, pharmaceutical powder compacts and feed pellets. Despite the significance of reliability in brittle materials, current literature is limited in pore–pore interaction effects on fracture statistics of brittle porous materials (BPMs). In this paper, a two-dimensional finite element (FE) simulation-based approach was developed to assess the pore–pore interactions and their impact on fracture statistics of isotropic microstructures. The classical fracture mechanics approach was combined with FE simulations that account for the interactions to predict the decrease in the fracture stress with increasing porosity. Rules were directly compared against experimental data for porous polycrystalline alumina, hydroxyapatite, and all the other data combined in Fig. 6. The maximum reliability of BPMs was shown to be limited by the underlying pore–pore interactions. Weibull modulus decreased more than threefold for a change in porosity from 1 to 2 vol.%. The Weibull moduli were between 7 and 18 in the range of 2–31 vol.% porosity. Even the microstructures with the same porosity level and size of pores showed substantial differences in fracture strength.     

Processing optimization,surface properties and wear behavior of HVOF spraying WC–CrC–Ni coating

In this work, the optimal coating process (OCP) designed by Taguchi program for high velocity oxy-fuel (HVOF) thermal spraying WC–CrC–Ni powder on Inconel 718 substrate (IN 718) is obtained by optimizing hardness (38 FMR oxygen flow rate, 53 FMR hydrogen flow rate, 25 g/min powder feed rate and 7 in. spray distance). Oxygen flow rate affects hardness mostly. The surface properties such as microstructure, crystalline phase, hardness, and porosity of WC–CrC–Ni coating have been investigated. The phase of coating has been changed during the OCP spraying because a portion of carbides, such as WC, Cr₇C₃, Ni₃C decomposes to W₂C, Cr, Ni and free carbon. Hardness (1150 ± 50 Hv) and porosity (1.2 ± 0.2%) of the OCP coating have been improved by optimization. The friction and wear behaviors of the WC–CrC–Ni coating, electrolytic hard chrome (EHC) plating and IN 718 have been studied comparatively. The lubrication due to free carbon and metal oxide debris results in a decrease of friction coefficients of the WC–CrC–Ni, compared to EHC and IN 718 at both 25 and 450 °C. It is concluded that HVOF WC–CrC–Ni coating performs more excellent anti-wear than others at both temperatures.     

Study on the microstructures,electrical resistance and mechanical properties of sputtering chromium target by HP,HIP and canning–HIP processes

In recent years, pure chromium (Cr) targets have been commonly used in metal surface coating treatment and flat-panel displays. In the traditional production of a pure Cr-target, there is a greater use of casting methods; however, the metal ingot frequently suffers from ingredient segregation, porosity and non-uniform microstructure defects. Powder metallurgy (PM) is a good method for fabricating high melting materials with better microstructure and properties. This study produced Cr targets using hot pressing (HP), hot isostatic pressing (HIP) and canning–HIP of PM technology. The experimental results showed that the Cr targets made by HP–HIP can further improve the density and mechanical properties. The relative density increases from 98.8% to 99.3%. Canning–HIP of the Cr target, in particular, can provide smaller grain size (50 μm), lower porosity about 0.3%, and increased relative density to 99.7%, with a TRS of up to 58 MPa. The canning–HIP process also shows the optimal electrical resistivity (8.003 × 10^− 5 Ω-cm), which suits for applications in the sputtering process.     

Pore formation mechanism and characterization of porous NiTi shape memory alloys synthesized by capsule-free hot isostatic pressing

Porous NiTi alloys with different porosities were fabricated by capsule-free hot isostatic pressing (CF-HIP) with ammonium acid carbonate (NH₄HCO₃) as a space-holder. The microstructure and porosity of porous NiTi produced with different NH₄HCO₃ contents and sintering temperatures were determined. Two different creep expansion models are used to explain the pore expansion mechanism during the sintering process, which involves slow and continuous reduction of the argon pressure at high temperatures. When the NH₄HCO₃ content is 30 wt.% and the sintering temperature is 1050 °C, an ideal porous NiTi alloy with 48 vol.% porosity and circular pores (50–800 μm) is obtained. Compression tests indicate that the porous NiTi alloys with 21–48% porosity possess not only lower Young’s moduli of 6–11 GPa (close to that of human bones) but also higher compression strength and excellent superelasticity. Cell cultures reveal that the porous NiTi prepared here has no apparent cytotoxicity. The porous materials are thus promising biomaterials in hard tissue replacements.     

Effect of annealing on percolating porosity in ultrafine-grained copper produced by equal channel angular pressing

Percolating porosity as a specific type of deformation-induced was discovered in ultrafine-grained (UFG) Cu produced by equal channel angular pressing (Ribbe et al., Phys Rev Lett 2009;102:165501). The stability of this defect type against annealing under various conditions is investigated for UFG Cu of different purity levels. The porosity is found to withstand the annealing treatments up to 1073 K for several hours in purified Ar atmosphere, despite significant microstructure transformation. Annealing at 1313 K in Ar removes the percolating porosity, as do relatively short heat treatments at 427 K in a hydrogen-containing atmosphere. Quasi-hydrostatic pressure applied at moderate temperatures, e.g. 1 GPa at 423 K, eliminates the percolating porosity, too. A model of porosity evolution, which accounts for the experimental findings, is suggested.     

Development of forging process design to close internal voids

In this study, the closing behavior of internal voids was examined by a deformation analysis involving the 2-D finite element method (FEM), which simulates voids in steel ingots in the compression process (upset process). In the compression process, a model experiment that uses internal voids was carried out to confirm the accuracy of the deformation analysis. By comparing the model experiment with the analytical results, it was confirmed to simulate the internal void behavior by this analysis. The relationship between the reduction ratio and the void shape/void position was investigated by the analysis. In the forging process, the closing evaluation value of internal voids (Q value) was calculated by a model experiment and 3-D FEM. Using the analysis results, a limit value of the closing behavior of voids was quantified, and it is now understood that the voids close at more than Q = 0.21. In addition, the forging process of filling the above-mentioned value was designed by the Taguchi method. The predicted Q value in the case of using the Taguchi method almost corresponds to the value calculated by the deformation analysis. It was clarified that the process is capable of being designed simply.     

Microstructure characterization of large TiC-Mo-Ni cermet tiles

A detailed characterization of large (150 mm × 150 mm), 6 to 12 mm thick, commercially produced tiles of a TiC-Mo-Ni cermet with ~ 13 vol% Ni binder and a microstructure consistent with processing via self-propagating high temperature synthesis (SHS) has been conducted. Many mechanical property defining attributes of the materials were highly reproducible, including the composition, phase content, TiC particle size distribution (with an average particle diameter of 8–10 μm), and density of 5.52 × 10³ kgm^− 3. However, sufficient variability in the distribution of metal elements within the carbide particles, interparticle contiguity, the distribution of porosity, and residual stress were discovered that the mechanical behavior is expected to exhibit significant variability. The spheroidal shaped TiC particles had a multilayered (onion ring like) composition with rings of locally higher Mo concentration, rather than the more usual Ti-rich core and a single Mo-rich rim that enhances wetting with the Ni-binder. The TiC particles also had a high contiguity factor of 0.30–0.47. Recent assessments of liquid phase sintered cermets indicate significant loss of fracture resistance as the contiguity increases above 0.25. Hot isostatic pressing (HIP) at 1250 °C and 100 MPa was unable to reduce the porosity, which remained as large pockets of insufficient metal binder material (a form of shrinkage porosity) between the spheroidal carbide particles. X-ray diffraction measurements indicated the presence of significant residual stress in the as-received and the HIP condition materials. A stress relief heat treatment at 900 °C succeeded in eliminating this residual stress consistent with its origination from thermal gradients associated with rapid cooling.     

Taguchi analysis on the effect of process parameters on densification during spark plasma sintering of HfB2-20SiC

Field assisted sintering (FAST) has emerged as a useful technique to densify ultra high temperature ceramics like HfB₂-20SiC to a high density at relatively low temperatures and shorter times. The effect of various process variables on the densification during spark plasma sintering of HfB₂-20SiC was studied using Taguchi analysis. The statistical analysis identified sintering temperature as the most significant parameter affecting the densification of HfB₂-20SiC material. A density of 99% was achieved on sintering at 2373 K for 8 min at 30 kN pressure and heating rate of 100 K/min.     

Performance of coated tungsten carbide tools on milling printed circuit board

This study investigates the performance of the ZrN tools coated by the pulsed-DC reactive magnetron sputtering on milling memory modules of PCB (printed circuit board). A speed-adjustable rotation system was designed to find out the optimal process parameters through grey relational analysis and Taguchi method, such as the sputtering distance, flow rate of nitrogen and rotational speed of tool. From the sputtering experiment, the two parameters having greatest effects are the ratio of argon to nitrogen as well as the rotational speed of tool. Within the range of the experimental equipments, the optimal ratio of argon to nitrogen flow rate is 10:4 (sccm), and the optimal rotational speed of tool is around 8 rpm. The average micro-hardness of the surface of the ZrN-coated tools are increased from 15 GPa to 21 GPa compared with uncoated tools by nano-indentator measurement. From the cutting experiments, the cutting tool life of an uncoated tungsten carbide tool is around 38 m, and that of a ZrN-coated tool is around 115 m. And the statistical process capability index (Cpk) of the coated tool also rises from 0.788 to 1.858. It is observed that a ZrN-coated tool not only can extend its used life, but also can improve the cutting quality.     

Effect of tool rotating rate on foaming properties of porous aluminum fabricated by using friction stir processing

A1050 porous aluminum is fabricated by the FSP route and the effect of the tool rotating rate on the porosity and morphology of the pores is investigated. To fabricate high-porosity porous aluminum with a uniform pore size distribution, a certain amount of stirring action is necessary; however, excessive stirring action is ineffective. A sufficiently uniform mixture is realized by traversing the FSP tool two times at a tool rotating rate exceeding 2200 rpm. The results indicate the minimum necessary amount of stirring action and will provide a guideline for improving productivity. Also, to improve the morphology of pores, optimizing the amount of Al₂O₃ is effective. Closed-cell porous aluminum with a porosity of about 80% was successfully fabricated by 2-pass FSP at 2200 rpm with the addition of 7 mass% Al₂O₃, a holding temperature of 998 K and a holding time of 10 min.     

Mechanical behavior of cellular borosilicate glass with pressurized Ar-filled closed pores

High strength borosilicate foams were fabricated by melting glass powder under high-pressure argon gas and subsequent heat treatment of the glass bulk at atmospheric pressure. In the first step, borosilicate glass powder was melted at 1100 °C for 1 h by capsule-free hot isostatic pressing (HIPing) under a high gas pressure of 10–70 MPa. Pressurized Ar-filled spherical pores were introduced into the glass, and argon atoms were dissolved in the glass network structure. The expansion of argon-filled pores and the release of the dissolved Ar gas resulted in the formation of pressurized Ar-filled closed pores by isothermal heat treatment at 800 °C for 10 min. A high porosity of up to 80% with a bimodal distribution of micro-size cells was obtained for the resultant cellular borosilicate glass. By increasing the total gas pressure from 10 to 70 MPa, the compressive strength and the Young’s modulus were increased considerably from 15 to 52 MPa and from 4.1 to 12.6 GPa, respectively, which can be substantially attributed to the high collapse stress from the high enclosed gas pressure. The cellular glass with a high porosity showed a large failure strain under uniaxial compression.     

The effect of additive and sintering mechanism on the microstructural characteristics of W–40Cu composites

In this research, effect of cobalt and nickel additives on the W–40wt.% Cu composites prepared by solid phase sintering and infiltration (SPS + I) as well as liquid phase sintering (LPS) processes has been investigated. For this purpose, three types of powder consist of pure tungsten, mixture of tungsten–1wt.%Co and mixture of tungsten-1wt.%Ni were separately prepared and compacted by cold isostatic pressing (CIP). In the SPS + I process, compacted specimens were sintered at 1100 °C for 1 h and subsequently infiltrated by liquid copper at 1250 °C for 1 h. In the LPS process, compacted samples were directly infiltrated without initial sintering. Density of samples was measured by Archimedes method. Microstructure (i.e. contiguity, porosity, grain size) and chemical composition were studied by SEM and EDS, respectively. It is found that microstructural characteristics of the W–40wt.%Cu composites depend on sintering mechanism as well as additive type. Density of samples prepared by LPS process was higher in compared with ones obtained via SPS + I process. This behavior was related to W–W contiguity as well as tungsten particles wettability.     

Micro cutting of V-shaped cylindrical grating template for roller nano-imprint

The roller nano-imprint technology is considered as efficient and low cost method for the large scale micro-pitch grating fabrication. The surface quality of the V-shaped grooves for roller grating template developed by diamond tool cutting process is investigated. The surface quality of the micro V-shaped grating is optimized with minimized burr formation by optimizing the cutting tool's shape design and the micro-cutting process, based on the chip morphology and non-free chip interference analyses. Burr shows different variation with the grating pitch of 10–20 μm and less than 2 μm. The burr formation mechanism for V-shaped micro-cutting is studied by material (H62 Copper) metallography and homogeneity analyses. Finally, high precision grating template with various pitches is successfully achieved.     

Microstructure and electrochemical properties of the bonding zone of AISI 316L steel joined with a Fe-based amorphous foil

The effect of temperature and holding time on the microstructure and corrosion resistance of the junction zone of AISI 316L stainless steel (SS) bonded to itself with Fe₇₅Cr₈P₁₀B₇ filler alloy was investigated. The brazing alloy was prepared in the laboratory in the form of amorphous ribbons and its melting temperature was determined by differential thermal analysis (DTA) to be 1571 K. The joining process was carried out in a chamber with controlled Ar atmosphere at two temperatures: 1173 and 1273 K for different holding times not exceeding 40 min. Joining took place by the mechanism of diffusion bonding. The joints produced at 1273 K for 40 min exhibited no porosity in the reaction zones and presented the best quality. Scanning electron microscopy (SEM) characterization of the bonding zone revealed an improvement in the quality of the joints brazed at 1173 K for 20 min and longer. These samples had continuous base metal–filler alloy interfaces with minimum porosity. At 1273 K the bonding interfaces diffused and for the samples held for 40 min completely vanished and porosity disappeared. Even the presence of particle precipitates the bonding zone showed acceptable resistance to localised corrosion in non-aggressive electrolytes. SEM study revealed that irregularly precipitated particles and other phases of about 10 μm in size formed in the interlayer during the joining process. The presence of σ-phase in samples bonded at 1273 K promoted preferential dissolution in the bonding zone in NaCl solution.     

Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering

Selective laser sintering (SLS) technique is capable of rapidly fabricating customized implants with porous structure. A simple encapsulation process was developed to coat 316L stainless steel (316L SS) powder with ethylene-vinyl acetate copolymer (EVA). Subsequently, porous 316L SS was prepared by SLS preforming of EVA-coated metal powders, debinding and sintering in hydrogen atmosphere. The effects of processing parameters on pore characteristics and mechanical properties were analyzed. The results indicate that the porosity of green body mainly depends on laser energy density, while the pore features and mechanical properties of sintered specimens are largely dominated by sintering temperature. After sintering at 1100–1300 °C, the average pore size and porosity are 160–35 μm and 58–28%, respectively. In addition, the elastic modulus and compressive yield strength are 1.58–6.64 GPa and 15.5–52.8 MPa, respectively. It is revealed that the pore structural parameters and mechanical properties of the as-sintered porous 316L SS can be controlled readily to match with those of cancellous bone by modification of SLS processing parameters and subsequent sintering temperature.     

Improving the uniformity in mechanical properties of a sintered Ti compact using a trace amount of internal lubricant

Large sintered powder compacts are likely to be associated with variability in mechanical properties; an improvement of the uniformity of the mechanical properties of sintered powder compacts is important for powder metallurgy. In this work 0.3–1 wt.% stearic acid (SA) or magnesium stearate (MgSt) was added to a 40 mm diameter Ti powder compacts with height to depth (H/D) ratio of unity to give a more uniform green density. Tensile test pieces were cut from selected positions in each sintered compact to obtain the distribution of mechanical properties. Results revealed that variations in mechanical properties are due to the pore morphology with respect to size, aspect ratio and preferred orientation. A trace amount of lubricant significantly improves the uniformity in mechanical properties by optimizing the porosity distribution and minimizing the pore size and aspect ratio of pores after sintering. Such an effect was achieved by reducing the initial green density inhomogeneity and the stress induced by the mismatch of sintering shrinkage. However a relatively high 1 wt.% SA addition with a large particle size created burnt-off pores in the top and bottom zones. MgSt is not recommended since it significantly increases the oxygen content. An addition of 0.6 wt.% SA is the best choice due to the even pore distribution, small pore size and acceptable level of oxygen pick up.     

Grain growth inhibition by connected porosity in sintered niobium

Pore–boundary interaction plays an important role in densification during solid-state sintering. This paper reports the evolution of porosity and grain size in niobium sintered at 2073 and 2273 K for different sintering times. The densification curves show a decrease in porosity up to 8 and 4 vol.% after 10,800 s for the samples sintered at 2073 and 2273 K, respectively. Grain growth is observed to take place together with this decrease in porosity. A new model for grain growth inhibition during sintering is proposed for connected porosity. This model considers that the moving grain boundaries and the outer surface of cylindrical pores remain in contact during grain growth and that energy dissipation takes place owing to the fact that the grain boundary is moving relative to the porosity. Our mechanism is akin to a friction between the grain boundary and the connected porosity at their contact region. In contrast to the traditional particle–grain boundary bypassing mechanisms, the present model is not a purely geometrical relationship but is material dependent. The model gives agrees well with experimental results obtained in this paper for sintered niobium as well as for other sintered materials reported in the literature. Our model is a novel approach to treating grain growth inhibition by pores during the sintering stage in which the porosity becomes interconnected.     

Effect of Ni content on the compression properties and abrasive wear behavior of the (TiB2–TiCxNy)/Ni composites

The (TiB₂–TiC_xN_y)/Ni composites were fabricated by the method of combustion synthesis and hot press consolidation in a Ni–Ti–B₄C–BN system. The effect of Ni content on the microstructure, hardness, compression properties and abrasive wear behavior of the composites has been investigated. The results indicate that with the increase in Ni content from 30 wt.% to 60 wt.%, the average size of the ceramic particles TiB₂ and TiC_xN_y decreases from 5 μm to ≤ 1 μm, while the hardness and the abrasive wear resistance of the composites decrease. The composite with the Ni content of 30 wt.% Ni possesses the highest hardness (1560.8 Hv) and the best abrasive wear resistance. On another hand, with the increase in the Ni content, the compression strength increases firstly, and then decreases. The composite with 50 wt.% Ni possesses the highest compression strength (3.3 GPa). The hardness and fracture strain of the composite with 50 wt.% Ni are 1251.2 Hv and 3.9%, respectively.     

Effect of starch filler content and sintering temperature on the processing of porous 3Y–ZrO2 ceramics

A direct casting process was used to produce porous 3Y–ZrO₂ ceramics using starch as a fugitive filler and binder. The compositions with low additions of starch had higher porosity than the volume fraction of starch initially in the green body (X_st), whereas, the compositions with high amounts of starch produced lower porosity than the predicted value. The well ordered structure consisted of spherical pores of 8–10 μm diameter, retained from the original starch particles, connected by channels. The interconnection between pores was dependent on the volume fraction of starch incorporated, as well as on the sintering temperature. Pore interconnection was observed for all the compositions sintered at 1000–1300 °C. Increasing the sintering temperature to 1400–1500 °C produced a marked dependence of the open to total porosity ratio on X_st. For a high porosity material, a bimodal channel size distribution was found at 1400 and 1500 °C. The primary pore channel diameter was 0.7 μm and the secondary one was close to 4 μm. As the sintering temperature increased, the volume of the connecting channels decreased; at 1500 °C only a minor volume of the larger channels was found.     

A TEM study on the crystallization behavior of an yttrium-doped Zr-based bulk metallic glass

Transmission electron microscopy (TEM) is used to conduct the systematic study of the annealing induced crystallization, both continuously and isothermally, of a Zr-based metallic glass. Through detailed microstructure analysis, it is found that the crystallization of this metallic glass is initialized by a nano-scaled primary crystallization process with a tetragonal structured crystal (a = 0.96 nm and c = 2.82 nm) as the primary phase. A eutectic crystallization is followed afterwards with two types of crystalline phases as the crystallization products, one of which is determined to be a metastable orthorhombic phase (a = 0.69 nm, b = 0.75 nm and c = 0.74 nm). Upon annealing at a raised temperature or isothermal treatments, a solid state phase transformation takes place and the orthorhombic metastable phase transforms into two types of tetragonal crystalline phases. The whole crystallization process of this metallic glass is in turn realized, and the thermal stability and nano-crystallization mechanism are discussed based on the microstructure and thermal analyses.