Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2006"

Quenching and partitioning martensite-a novel steel heat treatment

Abstract: A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.

Self-deployable origami stent grafts as a biomedical application of Ni-rich TiNi shape memory alloy foil

Abstract: This paper describes the design, manufacturing and properties of a new type of stent graft, the origami stent graft. Unlike conventional stent grafts which consist of a wire mesh stent and a covering membrane, the new origami stent graft is made from a single foldable foil with hill and valley folds. The Ni-rich titanium/nickel (TiNi) shape memory alloy (SMA) foil made by the newly developed ultrafine laminates method was used in order to produce the stent graft. The pattern of folds on the foil was produced by negative photochemical etching. The deployment of the stent graft is achieved either by SMA effect at the body temperature or by making use of property of superelasticity. A number of prototypes of the stent graft, which are the same size as standard oesophageal and aortal stent grafts, have been produced successfully. It was demonstrated that the stent graft deploy as expected.

Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features

Abstract: In the present work, the densification and microstructural evolution during direct laser sintering of metal powders were studied. Various ferrous powders including Fe, Fe–C, Fe–Cu, Fe–C–Cu–P, 316L stainless steel, and M2 high-speed steel were used. The empirical sintering rate data was related to the energy input of the laser beam according to the first order kinetics equation to establish a simple sintering model. The equation calculates the densification of metal powders during direct laser sintering process as a function of operating parameters including laser power, scan rate, layer thickness and scan line spacing. It was found that when melting/solidification approach is the mechanism of sintering, the densification of metals powders ( D ) can be expressed as an exponential function of laser specific energy input ( ψ ) as ln(1 − D ) = − Kψ . The coefficient K is designated as “densification coefficient”; a material dependent parameter that varies with chemical composition, powder particle size, and oxygen content of the powder material. The mechanism of particle bonding and microstructural features of the laser sintered powders are addressed.

Grid indentation analysis of composite microstructure and mechanics: Principles and validation

Abstract: Several composites comprise material phases that cannot be recapitulated ex situ, including calcium silicate hydrates in cementitous materials, hydroxyapatite in bone, and clay agglomerates in geomaterials. This requirement for in situ synthesis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. Current advances in experimental micro and nanomechanics have afforded new opportunities to explore and understand the effect of thermochemical environments on the microstructural and mechanical characteristics of naturally occurring material composites. Here, we propose a straightforward application of instrumented indentation to extract the in situ elastic properties of individual components and to image the connectivity among these phases in composites. This approach relies on a large array of nano to microscale contact experiments and the statistical analysis of the resulting data. Provided that the maximum indentation depth is chosen carefully, this method has the potential of extracting elastic properties of the indented phase which are minimally affected by the surrounding medium. An estimate of the limiting indentation depth is provided by asssuming a layered, thin film geometry. The proposed methodology is tested on a “model” composite material, a titanium-titanium monoboride (Ti–TiB) of various volumetric proportions. The elastic properties, volume fractions, and morphological arrangement of the two phases are recovered. These results demonstrate the information required for any micromechanical model that would predict composition-based mechanical performance of a given composite material.

Effect of block size on the strength of lath martensite in low carbon steels

Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.

Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

Abstract: Ultrafine grained steels with grain sizes below about 1 μm offer the prospect of high strength and high toughness with traditional steel compositions. These materials are currently the subject of extensive research efforts worldwide. Ultrafine grained steels can be produced either by advanced thermomechanical processes or by severe plastic deformation strategies. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained bcc steels, the corresponding microstructures, and the resulting spectrum of mechanical properties.

Three defect types in friction stir welding of aluminum die casting alloy

Abstract: For different tool plunge downforces, the optimum FSW conditions of aluminum die casting alloy were examined. The higher the tool plunge downforce is, the wider the range of the optimum FSW conditions is. The following three different types of defects are formed, depending on the FSW conditions. (1) A large mass of flash due to the excess heat input; (2) cavity or groove-like defects caused by insufficient heat input; and (3) cavity caused by the abnormal stirring. As for the abnormal stirring, it is clearly seen that the shape of the top part on the advancing side in the stir zone is completely different. For this type of defect, the effect of the tool plunge downforce is small, though the size of the defect due to insufficient heat input significantly decreases with the increasing downforce.

A review of wet impregnation—An alternative method for the fabrication of high performance and nano-structured electrodes of solid oxide fuel cells

Abstract: Development of solid oxide fuel cells (SOFC) for operation at intermediate temperatures of 600–800 °C with hydrocarbon fuels requires a cathode and anode with high electrocatalytic activity for O2 reduction and direct oxidation of hydrocarbon fuels, respectively. Wet impregnation is a well known method in the development of heterogeneous catalysts. Surprisingly, very few have concentrated on the application of the wet impregnation technique to deposit nano-sized particles into the established electrode structure of the SOFC. This paper reviews and discusses the progress in the application of the wet impregnation technique in the development of Ni-free Cu-based composite anodes, doped CeO2-impregnated (La, Sr)MnO3 (LSM) cathodes and Ni anodes, Co3O4-infiltrated cathodes and precious metal-impregnated electrodes. Enhancement in the electrode microstructure and cell performance is substantial, showing the great potential of the wet impregnation method in the development of high performance and nano-structured electrodes with specific functions. However, the long-term stability of the impregnated electrode structure needs to be addressed.

Effect of Zr and Sn on Young's modulus and superelasticity of Ti-Nb-based alloys

Abstract: Quaternary Ti–(20–26)Nb–(2–8)Zr–(3.5–11.5)Sn (wt%) alloys were investigated to evaluate the effects of Zr and Sn on Young's modulus and superelasticity of Ti–Nb-based alloys. X-ray diffraction analysis showed that solution-treated alloys have β + α″, β + ω, α″ + ω, α″, or β microstructures. Zr and Sn increase the lattice parameters of the β phase; for orthorhombic α″ matensite, they increase the lattice parameter a but decrease both b and c . The martensitic start temperature of the α″ is depressed by Zr and Sn additions, whereas the formation of athermal ω is dependent on Zr and Sn contents. Differential scanning calorimetry (DSC) measurements show that 1 wt% of Nb, Zr or Sn addition decreases the martensitic start temperature by 17.6, 41.2 or 40.9 K, respectively, due to their negative effect on lattice parameter ratios of the martensite ( c / a and b / a ). Tensile tests were used to evaluate Young's modulus and superelasticity of the solution-treated alloys. Of the studied alloys Ti–24Nb–4Zr–7.5Sn with single β microstructure has the lowest Young's modulus of 52 GPa and recoverable elastic strain of about 2% at room temperature after cyclic strain.

Friction stir welding of carbon steels

Abstract: In order to determine the effect of the carbon content and the transformation on the mechanical properties and microstructures of the FSW carbon steel joints, three types of carbon steels with different carbon contents (IF steel, S12C, S35C) were friction stir welded under various welding conditions. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. The strength of the S12C steel joints increases with the increasing welding speed (decreasing the heat input), while the strength of the S35C steel joints shows a peak near 200 mm/min. This can be explained by the relationship between the peak temperature and the A 1 and A 3 points. When friction stir welding is performed in the ferrite–austenite two-phase region, the microstructure is refined and the highest strength is then achieved.

Development and characterization of Ni-free Ti-base shape memory and superelastic alloys

Abstract: Recently the Ni-hypersensitivity and toxicity of Ni have stimulated the development of Ni-free shape memory alloys. The β-Ti alloys are the most attractive candidates for biomedical shape memory alloys. Ti–Nb–X (X = Zr, Ta, Mo, Au, Pd, Pt, Al, Ga, Ge, O) and Ti–Mo–X (X = Ta, Nb, Zr, Au, Pd, Pt, Al, Ga, Ge) alloys have been developed and their shape memory effect and superelasticity were investigated systematically by the present authors for about 5 years. Although shape memory effect and superelasticity observed in the Ti–Nb alloys, the low critical stress for slip deformation caused the superelasticity not to reveal a large strain at room temperature. However, low temperature annealing and an aging treatment were effective in improving superelasticity. Additions of alloying elements such as Zr, Ta, Mo, Au, Pt and Al were also effective in stabilizing the superelasticity. In this paper, the basic characteristics of Ti–Nb, Ti–Nb–Zr, Ti–Nb–Ta and Ti–Nb–O are to be briefly reviewed based on the recent works of the present authors.

MWCNTs/AZ31 surface composites fabricated by friction stir processing

Abstract: Multi-walled carbon nanotubes (MWCNTs) were successfully dispersed into a magnesium alloy (AZ31) using friction stir processing (FSP). Distribution of the MWCNTs was changed on the basis of the travel speed of the FSP tool. The grain size of the MWCNTs/AZ31 surface composites was smaller than that of the FSPed AZ31 without the MWCNTs. The addition of the MWCNTs appears effective for fabricating the composites consisting of fine matrix grains. The maximum microhardness of these composites was ∼78 Hv, which is almost double that of the AZ31 substrate (41 Hv). It is considered that both the grain refinement of the AZ31 matrix and the reinforcement by the MWCNTs increased the microhardness of the surface composites.

A continuum based fem model for friction stir welding—model development

Abstract: Although friction stir welding (FSW) has been successfully used to join materials that are difficult-to-weld or unweldeable by fusion welding methods, it is still in its early development stage and, therefore, a scientific knowledge based predictive model is of significant help for thorough understanding of FSW process. In this paper, a continuum based FEM model for friction stir welding process is proposed, that is 3D Lagrangian implicit, coupled, rigid-viscoplastic. This model is calibrated by comparing with experimental results of force and temperature distribution, then is used to investigate the distribution of temperature and strain in heat affect zone and the weld nugget. The model correctly predicts the non-symmetric nature of FSW process, and the relationships between the tool forces and the variation in the process parameters. It is found that the effective strain distribution is non-symmetric about the weld line while the temperature profile is almost symmetric in the weld zone.

Microstructure evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250°C

Abstract: The microstructural evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C, has been investigated using transmission electron microscopy. The decomposition of α-Mg supersaturated solid solution (S.S.S.S., cph) in the alloy with increasing aging time is as follows: β″ (D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of Mg–Gd–Y, Mg–Gd–Nd and Mg–Y–Nd alloys, but different from previously reported three stage sequence: S.S.S.S. → β″ (D019) → β′(cbco) → β(fcc). It is found that the metastable β″ and β′ phases coexist in the matrix at the very early stage of aging. Peak age-hardening is attributed to the precipitation of prismatic β′ plates in a triangular arrangement. At the over-aged stage, β1 phase appears to take place via an in situ transformation from a decomposed β′ phase but grows in a direction different from the previous one of β′ phase. Continued aging makes the β1 phase transform in situ to the equilibrium β phase and the orientation relationship between the precipitate and matrix phases is retained through the in situ transformation of the β1 phase.

Retardation of crack initiation and growth in austenitic stainless steels by laser peening without protective coating

Abstract: Laser peening without protective coating (LPPC) has been applied to water-immersed SUS304 (Type 304) and SUS316L (Type 316L) austenitic stainless steels. The surface residual stress of both materials was converted from tensile to compressive of several hundreds of megapascals by LPPC with a Q-switched and frequency-doubled Nd:YAG laser. The depth of the compressive residual stresses after LPPC exceeded 1 mm from the surface. Accelerating stress corrosion cracking (SCC) tests in a high-temperature and corrosive-water environment showed that LPPC completely prevented the SCC initiation of sensitized SUS304. SCC tests of pre-cracked samples were also performed for SUS304, which indicated that LPPC inhibits the propagation of the small pre-cracks. Rotating bending tests demonstrated that the fatigue strength of SUS316L with LPPC is enhanced by 1.4–1.7 times compared to that of the reference material at 108 cycles.

An experimental investigation of humidity and temperature effects on the mechanical properties of perfluorosulfonic acid membrane

Abstract: The mechanical properties of a perfluorosulfonic acid (PFSA) membrane have been investigated at different humidities and temperatures in a custom-designed environmental chamber. Tensile tests were conducted to determine Young's modulus, the proportional limit stress (“yield strength”), break stress, and break strain. In-plane dimensional changes of the membrane at different temperature and humidities were also determined. The results indicate that Young's modulus and the proportional limit stress of the PFSA membrane decrease as humidity and temperature increase. Higher temperature leads to lower break stress and higher break strain. However, humidity has little effect on the break stress and break strain. A nonparametric statistical analysis, Kruskal–Wallis test, is applied to the experimental results, which shows that the effects of temperature and humidity on Young's modulus and proportional limit stress are statistically significant.

Effect of friction stir processing with SiC particles on microstructure and hardness of AZ31

Abstract: SiC particles were uniformly dispersed into an AZ31 matrix by friction stir processing (FSP). The SiC particles promoted the grain refinement of the AZ31 matrix by FSP. The mean grain size of the stir zone with the SiC particles was obviously smaller than that of the stir zone without the SiC particles. The microhardness of the stir zone with the SiC particles was reached about 80 Hv due to the grain refinement and the distribution of the SiC particles. Additionally, the SiC particle/AZ31 region showed fine grains even at elevated temperatures (∼400 °C) resulting in the pinning effect by the SiC particles. In contrast, the microhardness was significantly decreased attributed to the abnormal grain growth of the FSPed AZ31 without the SiC particles.

Effect of nanosilica on characterization of Portland cement composite

Abstract: Both the filling effect and the pozzolanic reaction make siliceous materials as one of major ingredients of high-performance Portland cement-based composites. Hence, the introduction of nanosilica with finer particle size and larger silicon dioxide to the composite becomes a great deal of interest in recent years. In this study, a liquid-form of nanosilica particle with a spherical diameter of about 20 nm was incorporated into the Portland cement paste at five different dosages and analyzed at four different ages to identify the nanosizing effects on the microstructures and material properties of composite cement paste. Experimental results show that the Portland cement composite with 0.60% of added nanosilica by weight of cement has an optimum compressive strength, in which the increase of compressive strength is about 43.8%. Moreover, the corresponding nanosilica paste of one portion of water mixed with nanosilica of 1.08 wt.% of water has the maximum absolute value of zeta potential of 41.3 mV. Properties through the analyses of NMR, BET and MIP also indicate that the microstructure of Portland cement composite with nanosilica evidently has a more solid, dense and stable bonding framework.

An enhanced age hardening response in Mg–Sn based alloys containing Zn

Abstract: Additions of Zn and Zn + Na to an age hardenable Mg–1.3 Sn (at.%) alloy have been examined using hardness measurements and transmission electron microscopy. Zn additions resulted in a substantial increase (300%) in the hardening increment after aging at 200 °C but the time to peak hardness was relatively unaffected. The additions were found to have little effect on the number density of precipitates formed and no effect on the identity of precipitates but significant changes to particle morphology were observed. Combined additions of Zn and Na (an element previously illustrated to be an effective microalloying element for the Mg–Sn system) showed features typical of separate additions of Na and Zn: a large increase in the observed hardening increment, an acceleration of the time to peak hardness and a change in particle morphology. Furthermore, the combined Zn + Na additions resulted in synergistic effects on precipitate stability during overaging and the time to peak hardness. The stability of Mg 2 Sn precipitates formed in the Mg–Sn–Zn–Na alloy was much greater than those usually observed in the Mg–Sn–Na system and the time to peak hardness was much reduced from ∼58 h in the Mg–Sn–Na alloy to 7 h in the Mg–Sn–Zn–Na alloy.

Design of the friction stir welding tool using the continuum based FEM model

Abstract: In friction stir welding (FSW), the welding tool geometry plays a fundamental role in obtaining desirable microstructures in the weld and the heat-affected zones, and consequently improving strength and fatigue resistance of the joint. In this paper, a FSW process with varying pin geometries (cylindrical and conical) and advancing speeds is numerically modeled, and a thermo-mechanically coupled, rigid-viscoplastic, fully 3D FEM analysis able to predict the process variables as well as the material flow pattern and the grain size in the welded joints is performed. The obtained results allow finding optimal tool geometry and advancing speed for improving nugget integrity of aluminum alloys.

The investigation of typical welding defects for 5456 aluminum alloy friction stir welds

Abstract: The external factors on the friction stir welding defects are so abundant that the experiments of friction stir welding were conducted for 5456 aluminum alloy. With the changes of the tool tilt angle and material condition, defects can be generated. These defects can be conventional ones (lack of penetration or voids), or lazy S, which are unique to friction stir welding. However, the origin of the defects remains an area of uncertainty. In this study, an attempt has been made to investigate the formation of these defects. The typical welding defects of friction stir welding joint for 5456 aluminum alloy were analyzed and discussed, respectively, by using optical microscopy (OM), energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscope (SEM). The microscopic examination of the nugget zone and fracture location of the weld confirms that the tilt angle can change the plastic material flow patterns in the stir zone and accordingly control the weld properties. In addition, the oxide layer from the initial butt surface during FSW is dispersed at the grain boundary. These A12O3 particles are actually the major cause of failure of the joint.

Effect of tool shape on mechanical properties and microstructure of friction stir welded aluminum alloys

Abstract: Prospecting the optimal tool design for welding steels, the effect of the tool shape on the mechanical properties and microstructures of 5-mm thick welded aluminum plates was investigated. The simplest shape (column without threads), the ordinary shape (column with threads) and the triangular prism shape probes were used to weld three types of aluminum alloys. For 1050-H24 whose deformation resistance is very low, a columnar tool without threads produces weld with the best mechanical properties; for 6061-T6 whose deformation resistance is relatively low, the tool shape does not significantly affect the microstructures and mechanical properties. For 5083-O whose deformation resistance is relatively high, the weldablity is significantly affected by the rotation speed. For a low rotation speed (600 rpm), the tool shape does not significantly affect the microstructures and mechanical properties of the joints.

Microstructures and tensile behavior of carbon nanotube reinforced Cu matrix nanocomposites

Abstract: Carbon nanotubes (CNTs) have been considered as an ideal reinforcement to improve the mechanical performance of monolithic materials. However, the CNT/metal nanocomposites have shown lower strength than expected. In this study, the CNT reinforced Cu matrix nanocomposites were fabricated by spark plasma sintering (SPS) of high energy ball-milled nano-sized Cu powders with multi-wall CNTs, and followed by cold rolling process. The microstructure of CNT/Cu nanocomposites consists of two regions including CNT/Cu composite region, where most CNTs are distributed, and CNT free Cu matrix region. The stress–strain curves of CNT/Cu nanocomposites show a two-step yielding behavior, which is caused from the microstructural characteristics consisting of two regions and the load transfer between these regions. The CNT/Cu nanocomposites show a tensile strength of 281 MPa, which is approximately 1.6 times higher than that of monolithic Cu. It is confirmed that the key issue to enhance the strength of CNT/metal nanocomposite is homogeneous distribution of CNTs.

Growth of intermetallic layer in multi-laminated Ti/Al diffusion couples

Abstract: Solid-state reactive diffusion between Ti and Al was investigated in the temperature range of 520-650 degrees C by employing multi-laminated Ti/Al diffusion couples. In samples annealed up to 150 h intermetallic TiAl3 is the only phase observed in the diffusion zone and the preferential formation of this compound in Ti/Al diffusion couples was predicted using an effective heat of formation model. The present work indicated that both Ti and Al diffused into each other and the growth of the TiAl3 layers occurred mainly towards the Al side. The TiAl3 growth changes from parabolic to linear kinetics between 575 and 600 degrees C, characterized by activation energy of 33.2 and 295.8 kJ mol(-1), respectively. It is suggested that the low-temperature kinetics is dominated by the diffusion of Ti atoms along the grain boundaries of the TiAl3 layers, while the reaction at the TiAl3/Al interfaces in the high-temperature regime is limited by the diffusion of Ti atoms in the Al foils as a result of increased solubility of Ti in Al with increasing temperature. (c) 2006 Elsevier B.V. All rights reserved.

Design of powder metallurgy titanium alloys and composites

Abstract: Low cost and good performance are two major factors virtually important for Ti alloy development. In this paper, we have studied the effects of alloying elements, thermo-mechanical treatment and particle reinforcement on microstructures and mechanical properties of powder metallurgy (PM) Ti alloys and their composites. Our results indicate that low cost PM Ti alloys and their composites with attractive properties can be fabricated through a single compaction-sintering process, although secondary treatments are required for high performance applications. Three new PM Ti alloys and one TiC/Ti composite of high performance are developed, and new design principles are also proposed. For design of PM Ti alloys, addition of alloying elements has the beneficial effect of enhanced sintering and/or improved mechanical properties. For example, Fe element accelerates the sintering process, Mo and Al are good candidates for solution strengthening, and rare earth elements effectively increase the material ductility by scavenging oxygen from the Ti matrix. For the design of Ti-based composites, in situ formation of strengthening particles and solid solution hardening of the matrix both should be considered simultaneously for alloy development. Cr 3 C 2 is found to be a very suitable additive for processing particle reinforced Ti composites.

Synthesis and characterization of high volume fraction Al-Al2O3 nanocomposite powders by high-energy milling

Abstract: Al–Al2O3 metal matrix composite (MMC) powders with volume fractions of 20, 30, and 50% Al2O3 were synthesized by high-energy milling of the blended component powders. The particle sizes of Al2O3 studied were 50 nm, 150 nm, and 5m. A uniform distribution of the Al2O3 reinforcement in the Al matrix was successfully obtained after milling the powders for a period of 20 h at a ball-to-powder ratio of 10:1 in a SPEX mill. The uniform distribution of Al2O3 in the Al matrix was confirmed by characterizing these nanocomposite powders by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray mapping, and X-ray diffraction (XRD) techniques. © 2006 Elsevier B.V. All rights reserved.

Microstructure and crystallographic texture of the chitin-protein network in the biological composite material of the exoskeleton of the lobster Homarus americanus

Abstract: The exoskeleton of the lobster Homarus americanus is a multiphase biological composite material which consists of an organic matrix (crystalline α-chitin fibers and various types of non-crystalline proteins) and minerals (mainly calcite) In this study we discuss experimental data about the mesoscopic structure and the crystallographic texture (orientation distribution) of the α-chitin–protein fiber network in this material The synchrotron measurements reveal very strong crystallographic textures of the α-chitin According to these data, a large fraction of the α-chitin lattice cells is arranged with their longest axis parallel to the normal of the surface of the exoskeleton Additionally, a smaller fraction of the α-chitin cells is oriented with their longest axis perpendicular to the cuticle surface These structural investigations reveal the pronounced role of crystallographic orientation distributions in mineralized biological composite materials which may be of relevance for an improved understanding of biological and bio-inspired nano-composites

Mixed mode brittle fracture in PMMA—An experimental study using SCB specimens

Abstract: A series of mixed mode I/II fracture tests is conducted on polymethylmethacrylate (PMMA) in the full range from pure mode I to pure mode II using a semi-circular bend (SCB) specimen containing an edge crack. The fracture load and the path of crack growth are obtained from experiments for various crack angles. It is shown that the conventional mixed mode I/II fracture criteria such as the maximum tangential stress (MTS) criterion overestimate the fracture strength of PMMA when the SCB specimen is used for fracture tests, particularly for mode II dominant loading conditions. However, improved predictions of fracture load are achieved when a generalized MTS criterion is employed. While the path of crack growth is straight for pure mode I, it deviates significantly from the angle of fracture initiation for pure mode II and mode II dominant loading conditions. It is shown that the path of crack growth predicted by the generalized MTS criterion is also in a good agreement with the observed fracture path in the fractured SCB samples.

Effects of processing on microstructure and properties of SLS Nylon 12

Abstract: There currently exists the requirement to improve reproducibility and mechanical properties of SLS Nylon parts for rapid manufacturing (RM). In order to achieve this, further fundamental research is needed and this paper addresses this need by investigating effects of potential sources of the lack of reproducibility and reports effects in relation to crystal structure, microstructure, chemical structure (molecular weight) and mechanical properties. Different γ crystal forms were identified and related to the unmolten particle cores and the melted/crystallised regions of the microstructure. The melt point of the γ-form varied depending on processing conditions. Observable differences were also present when comparing the microstructure of the parts. Molecular weight of parts was significantly higher than virgin powder but used powder also showed an increase in molecular weight. This was related to improved elongation at break of parts built from the used powder, consistent with previous studies. Tensile strength showed some increase with machine parameters selected for improved strength but Young's modulus values were broadly similar.

Causes of breakdown of creep strength in 9Cr-1.8W-0.5Mo-VNb steel

Abstract: Premature breakdown of creep strength is a serious problem to be solved in long-term creep of advanced high Cr ferritic steels. The material studied was ASTM grade 92 steel crept at 550–650 °C for up to 63 151 h. Stress exponent for rupture life decreases from 17 in short-term creep to 8 in long-term creep, confirming the breakdown in the steel. The steel shows ductile to brittle transition with increasing rupture life, and the breakdown accords with the onset of brittle intergranular fracture. Creep cavities are nucleated at coarse precipitates of Laves phase along grain boundaries. These findings suggest the following story of the breakdown of creep strength. Laves phase precipitates and grows during creep exposure. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. The brittle fracture causes the breakdown. The coarsening of Laves phase can be detected non-destructively by means of hardness testing of the steel exposed to elevated temperature without stress.