As-processed Samples Research Articles

Strengthening and failure mechanisms during tension of a Ti–6Al–4V alloy-based nanocomposite processed by laser powder bed fusion

The present work aims to clarify the tensile behavior of a novel metallic matrix composite of Ti–6Al–4V alloy reinforced with 1 wt% nano-yttria-stabilized zirconia, processed by laser powder bed fusion technology. The as-processed samples were post-processed using notably hot isostatic pressing in order to investigate the most promising material for actual structural applications. The cracking behavior is governed by the activation of prismatic slip systems in primary α-Ti grains. Under horizontal loading configuration (i.e., perpendicular to building direction), average yield stress and ultimate tensile strength of 999 and 1195 MPa have been found, respectively. Strengthening mechanisms caused by (i) solid solution due to raising oxygen concentration and (ii) dispersion hardening have resulted in similar yield stress increases of 108 and 99 MPa, correspondingly. Furthermore, the effect of the texture on the mechanical properties of the investigated material was assessed via a statistical approach, which forecasts a strong strength anisotropy depending on the loading direction. In addition, a particular attention has been paid to the effect of nano-yttria-stabilized zirconia addition on strain-hardening ability, underlining that the enhanced amount of solute oxygen yielded a strong strain hardening in stage II of plastic deformation.

Mechanical properties and deformation mechanisms of martensitic Ti6Al4V alloy processed by laser powder bed fusion and water quenching

Martensitic Ti6Al4V samples were fabricated by laser powder bed fusion additive manufacturing, water quenching of wrought and the additively manufactured Ti6Al4V samples respectively, their microstructural characteristics, tensile properties and deformation behaviours have been investigated in this study.These samples showed differences in grain orientations, size of the prior β grains, thickness of the martensite (α′) laths, the lattice strain and presence of the β phase between the α′ laths. A relationship between the yield strength and the thickness of α′ laths and lattice strain has been developed.The significant difference in the ductility in terms of the total strain at fracture among the martensitic Ti6Al4V samples processed by the different routes has been observed and investigated. The additively manufactured Ti6Al4V showed the highest post-uniform plastic deformation (deformation after necking) with rapid reduction of stress, its failure mode was dominated by the inter-α′ quasi-cleavage and intra-α′ ductile fractures. The water quenched-wrought Ti6Al4V displayed the negligible post-uniform plastic deformation, the inter-prior β cleavage fracture was its dominant failure mode. The water quenched-additively manufactured Ti6Al4V produced the highest yield strength and medium post-uniform plastic deformation, during which the reduction of stress was much slower, its failure mode was dominated by the inter-α′ quasi-cleavage fracture.Plastic deformation was mainly taken by the sliding between α′ laths, however, grain reorientation and slip in α′ with the different scales in the martensitic Ti6Al4V processed by the different routes were also observed with the evidence of the changes in the relative intensities and shift of 2θ angles for the diffraction peaks in X-ray diffraction (XRD) spectra in the fractured samples compared to the as-processed samples.

Parameter identification of micron-sized freestanding stretchable electronic interconnects using integrated digital height correlation

For the development of reliable stretchable electronic systems, it is essential to comprehend and predict their mechanical behavior. It is important to test and analyze original as-processed samples, as opposed to standard tests on bulk material. Dedicated analysis methods are necessary for obtaining the material properties from the tests, as complex 3D deformations complicate the use of existing methods. This paper presents an integrated digital height correlation (IDHC) method for the mechanical characterization of a recently developed ultra-stretchable freestanding interconnect. Height maps from an out-of-plane loading experiment are correlated to a numerical model, with the aim to identify the material parameters in the plastic regime. The IDHC method is tested on a virtual test case, where it is shown that the algorithm converges for the considered three plasticity parameters. For the real experiment, simultaneous correlation of all three parameters is not possible due to an inherently flat residual landscape with many local minima. However, the initial yield strength and hardening exponent were still identified and estimated at 225–300 MPa and 0.15–0.2 respectively. Despite the moderate accuracy of the identification, the potency of the IDHC method for this extremely challenging case of micron-sized delicate freestanding stretchable electronic interconnects is demonstrated.

Radiation-induced effects on micro-scratch of ultra high molecular weight polyethylene biocomposites

Sterilization of ultra-high molecular weight polyethylene (UHMWPE) based composites used for acetabular cup liner via UV or gamma-irradiation before surgery is inevitable. Thus, understanding the alteration of mechanical properties and wear resistance of cup liner via irradiation process requires investigation. This paper aims to understand the effect of UV (0.03 J/cm2) and gamma (25 kGy) irradiation on mechanical properties and scratch resistance of compression molded UHMWPE composites reinforced with alumina (Al2O3), hydroxyapatite (HAp) and carbon nanotubes (CNTs). After irradiation, nearly 100% increased crystallinity of polyethylene, due to recrystallization, helped in enhancing the hardness and elastic modulus by ~1.5 times compared to that of as-processed UHMWPE (Hardness: ~70 MPa and Elastic modulus: ~1.25 GPa). The recrystallization was not favored by the presence of nano-reinforcements in irradiated UHMWPE based nanocomposites, and caused deterioration in the mechanical properties. The micro-scratch results revealed the remarkable wear resistance (i.e., 1.5 to 2 times lower wear rate) of irradiated samples than that of as-processed samples. Mouse fibroblast L929 in vitro cell culture test confirmed the cytocompatibility of irradiated samples. This study indicated that the UV and gamma irradiated UHMWPE nanocomposites are the challenging candidates as acetabular cup liner.

Development of process parameters for selective laser melting of a Zr-based bulk metallic glass

Parameters for selective laser melting of Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMZ4), allowing crack-free bulk metallic glass with low porosity, have been developed. The phase formation was found to be strongly influenced by the heating power of the laser. X-ray amorphous samples were obtained with laser power at and below 75 W. The as-processed bulk metallic glass was found to devitrify by a two-stage crystallization process within which the presence of oxygen was concluded to play an essential role. At laser powers above 75 W, the observed crystallites were found to be a cubic phase (Cu2Zr4O). The hardness and Young’s modulus in the as-processed samples was found to increase marginally with increased fraction of the crystalline phase.

Microstructural and morphological investigations on Mg-Nb2O5-CNT nanocomposites processed by high-pressure torsion for hydrogen storage applications

A combined deformation process of high energy ball milling and subsequent high-pressure torsion method was applied to synthesize nanocrystalline magnesium powders catalyzed by Nb2O5 and/or multiwall carbon nanotubes. The effect of the different additives on the kinetics of the milled powders and the bulk disks produced by simultaneous uniaxial compression and severe shear deformation was examined in a Sieverts’-type apparatus. The microstructure and the morphology of the as-processed samples and the additives were characterized by X-ray diffraction and high-resolution transmission electron microscopy, respectively. Microstructural changes and morphological alterations after several absorption-desorption cycles were also studied. It was found that high-pressure torsion has significantly changed the texture of magnesium and the shape of carbon nanotubes. The combined use of Nb2O5 and carbon nanotubes was found to improve the desorption kinetics of Mg. Influence of the additives and processing methods on the evolution of the microstructure will also be demonstrated.

On the Study of a TiB<sub>2</sub> Nanoparticle Reinforced 7075Al Composite with High Tensile Strength and Unprecedented Ductility

Strength and ductility are the two most important mechanical properties of a structural material. However, they are often mutually exclusive. In this study, a 6 wt. % TiB2 nanoparticle reinforced 7075Al (i.e. TiB2/7075Al) composite was designed and produced by the processing route combining casting, friction stir processing, hot extrusion and T6 heat treatment. The result of tensile testing demonstrates that the as-processed composite sample presents an ultimate tensile strength of 677 MPa and a total elongation to failure of around 15 %, being higher than any Al or Al based materials ever reported. The typical microstructure contains the TiB2 reinforcement nanoparticles uniformly distributed in the equiaxed Al grain matrix (2 μm in average grain size). In addition to the dispersed nanoprecipitates of the 7075Al (Al-Zn-Mg-Cu) matrix, the integrated TiB2 nanoparticles are systematically decorated by a shell corresponding to (Zn1.5Cu0.5)Mg. This finding challenges our understanding and opens a door for further enhancing strength and ductility being easily scalable for industrial applications.

Promising Tensile and Fatigue Properties of Commercially Pure Titanium Processed by Rotary Swaging and Annealing Treatment.

The effect of the grain refinement and texture on tensile and fatigue properties in commercially pure titanium (grade 2) processed by rotary swaging (RS) and an annealing treatment is investigated. The as-processed sample consists of band-like grains on the longitudinal section and equiaxed grains on the transversal section and revealed an obvious <10-10> fiber texture with respect to the rod axis. Through this technique, a sample with a high tensile strength of 870 MPa, a high uniform elongation of 8.5%, and a high fatigue limit of 490 MPa can be achieved, and the tensile and fatigue properties are almost the same as those of a conventional Ti-6Al-4V alloy. The enhanced mechanical properties and plastic deformation mechanism are discussed in terms of the observed ultrafine-grained microstructure and strong fiber texture.

Process-Defect-Structure-Property Correlations During Laser Powder Bed Fusion of Alloy 718: Role of In Situ and Ex Situ Characterizations

Components made by laser powder bed fusion (L-PBF) additive processes require extensive trial and error optimization to minimize defects and arrive at targeted microstructure and properties. In this work, in situ infrared thermography and ex situ surface roughness measurements were explored as methodologies to ensure Inconel® 718-part quality. For a given laser energy of 200 Watts, prismatic samples were produced with different exposure times (80 to 110 µs) and point spacings (80 to 110 µm). The infrared intensities from laser–material interaction zones were measured spatially and temporally. The conditions leading to higher IR intensity and lowest surface roughness values correlated well with less porosity and coarse solidification grain structure. The transition from highly columnar to misoriented growth is attributed to changes in thermal gradients and liquid–solid interface velocities. Hardness measurements and electron microscopy of the as-processed and post-processed heat-treated samples show complex transitions in microstructural states including the heavily dislocated FCC matrix, reduction of dislocation density, and copious precipitation, respectively. These results show that the geometry-process-structure-property correlations are dynamic, and they cascade depending on the transitions of phase states from powder to liquid to solid, as well as phase decompositions and deformations within the solid FCC phase. Validity of using analytical weld process models to describe the above phenomena is also highlighted.

Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy

The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19′) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing.

Effect of severe plastic deformation and post-annealing on the mechanical properties and bio-corrosion rate of AZ31 magnesium alloy

Abstract In this work, the effect of fine grain sizes on the mechanical and bio-corrosion properties of AZ31 magnesium alloy was studied. Bio-corrosion refers to the accelerated degradation of metal within the human body. Fine-grained (~1.5 µm) AZ31 was obtained through Severe Plastic Deformation (SPD) via three cycles of Constrained Groove-Pressing (CGP) under elevated temperature. The effects of CGP and post-annealing (at 473 K for 15 and 30 min) on bio-corrosion were preliminarily investigated by potentiodynamic polarization measurements and constant immersion tests. Results obtained show that the as-processed samples with annealing exhibited improvements in yield strength and ductility while the bio-corrosion rate in Hank’s solution remains fairly similar during the early stage.

In-Situ Aluminothermal Reduction Synthesis of Ti3AlC2 Aluminium Composite by Friction Stir Processing

Max phase Ti3AlC2 composite, which generally requires high energy ball milling and reactions at higher temperature, has been synthesized at lower temperatures by Friction Stir Processing (FSP). To enable this TiO2+C mixture is dispersed into aluminium matrix by FSP. FSP is a high strain rate plastic deformation process used for microstructural refinement in which a rotating non-consumable tool mixes the work piece material while moving forward. Six overlapping passes of FSP were done to disperse stoichiometric TiO2+C mixture in to Aluminium matrix. Optical microscopy and scanning electron microscopy was done to confirm uniform distribution of particles. X-ray diffraction (XRD) was used for phase identification. XRD of as-processed sample showed peaks of Al, TiO2 and C indicating absence of reaction during FSP. Differential Scanning Calorimetric (DSC) analysis done on this processed sample showed exothermic peaks at 633oC (below melting point of Al), 710oC (above melting point) and 951oC (above melting point). To evaluate the reactions at these temperatures, processed samples were heated to respective temperatures. For samples heated to 633oC, Ti3AlC2, Al3Ti, TiC, Al phases has been observed along with unreacted TiO2 and C. For samples heated to 710oC, peaks of Ti3AlC2, Al3Ti, Al and C were observed. For sample heated to 951oC, only Ti3AlC2 and Al2O3 phases were observed besides Aluminium (Fig. 1). Interestingly, TiC impurity normally associated with Ti3AlC2 synthesis was absent for the sample heated to 951oC. It is postulated that due to particle refinement during FSP, activation energy of the reaction is lowered. This lowering of activation energy also led to formation of Ti3AlC2 and TiC phases even for samples heated to 633oC. Based on the results a reaction path has been proposed.

Effect of grain boundary engineering on the microstructure and mechanical properties of copper containing austenitic stainless steel

The present investigation deals with grain boundary engineering of a modified austenitic stainless steel to obtain a material with enhanced properties. Three types of processing that are generally in agreement with the principles of grain boundary engineering were carried out. The parameters for each of the processing routes were fine-tuned and optimized. The as-processed samples were characterized for microstructure and texture. The influence of processing on properties was estimated by evaluating the room temperature mechanical properties through micro-tensile tests. It was possible to obtain remarkably high fractions of CSL boundaries in certain samples. The results of the micro-tensile tests indicate that the grain boundary engineered samples exhibited higher ductility than the conventionally processed samples. The investigation provides a detailed account of the approach to be adopted for GBE processing of this grade of steel.

Microstructure characterization of the stir zone of submerged friction stir processed aluminum alloy 2219

Abstract Aluminum alloy 2219-T6 was friction stir processed using a novel submerged processing technique to facilitate cooling. Processing was conducted at a constant tool traverse speed of 200 mm/min and spindle rotation speeds in the range from 600 to 800 rpm. The microstructural characteristics of the base metal and processed zone, including grain structure and precipitation behavior, were studied using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness maps were constructed on polished cross sections of as-processed samples. The effect of tool rotation speed on the microstructure and hardness of the stir zone was investigated. The average grain size of the stir zone was much smaller than that of the base metal, but the hardness was also lower due to the formation of equilibrium θ precipitates from the base metal θ′ precipitates. Stir zone hardness was found to decrease with increasing rotation speed (heat input). The effect of processing conditions on strength (hardness) was rationalized based on the competition between grain refinement strengthening and softening due to precipitate overaging.

Room-temperature deformation micro-mechanisms of polycrystalline nickel processed by spark plasma sintering

The present work focuses on room temperature mechanical properties and deformation mechanisms of bulk polycrystalline nickel processed from a high purity micrometre-sized powder by means of spark plasma sintering. Optimisation of the conditions yielded bulk samples having a relative density ranging from 97 to about 99% and an average grain size in the range 5–45 μm. EBSD experiments were carried out to characterise the microstructure of as-processed sample before and after room temperature compression tests. The microstructure investigations prior to compression tests revealed a high density of high-angle grain boundaries (HAGBs); a large fraction of them are Σ3 boundaries (20 to 52% depending on the sample), the majority of which are twin boundaries (TBs). Compression tests (at a strain rate of 10− 3 s− 1) at fixed amount of strains (5, 8 and 50%) result in an increase of the amount of low-angle grain boundaries, probably as a consequence of an intense dislocation activity. As for HAGBs, the more prominent effects occur for Σ3 boundaries and TBs for which a sharp decrease in the course of deformation is found. This was attributed to interaction with dislocations that possibly induce misorientation changes across boundaries (e.g., TBs) as well as their partial disruption as evidenced by EBSD investigations.

Measurement of effective carrier lifetime at the semiconductor–dielectric interface by Photoconductive Decay (PCD) Method

The semiconductor–dielectric interface is of key importance to the performance of Metal-Oxide-Semiconductor transistors (MOSFETs). The near-surface Photoconductance Decay (ns-PCD) method using probe contacts is shown in this study to be very useful in measuring effective carrier lifetime at the semiconductor–dielectric interface. By doing so, it provides direct information on the condition of the charge transport environment in the MOSFET channel without a need to fabricate a transistor. The way measurement is implemented depends on the thickness of dielectric. For dielectric layers thicker than about 5nm, etched windows in the dielectric layer are necessary to achieve an ohmic contact with the semiconductor layer. For dielectric layers thinner than about 5nm, however, the ohmic contact to the semiconductor substrate, essential to the performance of this measurement, is established using probes and electrical contact formation process. The measurements were performed on thermally oxidized Si–SiO2 structures as well as Si–Al2O3 (3nm) and Si–Ta2O5 (3nm) structures formed by means of Atomic Layer Deposition (ALD). The results obtained demonstrate that the PCD method adapted as discussed in this work can be very useful in monitoring condition of semiconductor – ultra-thin (<5nm) dielectric interface by measuring carrier lifetime in the as-processed samples, i.e. without subjecting it to any processing step beyond dielectric deposition.

Microstructural Characterizations of Maraging Steel Processed by Equal-Channel Angular Pressing

Microstructures of a maraging steel subjected to equal-channel angular pressing (ECAP) up to 16 passes at room temperature were investigated. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) examinations showed that the microstructures of the as-processed samples were characterized by a well-defined lamellar structure. With increasing the number of ECAP passes from 1 to 16, the lamellar spacing was significantly decreased from ~323 nm to ~54 nm, while the volume fraction of high-angle lamellar boundaries (HALBs, having the misorientation larger than 15o) was obviously increased form ~46% to ~80%. The real lamellar-type nanostructured microstructures were completely accomplished in the tested steel after 8 or more ECAP passes. In addition, the grain refinement mechanism of maraging steel during the processing was discussed in detail.

Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 μm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat.

Production of free standing Cu–Al intermetallics by cathodic arc plasma treatment

In this study a new approach, based on modification of surfaces by energetic ion bombardment produced through cathodic arc plasma was utilized for the production of bulk Cu–Al compounds. Alloying was done in a cyclic manner by depositing aluminum under low bias voltages, and then by enhancing diffusion/mixing by applying a high bias voltage (bombarding). During the bombarding step, substrate temperatures in a range of 750–800 °C can be achieved depending on the duration in a time scale of seconds where copper foils with 25 μm thickness were used as substrates. For homogenization of the as-processed samples, a vacuum annealing heat treatment was applied at 700 °C for 24 h. Single phase α1-Cu solid solution, and γ1-Al4Cu9, δ-Al2Cu3, and ζ2-Al3Cu4 intermetallics were successfully produced. Structural and surface/cross sectional investigations of the samples were conducted with XRD, SEM and EDS measurements. The micro-hardness of the phases, was also determined.

The microstructure and properties of single grain bulk Ag-doped Y–Ba–Cu–O fabricated by seeded infiltration and growth

Single grain, Ag-doped Y–Ba–Cu–O bulk superconductors of up to 30 mm in diameter have been fabricated successfully by a seeded infiltration and growth (IG) technique. These samples exhibit low porosity compared with samples grown by conventional top seeded melt growth (TSMG). The density of voids within as-processed IG samples can be reduced further by adding metallic Ag, rather than Ag 2O, to the Y 2BaCuO 5 (Y-211) pre-form. An inhomogeneous distribution of Y-211 has been observed in these samples. Finally, a maximum trapped magnetic field of 0.5 T was observed for Ag-doped Y–Ba–Cu–O of diameter 30 mm at liquid nitrogen temperature (77 K).