Recent Development in Micromanufacturing of Metallic Materials.

Product miniaturization is a trend for facilitating product usage, enabling product functions to be implemented in microscale geometries, and aimed at reducing product weight, volume, cost and pollution. Driven by ongoing miniaturization in diverse areas including medical devices, precision equipment, communication devices, micro-electromechanical systems (MEMS) and microsystems technology (MST), the demands for micro metallic products have increased tremendously. Such a trend requires development of advanced micromanufacturing technology of metallic materials for producing high-quality micro metallic products that possess excellent dimensional tolerances, required mechanical properties and improved surface quality. Micromanufacturing differs from conventional manufacturing technology in terms of materials, processes, tools, and machines and equipment, due to the miniaturization nature of the whole micromanufacturing system, which challenges the rapid development of micromanufacturing technology. Against such a background, the Special Issue "Micromanufacturing of Metallic Materials" was proposed to present the recent developments of micromanufacturing technologies of metallic materials. The papers collected in the Special Issue include research articles, literature review and technical notes, which have been highlighted in this editorial.micromanufacturing; metallic materials; miniaturization; micro products.

Effect of undissolved Nb carbides on mechanical properties of hydrogen-precharged tempered martensitic steel.

Nb carbides have attracted significant attention to enhance the resistance of tempered martensitic (TM) steel to hydrogen embrittlement (HE). However, previous studies have elucidated the role of Nb carbides in HE resistance without categorizing their types (i.e., undissolved and newly precipitated). This study focuses on the effect of "undissolved" Nb carbides on the tensile and fatigue properties of hydrogen-precharged TM steels. It validated the following two factors for the HE resistance of the TM steels containing undissolved Nb carbides: hydrogen-trapping by the carbides and refinement of prior austenite grain. The former factor rarely affected the HE resistance owing to the interfacial incoherency between the undissolved carbides and ferritic matrix. Such results are distinguished from previous studies focusing on the newly precipitated carbides. In contrast, the latter factor contributed significantly to the HE resistance via the decrease in hydrogen contents per unit surface of prior austenite grain boundaries.

Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy.

Superplasticity describes a material's ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10-4 s-1), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10-2 s-1, required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10-2 s-1 in an Al9(CoCrFeMnNi)91 (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity.

Hydrogen Embrittlement Behavior of 18Ni 300 Maraging Steel Produced by Selective Laser Melting.

A study was performed to investigate the hydrogen embrittlement behavior of 18-Ni 300 maraging steel produced by selective laser melting and subjected to different heat treatment strategies. Hydrogen was pre-charged into the tensile samples by an electro-chemical method at the constant current density of 1 A m-2 and 50 A m-2 for 48 h at room temperature. Charged and uncharged specimens were subjected to tensile tests and the hydrogen concentration was eventually analysed using quadrupole mass spectroscopy. After tensile tests, uncharged maraging samples showed fracture surfaces with dimples. Conversely, in H-charged alloys, quasi-cleavage mode fractures occurred. A lower concentration of trapped hydrogen atoms and higher elongation at fracture were measured in the H-charged samples that were subjected to solution treatment prior to hydrogen charging, compared to the as-built counterparts. Isothermal aging treatment performed at 460 °C for 8 h before hydrogen charging increased the concentration of trapped hydrogen, giving rise to higher hydrogen embrittlement susceptibility.

Comparative study on the effects of Cr, V, and Mo carbides for hydrogen-embrittlement resistance of tempered martensitic steel.

In this study, the ideal alloying element (among Cr, V, and Mo carbides) to enhance the resistance to hydrogen embrittlement (HE) in a tempered martensitic steel was investigated. Four types of steels were designed to contain cementites, Cr-rich M7C3 carbides, V carbides, and Mo carbides, respectively. These steels were tailored to possess a comparable tensile strength (~1.6 GPa). The HE resistances of these steels were evaluated through the slow strain rate test and cyclic corrosion test. The results showed an enhanced HE resistance, characterized by a high notch fracture strength after hydrogen charging, in the samples containing V carbides and Mo carbides. In particular, Mo carbide was regarded as the most ideal alloying element for HE resistance because of the high resistivity parameter, inhibited hydrogen penetration, and suppressed strength loss by internal hydrogen.

Simultaneous Improvement in the Strength and Formability of Commercially Pure Titanium via Twinning-induced Crystallographic Texture Control.

The rolling texture formed in the conventional cold rolling process of commercially pure titanium (CP-Ti) for producing a metal sheet significantly limits the potential applications of CP-Ti sheets in various industrial sectors by impairing the formability. Here, we report that by exploiting a twinning-induced crystallographic texture modification, the rolling texture can be weakened and dispersed effectively, leading to a simultaneous improvement in the formability and yield strength. A two-stage cold rolling process was designed with intermediate annealing at a late stage of the conventional cold rolling process to generate deformation twins. The intermediate annealing drove the activation of [Formula: see text] twin and [Formula: see text] - [Formula: see text] double twin in the second stage of the rolling process by removing the internal reaction stress developed in the first stage of the rolling process through recrystallization, and the crystallographic feature of the [Formula: see text] twinned region, i.e., [Formula: see text] twin texture, was effective for type slips and [Formula: see text] twinning to accommodate a through-thickness strain as well as for reducing the planar anisotropy. This enhanced thinning capability and reduced planar anisotropy in the [Formula: see text] twin texture led to an improvement of the formability. We demonstrated the feasibility of the suggested two-stage cold rolling process with ASTM grade 2 CP-Ti.

Tribological and corrosion behaviors of warm-and hot-rolled Ti-13Nb-13Zr alloys in simulated body fluid conditions.

Development of submicrocrystalline structure in biomedical alloy such as Ti-13Nb-13Zr (in wt%) through warm-rolling process has been found to enhance mechanical properties compared to conventional thermomechanical processing routes including hot-rolling process. The present study investigated the tribological and corrosion behaviors of warm-rolled (WR) and hot-rolled Ti-13Nb-13Zr alloys which have not been studied to date. Both tribological and corrosion experiments were carried out in simulated body fluid conditions (Hank's solution at 37°C) based on the fact that the investigated alloys would be used in a human body as orthopedic implants. The WR Ti-13Nb-13Zr demonstrated a submicrocrystalline structure that provided a significant enhancement in hardness, strength, and corrosion resistance. Meanwhile, there was no notable difference in wear resistance between the WR and hot-rolled samples despite the different microstructure and hardness. The present study confirmed the enormous potential of WR Ti-13Nb-13Zr with not only great mechanical properties but also high corrosion resistance in the simulated body fluid.

Surface modification of multipass caliber-rolled Ti alloy with dexamethasone-loaded graphene for dental applications.

Titanium (Ti) and its alloys with a high mechanical strength and a small diameter can be effectively exploited for minimally invasive dental implantation. Here, we report a multipass caliber-rolled Ti alloy of Ti13Nb13Zr (MPCR-TNZ) with a high mechanical strength and strong fatigue characteristics. For further dental applications, MPCR-TNZ was surface-modified with reduced graphene oxide (RGO) and loaded with osteogenic dexamethasone (Dex) via π-π stacking on the graphitic domain of RGO. The Dex-loaded RGO-MPCR-TNZ (Dex/RGO-MPCR-TNZ) resulted in significantly enhanced growth and differentiation of MC3T3-E1 cells into osteoblasts, which was confirmed by Alizarin red staining, alkaline phosphatase activity test, immunocytochemistry, and real-time PCR. Moreover, we could confirm the feasibility of Dex/RGO-MPCR-TNZ from the implantation test of a prototype of a dental implant to an artificial bone block for clinical dental applications.

Improved pre-osteoblast response and mechanical compatibility of ultrafine-grained Ti-13Nb-13Zr alloy.

OBJECTIVE: Metallic implantation materials having high yield strength, low elastic modulus, and non-cytotoxic alloying elements would be advantageous for the long-term stability of implants. This study assessed the surface and mechanical properties, and also in vitro osteoconductivity of ultrafine-grained (UFG) Ti-13Nb-13Zr alloy produced by dynamic globularization without any severe deformation for future biomedical applications as an endosseous implant material. MATERIAL AND METHODS: The surface characteristics and mechanical properties were investigated by orientation image microscopy, contact angle measurements, optical profilometry, and uniaxial tension tests. Mouse calvaria-derived pre-osteoblastic cell (MC3T3-E1) attachment, spreading, viability, alkaline phosphatase (ALP) activity, and quantitative analysis of osteoblastic gene expression on UFG Ti-13Nb-13Zr alloy were compared with coarse-grained (CG) Ti-13Nb-13Zr and CG Ti-6Al-4V alloys. RESULTS: Dynamic globularized Ti-13Nb-13Zr alloy has an ultrafine grain size (0.3 µm) and an excellent combination of yield strength and elastic modulus compared with CG alloys, which displayed significantly lower water contact angles compared with CG alloys (P<0.05). The UFG and CG Ti-13Nb-13Zr alloys displayed significantly increased cellular attachment compared with CG Ti-6Al-4V alloy (P<0.05). The UFG Ti-13Nb-13Zr supported better cell spreading and more numerous focal adhesions. ALP activity (P<0.05) and mRNA expressions of the osteoblast transcription factor genes (osterix, Runx2) and marker gene for osteoblast differentiation (osteocalcin) were markedly increased in cells grown on the UFG substrate compared with CG substrates at early incubation timepoints. CONCLUSION: Enhanced pre-osteoblast response to UFG Ti-13Nb-13Zr substrate is attributable to the non-cytotoxic alloying elements and the submicron scale grain size contributes to the superior surface hydrophilicity and abundant grain boundaries favorable for cell behavior. These findings indicate that dynamic globularized UFG Ti-13Nb-13Zr alloy is promising for load-bearing endosseous implant material because of excellent mechanical and biological compatibilites.

Enhanced osteoblast response to an equal channel angular pressing-processed pure titanium substrate with microrough surface topography.

This study investigated the surface characteristics and in vitro biocompatibility of ultrafine-grain pure titanium substrates produced by equal channel angular pressing (ECAP) using MC3T3-E1 pre-osteoblast cells, compared with those of conventional coarse-grain pure titanium (CP) and Ti-6Al-4V (Ti64) substrates. All Ti surfaces were grit-blasted with hydroxyapatite particles to produce microrough surfaces. The surface characteristics were evaluated by electron back-scattered diffractometry, scanning electron microscopy, contact angle and surface energy measurements, and optical profilometry. The morphology of spread cells, cell attachment, viability, alkaline phosphatase (ALP) activity, quantitative analysis of osteoblastic gene expression and mineralization nodule formation on different surfaces were evaluated. ECAP-processed substrates showed a significantly lower water contact angle and higher surface energy compared with coarse-grain CP and Ti64 substrates (p<0.05). They also showed enhanced cell spreading, attachment, viability and ALP activity compared with the CP and Ti64 surfaces (p<0.05). Real-time polymerase chain reaction analysis showed notably higher ALP, osteopontin and osteocalcin mRNA levels in cells grown on the ECAP surfaces than on the CP and Ti64 surfaces, and the ECAP surfaces showed significantly greater mineralization nodule formation compared with the CP and Ti64 substrates (p<0.05). These results demonstrate the superior osteoblast cell compatibility of microroughened Ti surface made of ECAP-processed ultrafine-grain pure Ti substrates over coarse-grain pure Ti and Ti64 substrates.

Osteoconductivity of hydrophilic microstructured titanium implants with phosphate ion chemistry.

This study investigated the surface characteristics and bone response of titanium implants produced by hydrothermal treatment using H(3)PO(4), and compared them with those of implants produced by commercial surface treatment methods - machining, acid etching, grit blasting, grit blasting/acid etching or spark anodization. The surface characteristics were evaluated by scanning electron microscopy, thin-film X-ray diffractometry, X-ray photoelectron spectroscopy, contact angle measurement and stylus profilometry. The osteoconductivity of experimental implants was evaluated by removal torque testing and histomorphometric analysis after 6 weeks of implantation in rabbit tibiae. Hydrothermal treatment with H(3)PO(4) and subsequent heat treatment produced a crystalline phosphate ion-incorporated oxide (titanium oxide phosphate hydrate, Ti(2)O(PO(4))(2)(H(2)O)(2); TiP) surface approximately 5microm in thickness, which had needle-like surface microstructures and superior wettability compared with the control surfaces. Significant increases in removal torque forces and bone-to-implant contact values were observed for TiP implants compared with those of the control implants (p<0.001). After thorough cleaning of the implants removed during the removal torque testing, a considerable quantity of attached bone was observed on the surfaces of the TiP implants.

Evaluation of bone healing with eggshell-derived bone graft substitutes in rat calvaria: a pilot study.

The purpose of this study was to investigate the potential effectiveness of a surface-modified natural calcium carbonate, hen eggshell (ES) as a bone graft substitute. The surface characteristics, cell viability on, and osteoconductivity of, particulated ES with and without hydrothermal treatment in phosphate solutions were evaluated. Hydrothermal treatment partially converted ES to calcium-deficient hydroxyapatite (HA) with surface microstructure. MTT assay indicated higher osteoblast viability on surface-modified ES compared with a commercially available bone substitute, anorganic bovine bone (Bio-Oss, BO) (p < 0.001). Histological and histomorphometric analysis showed significantly greater new bone formation and mineralized bone-to-graft contact of surface-modified ES, especially with hydrothermally treated ES, compared with BO in 5-mm diameter calvarial defects in rats at 4 and 8 weeks of healing (p < 0.01). Complete bony bridging was more frequently found with hydrothermally treated ES. The results of this pilot study indicate the potential efficacy of surface-modified particulated hen eggshell as an osteoconductive bone substitute in a rat calvarial defect model.