Lai-Chang Zhang

Ductile ultrafine-grained Ti-based alloys with high yield strength

The authors report on ductile ultrafine-grained (Ti0.72Fe0.28)100−xTax (0⩽x⩽4) alloys with not only high fracture strength but simultaneously high yield strength exceeding 2000MPa along with distinct plasticity, which are superior to high-strength Ti-based bulk metallic glasses and bimodal composites. All alloys mainly consist of β-Ti and FeTi solid solutions but display different microstructures. The alloys exhibit a high fracture strength >2500MPa and a high yield strength >2000MPa as well as large plasticity of ∼5%–7.5%. The microstructure-property correlation of these ultrafine-grained alloys is discussed.

Influence of Nb on the β → α″ Martensitic Phase Transformation and Properties of the Newly Designed Ti-Fe-Nb Alloys

A series of Ti-7Fe-xNb (x=0, 1, 4, 6, 9, 11 wt.%) alloys was designed and cast to investigate the β→α″ martensitic phase transformation, β phase stability, the resulting microstructure and mechanical properties. Phase analysis revealed that only Ti-7Fe-11Nb alloy shows a single body-centred cubic β phase microstructure while the others are comprised of β and orthorhombic α″ phases. Moreover, Nb addition up to 11 wt.% enhances the stability and volume fraction of β phase in the microstructure, hence reducing the propensity of the alloy system to form α″ phase during quenching. Compressive yield strength and hardness of the alloys are (985-1847) MPa and (325-520) Hv respectively. Additionally, Ti-7Fe-11Nb possesses the lowest Youngs modulus (84 GPa) and the highest deformability (42% strain) among the designed alloys due to the single β phase microstructure. This high deformability is also corroborated by the large plastic deformation zone underneath the Vickers indenter. In contrast, the fractured surfaces of Ti-7Fe and Ti-7Fe-1Nb alloys after compressive tests mostly contain shallow dimples, verifying their low ductility. The good combination of mechanical properties obtained for Ti-7Fe-11Nb renders it more desirable than commonly used CP-Ti and Ti-6Al-4V materials and makes it a promising candidate for biomedical application.

Glass formation in a (Ti,Zr,Hf)-(Cu,Ni,Ag)-Al high-order alloy system by mechanical alloying

In this work, glass formation under high-energy ball milling was investigated for a (Ti 0.33 Zr 0.33 Hf 0.33 ) 50 (Ni 0.33 Cu 0.33 Ag 0.33 ) 40 Al 10 high-order alloy system with equiatomic substitution for early and late transition-metal contents. For comparison, an amorphous alloy ribbon with the same composition was prepared using the melt-spinning method as well. Structural features of the samples were characterized using x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Mechanical alloying resulted in a glassy alloy similar to that obtained by melt spinning. However, the glass formation was incomplete, and a small amount of unreacted crystallites smaller than 30 nm in size still remained in the final ball-milled product. Like the melt-spun glass, the ball-milled glassy alloy also exhibited a distinct glass transition and a wide supercooled liquid region of about 80 K. Crystallization of this high-order glassy alloy proceeded through two main stages. After the primary nanocrystallization was completed, the remaining amorphous phase also behaved as a glass, showing a detectable glass transition and a large supercooled liquid region of about 100 K.

Thermal stability and crystallization kinetics of mechanically alloyed TiC∕Ti-based metallic glass matrix composite

A Ti-based metallic glass matrix composite with 10vol% TiC is synthesized by mechanical alloying. The thermal stability and crystallization kinetics of the metallic glass and composite powders are investigated by differential scanning calorimetry in the mode of isochronal heating and isothermal annealing. The isothermal transformation kinetics is analyzed by the Kolmogorov-Johnson-Mehl-Avrami equation. The values of the Avrami exponent calculated for low crystallization volume fractions imply that the crystallization of both types of powders is governed by diffusion-controlled three-dimensional growth. The mean activation energy of crystallization for the composite is slightly lower than that of the Ti-based metallic glass. The addition of 10vol% TiC particles into a Ti-based metallic glass matrix may slightly affect the crystallization kinetics of the glassy matrix.

Mechanically alloyed amorphous Ti-50(Cu0.45Ni0.55)(44-x)AlxSi4B2 alloys with supercooled liquid region

A high-energy ball milling procedure has been developed to produce amorphous alloys in Ti-50(Cu0.45Ni0.55)(44-x)AlSi4B2 (X = 0, 4, 8, 12) powder mixtures. The milling products were characterized using x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The Ti-based amorphous alloy powders prepared through this solid-state process exhibit a well-defined glass transition and a supercooled liquid region (DeltaT(x) = 64 K) close to the largest achieved so far for Ti-based undercooled melts. The substitution of Al for Cu and Ni has beneficial effects on stabilizing the supercooled liquid. Residual nanocrystals of the alpha-Ti structure are uniformly dispersed in the amorphous matrix. The composite alloy powders offer the potential for consolidation in the supercooled liquid region to bulk lightweight amorphous alloys and the possibility to attain desirable mechanical properties.

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

The key evidence for understanding the mechanical behavior of advanced high strength steels was provided by atom probe tomography (APT). Chemical overstabilization of retained austenite (RA) leading to the limited transformation-induced plasticity (TRIP) effect was deemed to be the main factor responsible for the low ductility of nanostructured bainitic steel. Appearance of the yield point on the stress-strain curve of prestrained and bake-hardened transformation-induced plasticity steel is due to the unlocking from weak carbon atmospheres of newly formed during prestraining dislocations.

Ultra-sustainable Fe 78 Si 9 B 13 metallic glass as a catalyst for activation of persulfate on methylene blue degradation under UV-Vis light

Stability and reusability are important characteristics of advanced catalysts for wastewater treatment. In this work, for the first time, sulfate radicals (SO4∙−) with a high oxidative potential (Eo = 2.5–3.1 V) were successfully activated from persulfate by a Fe78Si9B13 metallic glass. This alloy exhibited a superior surface stability and reusability while activating persulfate as indicated by it being used for 30 times while maintaining an acceptable methylene blue (MB) degradation rate. The produced SiO2 layer on the ribbon surface expanded strongly from the fresh use to the 20th use, providing stable protection of the buried Fe. MB degradation and kinetic study revealed 100% of the dye degradation with a kinetic rate k = 0.640 within 20 min under rational parameter control. The dominant reactive species for dye molecule decomposition in the first 10 min of the reaction was hydroxyl radicals (∙OH, Eo = 2.7 V) and in the last 10 min was sulfate radicals (SO4∙−), respectively. Empirical operating variables for dye degradation in this work were under catalyst dosage 0.5 g/L, light irradiation 7.7 μW/cm2, and persulfate concentration 1.0 mmol/L. The amorphous Fe78Si9B13 alloy in this work will open a new gate for wastewater remediation.

Review on manufacture by selective laser melting and properties of titanium based materials for biomedical applications

Titanium (Ti) based materials are deemed one type of the best metallic materials for biomedical application due to their good mechanical properties, high biocompatibility and corrosion resistance. Design and manufacture of complex shape Ti parts with good quality are highly demanded in biomedical areas. Selective laser melting (SLM), an additive manufacturing technology, is able to produce bulk structural parts almost without geometric constraints. This review paper briefly evaluates the work carried out on the significance of Ti materials, SLM technology and SLM manufacturing of Ti materials commonly used for biomedical applications including the microstructures and mechanical properties of resulting bulk dense parts (Ti, Ti–24Nb–4Zr–8Sn, Ti–6Al–4V, Ti–6Al–7Nb, Ti–TiC and Ti–TiB) as well as the porous structures successfully produced by SLM. This review indicates that SLM produced Ti materials are able to fulfill the biomechanical and biocompatibility requirements and can be considered as potential candidate for biomedical applications.

Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

For the first time, a porous and conductive Co0.85Se/graphene network (CSGN), constructed by Co0.85Se nanocrystals being tightly connected with each other and homogeneously anchored on few-layered graphene nanosheets, has been synthesized by a facile one-pot solvothermal method. Compared to unhybridized Co0.85Se, CSGN exhibits much faster kinetics and better electrocatalytic behavior for hydrogen evolution reaction (HER). The HER mechanism of CSGN is improved to Volmer-Tafel combination, instead of Volmer-Heyrovsky combination, for Co0.85Se. CSGN has a very low Tafel slope of 34.4 mV/dec, which is much lower than that of unhybridized Co0.85Se (41.8 mV/dec) and is the lowest ever reported for Co0.85Se-based electrocatalysts. CSGN delivers a current density of 55 mA/cm2 at 250 mV overpotential, much larger than that of Co0.85Se (33 mA/cm2). Furthermore, CSGN shows superior electrocatalytic stability even after 1500 cycles. The excellent HER performance of CSGN is attributed to the unique porous and conductive network, which can not only guarantee interconnected conductive paths in the whole electrode but also provide abundant catalytic active sites, thereby facilitating charge transportation between the electrocatalyst and electrolyte. This work provides insight into rational design and low-cost synthesis of nonprecious transition-metal chalcogenide-based electrocatalysts with high efficiency and excellent stability for HER.

A new insight into high-strength Ti62Nb12.2Fe13.6Co6.4Al5.8 alloys with bimodal microstructure fabricated by semi-solid sintering

It is well known that semi-solid forming could only obtain coarse-grained microstructure in a few alloy systems with a low melting point, such as aluminum and magnesium alloys. This work presents that semi-solid forming could also produce novel bimodal microstructure composed of nanostructured matrix and micro-sized (CoFe)Ti2 twins in a titanium alloy, Ti62Nb12.2Fe13.6Co6.4Al5.8. The semi-solid sintering induced by eutectic transformation to form a bimodal microstructure in Ti62Nb12.2Fe13.6Co6.4Al5.8 alloy is a fundamentally different approach from other known methods. The fabricated alloy exhibits high yield strength of 1790 MPa and plastic strain of 15.5%. The novel idea provides a new insight into obtaining nano-grain or bimodal microstructure in alloy systems with high melting point by semi-solid forming and into fabricating high-performance metallic alloys in structural applications.