Volume 108, Issue 8

As a biomedical hard tissue implant candidate, porous Ti–Mo alloy has received considerable attention because of its special porous structure, appropriate Young's modulus and compressive strength as well as good corrosion resistance. As a bioactive coating, hydroxyapatite is commonly used to cover the surface of bioinert metallic prostheses due to its excellent biocompatibility, bone-like structure and composition. This article reviews the current developments and the relationships between the fabrication methods, porous structure, mechanical properties, bioactive surface modification and corrosion behavior of porous Ti–Mo alloy used for hard tissue implant application. Furthermore, the future research directions are discussed to optimize the porous structure and improve the properties of porous Ti–Mo alloys.

In this paper, a systematic study of crystallization kinetics and deformation behavior is presented for (Cu 50 Zr 50 ) 94 Al 6 bulk metallic glass in the supercooled liquid region. Crystallization results showed that the activation energy for (Cu 50 Zr 50 ) 94 Al 6 was calculated using the Arrhenius equation in isothermal mode and the Kissinger–Akahira–Sunose method in non-isothermal mode. The activation energy was quite high compared with other bulk metallic glasses. Based on isothermal transformation kinetics described by the Johson–Mehl–Avrami model, the average Avrami exponent of about 3.05 implies a mainly diffusion controlled three-dimensional growth with an increasing nucleation rate during the crystallization. For warm deformation, the results showed that deformation behavior, composed of homogeneous and inhomogeneous deformation, is strongly dependent on strain rate and temperature. The homogeneous deformation transformed from non-Newtonian flow to Newtonian flow with a decrease in strain rate and an increase in temperature. It was found that the crystallization during high temperature deformation is induced by heating. The appropriate working temperature/strain rate combination for the alloy forming, without in-situ crystallization, was deduced by constructing an empirical deformation map. The optimum process condition for (Cu 50 Zr 50 ) 94 Al 6 can be expressed as T ∼733 K and ε.${\rm{\dot \varepsilon }}$ ∼ 10 −3 s −1 .

For high strength low alloys, intercritical heat treatment processes are usually used to obtain the dual phase steel. In this work, a micro–macroscopic model was developed for describing the hardening and fracture behavior of such steels after the heat treatments. In the microscopic model, a description of flow curves was established based on a 2D representative volume element from a real microstructure, whereas in the macroscopic model, the Gurson–Tveergard–Needleman approach was used to simulate the failure process. Furthermore, the simulation flow curves and true stress–strain curves were compared with the experimental observations. The results demonstrate that such a multiscale model can accurately predict the work hardening and fracture behavior of steels after the intercritical heat treatments.

The present study attempts to discuss the time evolution of the slurry microstructure of semisolid magnesium alloy AZ91D so as to control the forming process more effectively. We calculated the instantaneous shear stress and the time evolution of average agglomerate size of a real isothermal shearing process containing increasing shear rate, holding the maximal shear rate and decreasing shear rate. The calculated results demonstrate that we should adopt as great a maximal shear rate as possible to achieve a finer slurry microstructure and reduce the resulting larger startup shear stress by increasing the time interval required to increase the shear rate to maximum. Moreover, we also should shorten the time interval required to decrease the shear rate to zero as far as possible, so as to weaken agglomeration during decreasing shear rate and achieve a better final slurry microstructure.

Synthesis of an A356 alloy slurry with 30 wt.% solid fraction was carried out by the rheometal process, previously known as a rapid slurry formation process. A detailed study of the A356 alloy at different slurry stirring speeds (400, 600, 800, 1 000, and 1 200 rpm) was performed. Variation of the surface morphology of the enthalpy exchange material at different stirring times (0, 5, 10, 15, 20 and 25 s) was also investigated. The evolution of the cast microstructure was observed using optical and scanning electron microscopy. The microstructure exhibited a network of eutectic Si around the primary α-Al phase. The average grain size of the primary α-Al phase decreases with stirring speeds up to 1 000 rpm. For 1 200 rpm, average grain size decreased, however, high turbulence led to entrapment of gas porosity. The experimental results revealed that as stirring times increased under isothermal conditions, the α-Al phase changed its morphology from dendritic to rosette to globular. The effect of flow pattern was investigated with the introduction of four baffles along the crucible wall. These baffles modified the fluid flow pattern such that the average grain size of α-Al phase was further refined.

Investigation into the reactions and phase equilibria of transition metal elements (i. e. Mo, Zr, Cr, V and Ti), Al and rare earths is academically and industrially important for the development of both refractory alloys and lightweight high-temperature materials. In this work, the equilibria of the Mo–Al–Ho ternary system at 773 K have been determined by using X-ray powder diffraction and scanning electron microscopy equipped with energy dispersive X-ray analysis. A new ternary phase Al 4 Mo 2 Ho has been found and the other ternary phase Al 43 Mo 4 Ho 6 is observed. Ten binary phases in the Al–Mo and Al–Ho systems, including Al 17 Mo 4 rather than Al 4 Mo, have been determined to exist at 773 K. The homogeneity ranges of AlMo 3 and Al 8 Mo 3 phase are 7.5 at.% and 1 at.%, respectively. According to the phase-disappearing method, the maximum solubility of Al in Mo is about 16 at.%.

The phase equilibria in the Zr–Cu–Ni ternary system are investigated combined with X-ray diffraction, electron probe micro-analysis and differential scanning calorimetry. Two isothermal sections of the Zr–Cu–Ni ternary system at 1 000 °C and 1 100 °C are experimentally established. Most of the binary intermetallic compounds, e. g. Zr 7 Ni 10 , ZrNi, ZrNi 5 , Zr 14 Cu 51 , and Zr 2 Cu 9 , show a remarkable ternary solubility. A new ternary compound named τ 3 (Zr 3 1.1 – 30.7 · Cu 28.5 – 4 0. 3 Ni 40.4 – 29.0 ) is detected at 1 000 °C and dissolved at 1 020 °C because the nearby large liquid phase field further expands. The newly determined phase equilibria will provide important information for both thermodynamic assessment and alloy design of Zr-based metallic glass.

Ni 3 Al–Al 2 O 3 –Al 2 O 3 /MgO three-layered coatings with thicknesses of 50, 100, and 150 μm for Al 2 O 3 /MgO and 100 μm for the other layers were deposited on 316L stainless steel using plasma spraying. X-ray diffraction, atomic force microscopy, furnace hot corrosion testing in the presence of a mixture of Na 2 SO 4 and V 2 O 5 corrosive salts and scanning electron microscopy were used to determine the structural, morphological and hot corrosion resistance of samples. Results revealed that the crystalline grains of MgO and Al 2 O 3 coating were very small. Weight loss due to hot corrosion decreased from approximately 4.267 g for 316L stainless steel without coating to 2.058 g. The samples with 150 μm outer coating showed improved resistance with the increase in outer layer thickness. Scanning electron microscopy of the coated surface revealed that the coating's resistance to hot corrosion is related to the thickness and the grain size of Al 2 O 3 /MgO coatings.

In this study, B 4 C–TiB 2 –TiC–Ni coating was sprayed on the surface of 4130 steel by high velocity oxy-fuel torch. The tribological behavior of samples was studied by ball on disk wear testing. Structural evolution of the coating was analyzed by X-ray diffractometry. The microstructure of the coating, wear track and Al 2 O 3 ball was investigated by scanning electron microscopy, field emission scanning electron microscopy and optical microscopy. Elemental analysis of the wear track was done by energy dispersive X-ray spectroscopy. It was found that a cermet coating containing B 4 C, TiB 2 , TiC and Ni phases with good bonding to the 4130 steel substrate with no sign of any cracking or pores was formed. The wear mechanism of the composite coating was delamination. The friction coefficient of samples was decreased with increasing load because of higher frictional heat and creation of more oxide islands.

Anodic alumina oxide membrane was used to electrodeposit Fe–Ni alloy nanowires by varying the potential. The morphology of electrodeposited alloy nanowires was studied by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterization techniques. The results indicate that Fe–Ni alloy nanowires have stable fcc phase at −1.4 V and −0.7 V and the content of iron inside nanopores increased with increasing applied potential during deposition. This can be verified by the current density vs time graphs of depositing Fe and Ni nanowires, the current density ratio of Ni to Fe at lower applied potential is greater than at comparatively higher applied potential.

The mechanisms of nano-sized ZrC whisker formation by a vapor–liquid–solid process are investigated, which produces a very high purity, single crystal whisker. Rectangular ZrC whiskers with a cross-sectional diameter of 100–200 nm and lengths up to tens of microns are formed under the catalytic effect of nickel. The ZrC whiskers are characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. ZrC whiskers can be used as a potential reinforcing and strengthening phase for ceramic composites.