Yunping Li

Superthermostability of nanoscale TIC-reinforced copper alloys manufactured by a two-step ball-milling process

A Cu-TiC alloy, with nanoscale TiC particles highly dispersed in the submicron-grained Cu matrix, was manufactured by a self-developed two-step ball-milling process on Cu, Ti and C powders. The thermostability of the composite was evaluated by high-temperature isothermal annealing treatments, with temperatures ranging from 727 to 1273 K. The semicoherent nanoscale TiC particles with Cu matrix, mainly located along the grain boundaries, were found to exhibit the promising trait of blocking grain boundary migrations, which leads to a super-stabilized microstructures up to approximately the melting point of copper (1223 K). Furthermore, the Cu-TiC alloys after annealing at 1323 K showed a slight decrease in Vickers hardness as well as the duplex microstructure due to selective grain growth, which were discussed in terms of hardness contributions from various mechanisms.

Detwining in Mg alloy with a high density of twin boundaries

Abstract To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.

Effects of surface friction treatment on the in vitro release of constituent metals from the biomedical Co-29Cr-6Mo-0.16N alloy.

Due to the ignorance by many researchers on the influence of starting microstructure on the metal release of biomedical materials in human body after implant, in this study, the effect of surface friction treatment on the in vitro release of the constituent elements of the biomedical Co-29Cr-6Mo-0.16N (CCM) alloy is investigated for the first time by immersion test in lactic acid solution combined with electron backscatter diffraction, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-EOS). The results indicate that friction treatment on the as-annealed CCM alloy sample surface leads to a planar strain-induced martensitic transformation (SIMT) on sample surface; this greatly accelerates the release of all the constituent elements and, in particular, that of Co as indicated by the ICP-EOS analysis. This increase can be ascribed to a localized deformation that occurred over the entire sample surface, with the dislocation density being high within the SIMTed phase and low in the alloy matrix.

Effects of carbon addition on wear mechanisms of CoCrMo metal-on-metal hip joint bearings

The wear behaviors of biomedical CoCrMo prosthetic alloys containing various amounts of carbon were investigated using a standard hip joint simulator in a simulated body fluid. A few chunks and punctate σ-phase precipitates were observed in the low-carbon (LC) alloy; these were responsible for the abrasion and run-in wear. Increasing the carbon content led to greater precipitation of globular M23C6-type carbides. As a result, lower wear loss was observed in the high-carbon (HC) alloy. However, the Students t-test analysis on wear loss indicated that there was no significant difference in wear loss between the LC-LC and HC-HC combinations. Surface fatigue caused by torn-off of Mo-rich carbides was the dominant wear mechanism in the HC alloy. Further, Cr-rich carbides prevent three-body abrasion and increase the wear resistance.

Ultrahigh Oxidation Resistance and High Electrical Conductivity in Copper-Silver Powder

The electrical conductivity of pure Cu powder is typically deteriorated at elevated temperatures due to the oxidation by forming non-conducting oxides on surface, while enhancing oxidation resistance via alloying is often accompanied by a drastic decline of electrical conductivity. Obtaining Cu powder with both a high electrical conductivity and a high oxidation resistance represents one of the key challenges in developing next-generation electrical transferring powder. Here, we fabricate a Cu-Ag powder with a continuous Ag network along grain boundaries of Cu particles and demonstrate that this new structure can inhibit the preferential oxidation in grain boundaries at elevated temperatures. As a result, the Cu-Ag powder displays considerably high electrical conductivity and high oxidation resistance up to approximately 300 °C, which are markedly higher than that of pure Cu powder. This study paves a new pathway for developing novel Cu powders with much enhanced electrical conductivity and oxidation resistance in service.

Uneven damage on head and liner contact surfaces of a retrieved Co–Cr-based metal-on-metal hip joint bearing: An important reason for the high failure rate

Detailed metallurgical investigations have been performed on a used Co-Cr-based metal-on-metal (MoM) hip joint bearing containing a type of liner that is commonly used in such joints. The damage on the metal-liner sliding surface was considerably more severe than that on the metal head counterpart, in terms of wear-scar density and width and microcrack frequency. Cross-sectional transmission electron microscopy revealed that a thick (>3 μm) nanocrystalline layer formed on the sliding surface of the head, whereas the liner had coarse carbides embedded in it and nanocrystals were formed in a very limited region no deeper than 1 μm. Comparative investigation of an unused head and a liner of identical type showed that although the chemical compositions of the liner and head were nearly identical, their microstructures were significantly different. Specifically, the grain size in the liner was larger than that in the head on average, and the grain boundaries of the liner were decorated with coarse carbides. Moreover, X-ray diffraction analysis revealed a large tensile residual stress only in the liner. These differences are possibly responsible for the wear damage on the liner being more serious than that on the head.

Effects of Cu addition on the corrosion behavior of NiCoCrMo alloys in neutral chloride solution

The influence of Cu addition (0–4 mass%) on the corrosion behavior of Ni–30Co–16Cr–15Mo alloy in neutral chloride solution is investigated by electrochemical measurements. The results indicate that alloy with 0.5 mass% Cu shows inferior corrosion resistance compared to Cu-free alloy under open-circuit potential, which is possibly ascribed to the galvanic corrosion from the insufficient coverage of Cu on the passive film; a higher fraction of Cu results in a significant improvement on corrosion resistance, and Cu is believed capable of strengthening the passivation and postponing the oxidization of dominant Cr(III) in the passive film into more soluble Cr(VI).

Effects of Temperature and Pressure of Hot Isostatic Pressing on the Grain Structure of Powder Metallurgy Superalloy

The microstructure with homogeneously distributed grains and less prior particle boundary (PPB) precipitates is always desired for powder metallurgy superalloys after hot isostatic pressing (HIPping). In this work, we studied the effects of HIPping parameters, temperature and pressure on the grain structure in PM superalloy FGH96, by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and Time-of-flight secondary ion spectrometry (ToF-SIMS). It was found that temperature and pressure played different roles in controlling PPB precipitation and grain structure during HIPping, the tendency of grain coarsening under high temperature could be inhibited by increasing HIPping pressure which facilitates the recrystallization. In general, relatively high temperature and pressure of HIPping were preferred to obtain an as-HIPped superalloy FGH96 with diminished PPB precipitation and homogeneously refined grains.

Nitinol powders generate from Plasma Rotation Electrode Process provide clean powder for biomedical devices used with suitable size, spheroid surface and pure composition

The nickel-titanium alloy (57Ni-43Ti in wt%) was atomized by the plasma rotating electrode process (PREP). The PREP parameters such as plasma arc current, rotating electrode speed with corresponding PREP powder size range in weight percentage analysis, powder morphology and biocapability of cells were studied by scanning electron microscopies, Inductively Coupled Plasma and X-ray diffraction techniques. From the electrode of the produced powders, the composition has no obviously changes. Weight percentage up to 31.8% of the range under 300 μm while the rotation electrode speed increase to 12k rpm. Spherical and flat with smooth surface were observed in different size range. Brittle phase was not observed of XRD data. The nitinol powder has high biocapability with cells showed no cytotoxicity and well cell adhesion in the in vivo assay.