Junmei Chen

Surface characteristics and corrosion properties of selective laser melted Co–Cr dental alloy after porcelain firing

OBJECTIVE We examined the surface characteristics and corrosion properties of selective laser melted (SLM) cobalt-chromium (Co-Cr) dental alloys before and after porcelain-fused-to-metal (PFM) firing. METHODS Samples were manufactured utilizing SLM techniques and control specimens were fabricated using traditional casting methods. The microstructure and surface composition were examined using metallographic microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Corrosion properties were evaluated using electrochemical impedance spectroscopy. Students t-test was used to evaluate differences in numerical results of electrochemical corrosion tests between SLM and cast specimens before or after PFM firing. The results of electrochemical corrosion tests of the SLM and cast samples before and after firing were analyzed using one-way ANOVA. RESULTS Although PFM firing altered the microstructure of the SLM specimens, they still exhibited a compact and homogeneous structure, and XPS analysis indicated that there were no significant differences in the surface composition of the specimens after firing. In artificial saliva at pH 5, the Rp value of the SLM specimens was 6.21MΩcm(-2) before firing and 2.84MΩcm(-2) after firing, suggesting there was no significant difference in electrochemical corrosion properties (P>0.05). In artificial saliva at pH 2.5, the Rp value of the SLM group was 4.80MΩcm(-2) before firing and 2.88MΩcm(-2) after firing, again indicating no significant difference in electrochemical corrosion properties (P>0.05). At pH 2.5, there was a significant difference in corrosion behavior between the cast and SLM groups, with the Rp value of the cast group being 0.78MΩcm(-2) vs. 2.88MΩcm(-2) for the SLM group. SIGNIFICANCE The improved post-firing corrosion resistance of SLM specimens provides further support for their use in prosthodontic applications, as the oral environment may become temporarily acidic following meals.

Raman study of phonon modes and disorder effects in Pb1−xSrxSe alloys grown by molecular beam epitaxy

We report phonon modes and alloy disorder effects of Pb1−xSrxSe alloys (x⩽0.3) grown on BaF2 (111) substrates by Raman spectroscopy measurements. On the basis of phonon modes in binary PbSe and SrSe, first-, second-, and high-order Raman scattering phonon frequencies of ternary Pb1−xSrxSe are observed, together with the deduction of the disorder activated modes due to the breakdown of the selection rule. The alloy disorder is found to play a more important role than the strain effect in Pb1−xSrxSe, and has been further investigated by employing the special correlation model. The PbSe-like second-order features are also shown to broaden slightly and diminish in intensity with increasing Sr concentration, which has been attributed to a weak breakdown in the long-range order.

Effects of weld metal strength and transformation temperature on welding residual stress

Abstract In this study, we assess the effects of weld metal strength and transformation temperature on the residual stress in 2·25Cr–1·6W steel welded joints. We find that the stress levels in the weld and the peak residual stress position in the base metal can be controlled by the appropriate selection of filler metal. Low transformation temperature weld metals are capable of inducing compressive residual stresses in the weld bead. Meanwhile, for stresses and bending moment equilibrium, the high strength weld metal pushes the peak tensile stress outward to reside at the base metal, keeping off the weak region. A reasonable agreement is obtained between the numerical modelling considering phase transformation and the accurate local stresses measurement associated to weld microstructure regions.

Observation of periodical negative differential conductivity in nanocrystalline silicon/crystalline silicon heterostructures

We report the observation of periodical negative differential conductivity (NDC) in hydrogenated nanocrystalline silicon/crystalline silicon diode heterostructures by low-temperature current–voltage measurements. The NDC-related series of spike-like current peaks is found to result from the accumulation and depletion of electrons tunnelling through the nanodot layers in the neutral region. The observation has been further supported by the facts of onset-voltage blueshift, number variation in current peaks, and inter-Landau-level tunnelling in the temperature- and magnetic field-dependent experiments. We also discuss the differences between the present periodical NDC and the electric field domain in superlattice structures.

Ductility dip cracking mechanism of Ni-Cr-Fe alloy based on grain boundary energy

Abstract Inconel 690 and the companion filler metal 52M (FM-52M) have susceptibility of ductility dip cracking (DDC) in welding procedure. According to test results, DDC was not only preferential to propagate on grain boundaries vertical to loading direction but also related to grain boundary structure. To reveal the relation of grain boundary angle and cracking, a three-dimensional polycrystalline model was constructed to simulate the formation of DDC. In this work, the initiation and propagation of DDC were studied based on extended Read–Shockley formula. The results indicate that grain boundaries with approximate 45° disorientation are most prone to cracking, and the simulated DDC morphology shows agreement with experimental results. The study provides an extra method to predict DDC propagation and helps to evaluate DDC susceptibility in grain level.

Relation between Kirkendall voids and intermetallic compound layers in the SnAg/Cu solder joints

Several different kinds of intermetallic compound (IMC) layers were fabricated through adding trace elements into the Sn-3.5Ag solder, to investigate the relationship between the Kirkendall void (KV) and the IMC layers. The results show that a considerable amount of KVs were observed at the Cu3Sn/Cu interface in the Sn-3.5Ag/Cu joint after isothermal aging. A proper amount of Zn (0.5 wt%) and Ge (0.1 and 0.3 wt%) were found to effectively suppress the formation of the Cu3Sn layer, and no obvious KVs were observed at the Cu6Sn5/Cu interface, while more Zn induced the formation of the Cu6Sn5 plus Cu–Zn mixed IMC layer, and voids (not KVs) were observed at the Cu6Sn5/Cu–Zn interface. (Cu,Ni)6Sn5 IMC layer replaced the initial Cu6Sn5 at the SnAg-Ni/Cu joints, likewise, the Cu3Sn was suppressed at the thermal aging stage. Moreover, voids were found at the IMC/solder interface, while not at the IMC/Cu. Therefore, the formation of KVs is greatly determined by the characteristic of the IMC layer, this is consistent with the previous reports. On other hand, the KV can be suppressed by controlling the interfacial phase through adding trace elements into the solder.

Band offsets and transport mechanisms of hydrogenated nanocrystalline silicon/crystalline silicon heterojunction diode: Key properties for device applications

We have studied the electrical properties of hydrogenated nanocrystalline silicon/crystalline silicon heterojunction diode, focusing on the band offsets and electron transport mechanisms. Capacitance-voltage (C-V) analysis reveals that the band discontinuity mainly exists on the valence-band side, and an interface charge density on the order of 1011cm−2 is estimated via the numerical C-V matching technique. Temperature- and bias-dependent transport mechanisms have been clarified by dark current-voltage-temperature measurements, and the extracted parameters indicate a transition from nontunneling to tunneling dominant transport from 350to20K.

Finite element simulation of butt welded 2·25Cr–1·6W steel pipe incorporating bainite phase transformation

Abstract This article presents a newly developed global optimisation method for the finite element simulation of welding process considering bainite transformation. In this method, the pattern search algorithm was applied to determine kinetic parameters in Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation during a continuous cooling process. Meanwhile, the JMAK equation was modified into an explicit form as a function of welding temperature field to improve calculation efficiency in the optimisation process. This methodology improves the accuracy as calculating the temperature dependent volume fraction of bainite transformation in finite element simulation. The calculated welding residual stresses considering phase transformation effects exhibited better agreement with the measured results than those calculated without phase transformation. The influences of variable cooling rates on welding residual stresses were also investigated.

Assessment of ductility dip cracking susceptibility on Ni based alloy by FEM simulation

Abstract Alloy 690 is a Ni–Cr–Fe type alloy with a widespread application in nuclear power plants. However, the companion filler metal 52M (FM-52M) is susceptible to ductility dip cracking (DDC), which seriously affects safety and life extension in nuclear application. To provide a better understanding of DDC formation in the varestraint test, local strain is adopted to assess materials resistance of DDC instead of average bending strain. In this work, spot varestraint tests of FM-52M were performed with an average strain (0·25–7%). Then, a thermomechanical simulation of the spot varestraint tests was modelled based on the experiment. The simulation obtained preferential propagation orientation of DDC and accumulated strain increment during a susceptible temperature range. Simulated local strain caused at an elevated temperature is much larger than the average strain. Besides, prediction of DDC distribution agrees well with experimental results. The finite element model presents a clear local strain distribution and provides an extra understanding of DDC formation.

Current Crowding and its Effects on Electromigration and Interfacial Reaction in Lead-Free Solder Joints

The current density distribution in a line-to-bump structure as a function of cross-sectional area ratio of the electrical conductor was investigated, as well as the effects of current crowding on electromigration and interfacial reaction in lead-free solder joints. Finite element analysis shows that the crowding factor is directly proportional to the cross-sectional area ratio between Cu line and contact opening at the cathode side. Cu∕Sn‐3.0Ag‐0.5Cu∕Cu solder joints with Cu line in different widths were designed and tested under 1×103A∕cm2 at 60°C for 430h. The experiment results show that big voids induced by electromigration are only formed at the structure with a narrower line. Moreover, the growth of intermetallic compound layers, as well as dissolution of Cu at the cathode side, is accelerated by smaller current crowding, whereas impeded by a bigger one.