Suppression Of Void Formation Research Articles

Formabilities of C-Si-Al-Mn Transformation-Induced Plasticity-Aided Martensitic Sheet Steel

The substitution of Si with Al in 0.2%C-1.5%Si-1.25%Mn-0.2%Cr ultrahigh strength transformation-induced plasticity (TRIP)-aided martensitic (TM) sheet steel improves galvanization. The effect of Al content on the microstructure and formabilities of the TM steel was therefore investigated. Replacement of Si with Al maintained the high volume fraction of the retained austenite and the high stretch-formability and stretch-flangeability, whereas it decreased the tensile strength. Complex addition of Si and Al yielded the best formabilities with 1.5 GPa tensile strength grade. The superior formabilities of Si-Al bearing TM steel were attributed to the strain-induced transformation of the metastable retained austenite and the relatively soft lath-martensite structure matrix. The former leads to plastic relaxation of the localized stress concentrations, thus suppressing void formation.

3C-SiC carbonization optimization and void reduction on misoriented Si substrates: from a research reactor to a production scale reactor

In this work, we studied how to optimize the carbonization process in order to deposit cubic SiC (3C-SiC) on misoriented Si substrates. The carbonization process is a key step to obtain device-quality layers and to partially release the strain due to the lattice mismatch between the substrate and the thin 3C-SiC film. One of the common problems in SiC/Si epitaxy is the presence of interfacial voids that can degrade the structural quality of the deposited film and the electrical properties in vertical devices. Carbonization optimization requires careful control of several parameters, in particular temperature, precursor flows, and heating ramps. Precise tailoring of this process on a production reactor with the understanding of all the involved variables would require huge effort in terms of time and money, so it is desirable to perform part of this work on a smaller scale research reactor, which is more versatile and economically viable. Starting from a baseline process for 3C-SiC power devices on 100 mm Si already developed on a production reactor, we used a small research reactor to investigate how the thermal heating profile during the carbonization ramp influences the void density on the Si substrate. We have also studied how the introduction of silane during the carbonization ramp can effectively suppress void formation. After having obtained a comprehensive understanding of the variables involved in the process, the key ideas were successfully implemented in the production reactor, resulting in a significant reduction of interfacial voids.

Neutron irradiation effects on the microstructural development of tungsten and tungsten alloys

Data on the microstructural development of tungsten (W) and tungsten rhenium (Re) alloys were obtained after neutron irradiation at 400–800 °C in the Japan Materials Testing Reactor (JMTR), the experimental fast test reactor Joyo, and the High Flux Isotope Reactor (HFIR) for irradiation damage levels in the range of 0.09–1.54 displacement per atom (dpa). Microstructural observations showed that a small amount of Re (3–5%) in W–Re alloys is effective in suppressing void formation. In W–Re alloys with Re concentrations greater than 10%, acicular precipitates are the primary structural defects. In the HFIR-irradiated specimen, in which a large amount of Re was expected to be produced by the nuclear transmutation of W to Re because of the reactor's high thermal neutron flux, voids were not observed even in pure W. The synergistic effects of displacement damage and solid transmutation elements on microstructural development are discussed, and the microstructural development of tungsten materials utilized in fusion reactors is predicted.

Development of a Plane Strain Tensile Geometry to Assess Shear Fracture in Dual Phase Steels

A geometrically modified sample capable of generating a triaxial stress state when tested on a standard uniaxial tensile frame was developed to replicate shear fractures observed during stretch bend tests and industrial sheet stamping operations. Seven commercially produced dual phase (DP) steels were tested using the geometrically modified sample, and the modified sample successfully produced shear fractures on a unique shear plane for all steels. For each steel, void densities were determined, based on metallographic analyses, as a function of imposed displacement. Microstructural properties of ferrite and martensite grain size, martensite volume fraction (MVF), retained austenite content, Vickers hardness, average nanoindentation hardness, average ferrite and martensite constituent hardness, and tensile properties were obtained in order to evaluate potential correlations with void data. A linear correlation was observed between Vickers hardness and the average nanoindentation hardness, verifying the ability of nanoindentation to produce data consistent with more traditional hardness measurement techniques. A linear relationship was observed between the number of voids present at 90% failure displacement and the martensite/ferrite hardness ratio, indicating that a decrease in relative hardness difference in a microstructure can suppress void formation, and potentially extend formability limits. The void population appeared independent of MVF, grain size, and tensile properties suggesting that constituent hardness may be a dominant parameter when considering suppression of void nucleation in DP steels.

Neutron irradiation effects on tungsten materials

To understand the irradiation response of tungsten in the ITER and a DEMO-like reactor, irradiation effects on microstructure development, hardening and electrical resistivity of pure W and W-Re-Os alloys are studied using fission reactor irradiation up to about 1–1.5dpa in the temperature range from 400 to 800°C. Microstructural development and hardening behavior are summarized here based on the irradiation data. Voids are the major damage structure in pure W, but Re addition clearly suppressed void formation. Irradiation hardening was also suppressed in lower Re content alloys. The hardening was caused by irradiation-induced precipitation of WRe (σ-phase) and WRe3 (χ-phase), and the hardening behavior strongly depended on the neutron spectrum of the irradiation field. On the basis of these results, a damage structure development prediction map in fusion devices is suggested.

Suppression of Void Formation in Si0.5Ge0.5 Alloy Nanowire during Ni Germanosilicidation

ABSTRACTThe growth mechanism of Ni‐germanosilicide in Si0.5Ge0.5 NW with and without silicon dioxide shell encapsulation was investigated using transmission electron microscope imaging with in situ annealing of SiGe NW array samples capped with Ni. At 200 and 400 °C, the interfacial reaction is minimal while at 600 °C, we can observe the gradual formation of a thin Ni‐germanosilicide filament for the unconstrained SiGe NW. This observation is concomitant with material loss from the sides of the SiGe NW, which eventually leads to void formation. Energy dispersive X‐ray spectroscopy analysis along the SiGe NW shows that Ge atoms out‐diffused from the NW toward the Ni source during annealing. Excessive out‐diffusion of Ge atoms and void formation are shown to be retarded in the encapsulated SiGe NW with compressive stress.

Void control during plating process and thermal annealing of through-mask electroplated copper interconnects

The quality of the sputtered copper film, which serves as the seed layer for sequent electroplating, becomes critical when the size of crack on the surface of the sputtered film is close to the feature size of the electroplated copper interconnect. The crack results in void formation in electroplated copper before thermal annealing and this phenomenon limits attainable highest anneal temperature. To solve this problem, the sputtered seed layer was slightly etched before electroplating process and a TaN passivation layer was deposited on the electroplated Cu interconnect before thermal annealing. Those processes not only suppressed void formation during the electroplating and annealing process at 300°C, but also resulted in lower electrical resistance in the copper interconnects.

Fracture Toughness of an Advanced Ultrahigh-strength TRIP-aided Steel

The fracture toughness of an advanced ultrahigh-strength 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (in mass%) transformation-induced plasticity (TRIP)-aided steel with a bainitic ferrite and/or martensite structure matrix was investigated for applications in automobiles, construction machines, and pressure vessels. After the steel was austenitized and isothermally transformed via heat treatment at temperatures between 200°C and 350°C below the martensite-finish temperature, it exhibited a good combination of tensile strength (1.4 GPa) and total elongation (15%). In addition, the steel achieved a much higher plane-strain fracture toughness (KIC = 129–154 MPa m 1/2 ) than conventional structural steel such as SCM420 steel (KIC = 57–63 MPa m 1/2 ). Surprisingly, the fracture toughness was nearly the same as that of a maraging steel. Our results indicate that the high fracture toughness was associated with (1) a softened wide lath-martensite matrix with a low carbide content and carbon concentration and (2) effective plastic relaxation of localized stress concentration by the strain-induced transformation of fine metastable retained austenite in the narrow lath-martensite and retained austenite mixture, which suppresses void formation and cleavage crack initiation at the pre-crack tip.

Effects of Re Content and Fabrication Process on Microstructural Changes and Hardening in Neutron Irradiated Tungsten

The effects of the material fabrication process and rhenium (Re) content on the irradiation-induced changes in the microstructure and hardness of pure tungsten (W) and W­Re alloys were investigated. Neutron irradiation of pure W and W­Re alloys (Re concentration 3­26%) was carried out in the experimental fast reactor JOYO. The irradiation conditions were 0.44 displacement per atom (dpa) at 531°C and 0.47dpa at 583°C for pure W and W­Re alloys, respectively. After irradiation, microstructural observations using a transmission electron microscope (TEM) and Vickers microhardness tests were performed. Voids and dislocation loops were observed in both pure W and W­Re alloys after irradiation. The number density of voids in pure W was higher than that in W­3%Re, W­5%Re and W­10%Re. Only in the case of W­26%Re irradiated to 0.47dpa at 583°C were there no voids observed, but irradiation-induced fine precipitates and a few dislocation loops were observed. The irradiation hardening of pure W was greater than that of the W­Re alloys. It was considered that irradiation hardening of pure W was caused mainly by the higher number density of voids. The addition of Re suppressed void formation and irradiation hardening of the W­Re alloys. Irradiation hardening of W was also suppressed in hot-rolled W compared with arc-melted as-cast W. [doi:10.2320/matertrans.MBW201110]

Stress control during thermal annealing of copper interconnects

Grain growth of Cu interconnects in an ultralow k dielectric was achieved at an elevated anneal temperature of 300 °C without stress voiding related problems. For this, a TaN metal passivation layer was deposited on the Cu interconnect surface prior to the thermal annealing process, which suppressed void formation within the Cu features during the anneal process and reduced inelastic deformation within the interconnects after cooling down to room temperature. As compared to the conventional anneal process at 100 °C, the passivation layer enabled further Cu grain growth at elevated temperatures, which then resulted in lower electrical resistance in the Cu interconnects.

Defect evolution in steels: Insights from positron studies

AbstractPositron annihilation studies on austentic and ferritic/martensitic and oxide‐dispersion steels are highlighted so as to emphasize the sensitivity of the technique to defect annealing as well as precipitates. The results obtained on austenitic D9 steels are discussed with a view to emphasize the role of small TiC precipitates in the matrix, which play a critical role in void‐swelling behaviour. Further, the role of Ti/C ratio as well as minor alloying elements on swelling behaviour is discussed. The current focus of research is on the use of nanoclusters in ODS alloys towards the suppression of void formation under irradiation. Recent results on the investigation on nano Yttria particles in Fe – model oxide dispersion ferritic steel, are presented with supporting ab‐initio lifetime calculations. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

電気めっきプロセスを用いた Pt-Ir 拡散コーティング材の耐酸化特性

Oxidation resistance of electroplated Pt and Pt-Ir alloy coatings followed by simple annealing heat treatments was investigated. The so-called 2nd generation Ni-based single crystal superalloy TMS-82+ and 4th generation TMS-138 were used as substrate materials. The cyclic oxidation tests and corresponding surface observations suggested the comparable oxidation resistance of Pt-coated TMS-82+ and Pt-Ir-coated TMS-138. However, cross-sectional microstructural analysis revealed that Pt-coated TMS-82+ exhibited the accelerated formation of voids, indicating the beneficial effect of Ir addition in terms of suppressed void formation. This study also revealed the importance of cross sectional analysis for thoroughly evaluating specimens subjected to the cyclic oxidation tests, and confirmed that oxidation resistance of the electroplated coatings was dependent on the quality of films deposited, which was drastically affected by the substrate surface finishing and substrate composition.

Effects of Minor Additions of Zn on Interfacial Reactions of Sn-Ag-Cu and Sn-Cu Solders with Various Cu Substrates during Thermal Aging

The effects of Zn additions to Sn-0.7Cu and Sn-3.8Ag-0.7Cu (all in wt.% unless specified otherwise) Pb-free solders on the interfacial reactions with Cu substrates were investigated. The study was focused on the intermetallic compound (IMC) growth, Cu consumption and void formation as a function of thermal aging and solder composition. Four different kinds of Cu substrates (high-purity Cu, oxygen-free Cu, vacuum-sputtered Cu, and electroplated Cu) were prepared to compare their interfacial reaction behaviors with Zn-added solders. Thermal aging was performed at 150°C for up to 1000 h to accelerate the interfacial reactions between solders and Cu substrates. Growth of IMCs (Cu6Sn5 and Cu3Sn) in Zn-added solders was slower than those without Zn additions. The growth of the Cu3Sn phase, in particular, was drastically reduced in the Zn-added solders on all four Cu substrates. On an electroplated Cu substrate, numerous voids were observed in the Cu3Sn phase for Sn-Cu and Sn-Ag-Cu solders aged at 150°C for 1000 h. However, these voids were largely eliminated in the Zn-added solders. On the other three Cu substrates, the conditions which produce a high density of voids were not found after aging both solders with and without Zn. The Cu consumption with Zn-added solders was also significantly lower. The beneficial effects of Zn additions on interfacial reaction behaviors are reported, and the corresponding mechanisms in suppressing void formation and Cu consumption due to Zn additions will be discussed.

Void swelling behavior in electron irradiated Fe–Cr–Ni model alloys under temperature variation

We examined the effect of short HVEM pre-irradiation at lower or higher temperature on void swelling behavior in electron irradiated austenitic model alloys, Fe–15Cr– xNi ( x = 15, 20, 25 or 30 wt%). By 6 dpa irradiation at constant temperature, voids were formed at 200–400 °C in 15, 20, and 25Ni alloys but did not form above 350 °C in the 30Ni alloy. In the temperature variation irradiation, an initial 0.6 dpa irradiation at 200–500 °C was followed by irradiation of 5.4 dpa at 400 °C. The effect of pre-irradiation at lower temperature was a slight increase in the void number density, and that at higher temperature was a suppression of void formation. SFT induced by irradiation at lower temperature were not effective void nuclei in the subsequent irradiation at higher temperature.

Analysis of the Degradation Mechanism during Repeated Overwrite of Phase-Change Discs

Rewritable optical digital versatile discs (DVDs) were analyzed to identify alterations in layer composition after repeated overwrite cycles. Elemental depth profiling revealed the presence of S in the phase-change layer in areas with high jitter values. This we relate to a decomposition of the ZnS:SiO2 dielectric layer during repeated overwrite. The S concentration in the phase-change layer at the location of the repeatedly overwritten tracks is estimated to range between 0.1 and 1 at. %, depending on whether or not a caplayer is present between the ZnS:SiO2 layer and the phase-change layer. From electron microscope images we established that repeated overwrite cycles induce voids in the phase-change layer. By analyzing the repeatedly overwritten areas of various recording stacks we collected evidence that a caplayer may suppress void formation. Both S diffusion and void formation will limit the overwrite cyclability of optical discs, which explains the frequently reported beneficial effect of a caplayer on overwrite performance.

Theoretical study of vacancy supersaturation during silicon crystal growth and nitrogen-doping effects

We performed thermodynamical simulations of the vacancy supersaturation during silicon crystal growth and investigated nitrogen-doping effects on the suppression of void formation and the enhancement of oxygen precipitates. Although the mechanism with the N 2V complex has been proposed to explain the suppression, we found that it cannot reproduce the suppression even with nitrogen density as high as 10 20 cm –3. On the other hand, the mechanism with the N 2V 2 complex can reproduce the suppression with about 10 15 cm –3 of nitrogen, which is consistent with the experiments. It also explains the enhancement of oxygen precipitates. Therefore, the N 2V 2 complex plays the dominant role in the vacancy aggregation process.

Suppression of void coalescence in thermal aging of tin-silver-copper-X solder joints

Addressing the potential for drop impact failure of Pb-free interconnects, the shear ductility after extensive aging of Sn-Ag-Cu (SAC) solders has been improved radically by Co or Fe modifications. Several other SAC+X candidates (X=Mn, Ni, Ge, Ti, Si, Cr, and Zn) now have been tested. Solder joint microstructures and shear strength results show that new SAC+X alloys also suppress void formation and coalescence at the Cu (substrate)/Cu3Sn interface (and embrittlement) after aging at 150°C for up to 1,000 h. Microprobe measurements of 1,000 h aged samples suggest that Cu substitution by X is usually accentuated in the intermetallic layers, consistent with X=Co and Fe results.

Ni-Salicided Poly-Si/poly-SiGe-Layered Gate Technology for 65-nm-node CMOSFETs

A poly-Si/poly-SiGe-layered gate electrode with a high Ge content that is suitable for Ni silicidation has been examined. The optimum Ge content for suppressing gate depletion was found to be 30%. It was found that the in situ deposition of a thin Si layer after SiGe deposition was effective in suppressing void formation. A smooth Si0.7Ge0.3 film without voids was obtained as either polycrystalline or amorphous. The enhancement of boron activation in poly-Si0.7Ge0.3 was confirmed while gate depletion became large for α-Si0.7Ge0.3 because of its lower boron diffusivity. Stable Ni silicidation over the 300 mm wafers was achieved by using a poly-Si/poly-Si0.7Ge0.3-layered gate electrode.

The impact of pultrusion processing parameters on resin pressure rise inside a tapered cylindrical die for glass-fibre/epoxy composites

Pultrusion is a continuous process with application in the manufacture of fiber-reinforced composites. This work is focused on understanding the impact of various pultrusion process control parameters on the pressure-rise phenomenon inside a cylindrical die for glass-fibre/epoxy composites. The important process control parameters in pultrusion manufacturing are the pull speed, fiber volume fraction, viscosity, die heating zones temperature settings, and pre-form plate area ratio (compaction ratio). The pressure rise in the die inlet contributes to a major extent in enhancing fiber wet-out and suppressing void formation in the manufactured composite. A numerical model for evaluating the pressure-rise behavior inside the die inlet is developed on the basis of Darcy's law since the resin flow through the fibers is similar to the flow through porous media. A fixed control-volume-based finite-difference method known as the finite-volume method is employed to solve the governing equations of a three-dimensional axis-symmetric cylindrical die inlet geometry. Also, a variable viscosity model is used to compute the viscosity in the straight portion of the die.

Die Inlet Contour Impact on the Pressure Rise in a Pultrusion Die

The quality of a pultruded composite depends on many process control parameters and variables. The pressure rise in the pultrusion die inlet is one such variable which can impact the quality of a pultruded product. An appreciable die inlet pressure rise can suppress void formation and enhance fiber "wet out." In this study, a model based on Darcy's law for flow in a porous medium and which employs the finite volume solution method is developed to predict the pressure and velocity fields as a function of various die inlet contours. Investigating the behavior of the pressure rise in the pultrusion die inlet as a function of the shape of the die inlet contour can be a very useful and effective tool for analyzing and designing pultrusion die inlets. Various die inlet contours such as the wedge, circular tapered die inlet, and some hypothetical die inlet contours were investigated to determine the impact on die inlet pressure rise. The original contributions of this paper are results which are useful for designing pultrusion dies and for producing an optimum pressure rise which will enhance the quality of the pultruded product.