Impact of selective oxidation during inline annealing prior to hot-dip galvanizing on Zn wetting and hydrogen-induced delayed cracking of austenitic FeMnC steel

The present study discusses the impact of selective oxidation during in-line annealing of Fe–23%Mn–0.6%C–0.3%Si steel on surface and sub-surface properties and is focused on hot-dip galvanizability and susceptibility to hydrogen-induced delayed cracking. Annealing temperature (700–1100 °C) and dewpoint DP (? 15/?30/?50 °C) of the 5%H₂–N₂ annealing atmosphere were varied in order to investigate Zn wetting in dependence on selective oxidation of Mn and Si. Sub-surface microplasticity (hardness, pop-in frequency, pop-in activation load) was examined by electrochemical nanoindentation in-situ to hydrogen charging (ECNI) to assess hydrogen/material interactions. Zn wetting fails if external Mn and Si oxidation is not avoided by performing high reductive bright annealing (1100 °C/DP ? 50 °C). Zn wetting will however turn to increase if a roughly globular MnO layer appears and Si is internally oxidized (700–900 °C/DP ? 15 °C). Selective oxidation further affects hydrogen/material interactions by influencing the local distribution of solid-soluted Mn: ECNI results indicate hydrogen-induced dislocation demobilization (HEDE mechanism) or dislocation mobilization (HELP mechanism) in dependence on the local amount of solid-soluted Mn within the sub-surface. Macroscopic delayed cracking seems to occur earlier if HELP is predominating. The gained results benefit understanding the impact of selective oxidation on galvanizability and susceptibility to hydrogen-induced failure of austenitic FeMnC steel and advance further developments in processing high Mn alloyed steels.     

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Reactive vs non-reactive wetting of ZrB2 by azeotropic Au–Ni

The equilibrium contact angles and spreading kinetics of Au–Ni droplets of azeotropic composition on ZrB₂ substrates were measured by the dispensed drop technique in high vacuum. Experiments were performed at two temperatures, 980 and 1170 °C, leading to different interfacial reactivities, namely, simple substrate dissolution (1170 °C) and compound formation at the interface (980 °C). The contact angle of Au–Ni on unreacted ZrB₂ is evaluated and related to the type of bonding established at metal/metal-like ceramic interfaces. The experimental results on reactive wetting are compared with predictions of two different approaches proposed recently in order to explain the thermodynamics and kinetics of this type of wetting.     

Generation and characterization of T40/A5754 interfaces with lasers

Laser-induced reactive wetting and brazing of T40 titanium with A5754 aluminum alloy with 1.5 mm thickness was carried out in lap-joint configuration, with or without the use of Al5Si filler wire. A 2.4 mm diameter laser spot was positioned on the aluminum side to provoke spreading and wetting of the lower titanium sheet, with relatively low scanning speeds (0.1–0.6 m/min). Process conditions did not play a very significant role on mechanical strengths, which were shown to reach 250–300 N/mm on a large range of laser power and scanning speeds. In all cases considered, the fracture during tensile testing occurred next to the TiAl₃ interface, but in the aluminum fusion zone. The interfacial resistance was then evaluated with the LASAT bond strength tester, based upon the generation and propagation of laser-induced shock waves. A 0.68 GPa uniaxial bond strength was estimated for the T40/A5754 interface under dynamic loading conditions.     

Use of pre-oxidation to improve reactive wetting of high manganese alloyed steel during hot-dip galvanizing

The present study discusses hot-dip galvanizing of a Fe–23% Mn–0.6% C–0.3% Si steel using a Zn–0.22%Al bath. The paper concentrates on reactive Zn wetting on top a covering external oxide layer occurring after in-line annealing. Annealing was performed by soaking at 800 °C/60 s in 5% H₂–N₂ at different dewpoints. In-line pre-oxidation at 600 °C/10 s in 1.8% O₂–N₂ was further performed and the impact on selective oxidation as well as reactive Zn wetting was examined. After conventional annealing Zn wetting turns to increase if a roughly globular MnO layer appears on the external steel surface and Si is internally oxidized. Reactive wetting including the formation of Fe₂Al₅ crystals occurs on top of the MnO layer, because metallic-bond Fe exists incorporated within this MnO layer (→MnO·Fe_metall layer). The amount of metallic-bond Fe within the MnO·Fe_metall layer increases considerably if pre-oxidation is conducted. This results in an intensified Fe₂Al₅ formation on top of a MnO·Fe_metall, which improves liquid Zn wetting. Brittle Fe–Zn intermetallics were absent in all trials. These results offer a new way for hot-dip galvanizing (high) Mn alloyed steels. The absence of Fe–Zn intermetallics and (partial) MnO reduction implies that the currently discussed model of aluminothermic MnO reduction during hot-dip galvanizing Mn alloyed steel seems not to be dominating in the present case of reactive Zn wetting on top of a covering MnO·Fe_metall layer.     

Structural and electronic properties of Si nanocrystals embedded in amorphous SiC matrix

Si-rich hydrogenated amorphous silicon carbide thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited films were subsequently annealed at 900 °C and 1000 °C to form Si nanocrystals embedded in amorphous SiC matrix. Raman spectra demonstrate the formation of Si nanocrystals with size around 7–9 nm. For the sample annealed at 1000 °C, the crystallinity can be reached to 70%. As increasing the annealing temperature, the dark conductivity is increased accompanying with the increase of crystallinity of the film. The dark conductivity reaches to 1.2 × 10^?6 S cm^?1 for the sample annealed at 1000 °C, which is 4 orders of magnitude higher than that of as-deposited film. It is found that the carrier transport process is dominated by the thermally activated transport process according to the temperature-dependent conductivity results.     

Thermal properties of W–Cu composites manufactured by copper infiltration into tungsten fiber matrix

W–Cu composites were produced by the technique of copper infiltration into tungsten fiber preforms (CITFP) under vacuum circumstance. Fibrous structure preforms with various volume fraction of tungsten fiber were fabricated by the process of mold pressing and sintering. The molten copper was infiltrated into the open pores of the preforms under vacuum at 1473 K to 1573 K for 1 h to produce W–Cu composites with compositions of 10–30 wt.% copper balanced with tungsten. The microstructure, relative densities, and thermal properties of the composites were investigated and measured. The relative as-sintered density was enhanced with the increase of the sintering temperature. The thermal conductivity of the W–Cu30 composite with 28.2 wt.% Cu was 241 W/(m · K) at 298 K, 10% higher than that of the W–Cu alloy with similar copper content produced by conventional powder metallurgy process. The thermal expansion of the composites was decreased with the increase of tungsten content, keeping the same tendency as the prediction by the rule of weighted average of volume ratio of compositions.     

Wetting behavior of Al alloys on a TiH2 substrate

The wetting behavior of Al–Ge alloys on TiH₂ substrates was investigated by an improved sessile drop method under high vacuum and in a temperature range of 773–818 K. Results indicate that the equilibrium contact angles of Al–Ge/TiH₂ increase linearly with temperature according to the following formula: θ = 0.2882T ? 85.04, and decrease linearly as the Ge content increases from 25.2% to 36.2% according to the formula: θ = 214 ? 200Ge (wt.%). The worst wetting behavior was obtained for a pure Al/TiH₂ system at its foaming temperature (973 K). TiH₂ particles were prone to aggregate and were thus difficult to disperse. This could be one of the reasons for closed-cell aluminum foam products having non-uniform pores.     

Wetting and interfacial phenomena in Ni–HfB2 systems

The wettability and reactivity between polycrystalline hot-pressed HfB₂ and liquid Ni, Ni–Ti and Ni–B alloys have been investigated by the sessile drop method up to 1520 °C. Independently of the sintering aids (B₄C, HfSi₂), high-temperature interactions led to the formation of a bimodal interface profile, due to the competition between the strong dissolution of HfB₂ in the liquid phase and drop spreading along the substrate surface. Ni demonstrates good wetting with the HfB₂ substrates (θ = 20°). Compared to Ni, high Ti-containing alloys reveal much faster wettability kinetics with a final contact angle below 10°, accompanied by the formation of an interfacial Ti-rich reaction product. The eutectic NiB alloy shows complete wetting and fast spreading on HfB₂ and minor dissolution of the ceramic into the liquid, a clear indication that higher B contents could optimize the interfacial structure. These results are of practical interest for liquid-assisted composite synthesis and joining of HfB₂-based ceramics.     

Phase and microstructural evolution of Ir–Si binary alloys with fcc/silicide structure

To search hardening approach or new probable phases benefiting to high temperature behavior of Ir-based superalloys, Ir alloyed with Si was employed. Investigations on phase and microstructural evolution of a series of Ir–xSi (x=2.5, 5, 15, 20, 30, 36 and 45 mol%) binary alloys were carried out by XRD, EPMA and SEM analysis. A schematic plot of the Ir–Si binary diagram with the nominal Si content ranging from 0 to 50 mol% was primarily drafted. Room temperature mechanical properties, the Vicker hardness and Young's modulus, of bulk material or each kind of phases were also measured. Researches reveal that with Si addition up to 50 mol%, the microstructures are respectively composed of primary Ir solid solution fcc+peritectic Ir₃Si silicide (nominal Si content: 0–25 mol%), primary Ir₃Si+eutectoid silicide (Si: 25–33.3 mol%), Ir₃Si₂+eutectoid silicide (Si: 33.3–40 mol%) and primary IrSi+Ir₃Si₂ silicide (Si: 40–50 mol%). With plastic characteristic, the fcc phase has the low Vickers hardness and Young's modulus, while both of the silicides are high and the silicides behave brittle. For the high temperature applications over 1400 °C, Ir-based alloys with Si dropping must avoid the appearance of any kind of Ir/Si silicides in microstructure because the melting points of silicides (Ir₃Si, Ir₂Si and Ir₃Si₂) are close to 1400 °C; instead, solid solution hardening on Ir by Si is recommended.     

High temperature oxidation of Mo–Si–B alloys: Effect of low and very low oxygen partial pressures

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10^?19 and 10^?12 bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10^?6–10^?4 bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

The novel eutectic microstructures of Si–Mn–P ternary alloy

The microstructures of Si–Mn–P alloy manufactured by the technique of combining phosphorus transportation and alloy melting were investigated using electron probe micro-analyzer (EPMA). The phase compositions were determined by energy spectrum and the varieties of eutectic morphologies were discussed. It is found that there is no ternary compound but Si, MnP and MnSi_1.75?x could appear when the Si–Mn–P alloy's composition is proper. Microstructure is greatly refined by rapid solidification technique and the amount of eutectic phases change with faster cooling rates. Moreover, primary Si or MnP are surrounded firstly by the binary eutectic (Si + MnP) and then the ternary eutectic (Si + MnSi_1.75?x + MnP) which also exhibit binary structures due to divorced eutectic determined by the particularity of some Si–Mn–P alloys.     

Columnar to equiaxed transition in directional solidification of inoculated melts

Experiments on transient directional solidification were carried out with Al–3% Si and Al–7% Si alloys to study the modification by melt inoculation of the as-cast ingot macrostructure with columnar and equiaxed zones into a completely refined equiaxed structure. Without inoculation the macrostructure consists of typical columnar and equiaxed zones, separated by a relatively thin region of columnar to equiaxed transition. As the inoculant Al–3% Ti–1% B was added to the melt in a series of experiments relatively small equiaxed grains were observed in the as-cast macrostructure. For larger inoculant additions the number of these equiaxed grains increased. The macrostructure became completely refined and equiaxed as the Ti concentration in the melt increased in the small range 0.002 < %Ti < 0.01 for Al–3% Si, and of 0.029 < %Ti < 0.075 for Al–7% Si. The larger inoculant additions for the Al–7% Si alloy are attributed to Si poisoning of the inoculant. This macrostructure modification occurred with an increasingly large fraction of fine equiaxed grains mixed with columnar grains, rather than by a significant decrease in the length of the columnar grains. Solidification paths were calculated from the measured cooling curves and superimposed on dimensionless growth maps, enabling good qualitative predictions of the macrostructure, especially the existence of the mixed region. The maps, however, predict that the position where columnar grains are completely blocked is closer to the ingot base than that observed experimentally. This discrepancy might be related to the restrictive hypothesis assumed to construct the growth maps, not valid in the present experiments, of steady-state solidification conditions.     

Thermodynamic assessment of chemical and electrochemical stability of nickel–silicon system alloys

Phase and chemical equilibria in Ni–Si system at 298 K are considered. The possible maximum solid solubility of Si in fcc-Ni at 298 K is estimated.The Ni–Si–O state diagram at 298 K is plotted. The Ni–Si–O system invariant conditions are calculated. The potential–pH diagram of the Ni–Si–H₂O system at 298 K, air pressure of 1 bar and activities of ions in solution, equal to 1 mol/l is plotted. Basic chemical and electrochemical equilibria in Ni–Si–H₂O system are considered.     

Characterization studies of pulse magnetron sputtered hard ceramic titanium diboride coatings alloyed with silicon

The composition, structure and mechanical properties of pulsed-DC unbalanced magnetron sputtered Ti–Si–B thin films—hard coatings with the potential for excellent thermal stability and oxidation resistance—are investigated and reported in this paper. Fully dense, hard (19–37 GPa) Ti–Si–B coatings were deposited at substrate bias voltages (V_s) ranging from floating potential to −150 V which resulted in substrate temperatures of ∼90–135 °C. We found that variation of substrate biasing conditions critically affected film composition, structure and resultant mechanical properties. For instance, concentration of Si in films decreased from 18.4 at.% to 3.8 at.% as V_s was increased from floating potential to −150 V; composition profile analysis of the near-surface region of films (0–10 nm) revealed them to be rich in Si with significant differences among specimens produced at different substrate bias conditions. Variation of substrate biasing conditions provided coating structures that ranged from completely amorphous at floating substrate potential to nanocrystalline at V_s = −50 to −100 V and crystalline nanocolumnar at V_s = −150 V. We found that each of the structures obtained exhibited different specific values of hardness and elastic modulus, which is also in a good agreement with results reported for other coatings possessing similar micro- and nano-structures. Film structure was analyzed in detail by conventional and analytical transmission electron microscopy. Coatings that exhibited the highest values of hardness (37 GPa) were found to possess features such as crystalline nanocolumnar grains a few nanometres in diameter and disordered intergranular regions of different chemical composition, thus qualifying as nanocomposite films. Results of this work allowed relationships to be drawn between deposition parameters and Ti–Si–B coating composition, structure and mechanical properties. Qualitatively similar relationships are also expected for other biased plasma-assisted physical vapour deposited transition-metal-based ceramic coatings alloyed with Si (e.g. Ti–Si–N, Cr–Si–N, Cr–Al–Si–N).     

Thermoelectric properties of Sb-doped Mg2Si semiconductors

The thermoelectric properties of Sb-doped Mg₂Si (Mg₂Si:Sb = 1:x(0.001 ≦ x ≦ 0.02)) fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. Sb-doped Mg₂Si samples are n-type in the measured temperature range. The electron concentration of Sb-doped Mg₂Si at 300 K ranges from 2.2 × 10¹⁹ for the Sb concentration, where x = 0.001, to 1.5 × 10²⁰ cm⁻³ for x = 0.02. First-principles calculation revealed that Sb atoms are expected to be primarily located at the Si sites in Mg₂Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Sb concentration. The sample x = 0.02 shows a maximum value of the figure of merit ZT, which is 0.56 at 862 K.