Wt Pct Si Research Articles

Solidification in the system Al-Ge-Si: The phase diagram, coring patterns, eutectic growth, and modification

In the ternary system Al-Ge-Si, the binary eutectic reactions, L (liquid) ⇔ (Al) + (Si) and L ⇔ (Al) + (Ge) are connected by a monovariant valley, L ⇔ (Al) + (SiGe), falling from 578 °C at 12.7 wt pct Si to 424 °C at 53 wt pct Ge. The binary, solid eutectic surface extends from the Al corner of the ternary phase diagram (1.65 wt pct Si to 5.2 wt pct Ge) across to the continuous (SiGe) solid solution which contains very little Al: several tie triangles, L-(A1)-(SiGe), have been determined using thermal analysis and electron microprobe analysis (EMPA). Optical and scanning electron microscopy (SEM) examination reveals that coring in the normal (SiGe) phase is discontinuous, showing composition banding, which indicates that stationary 111 facets of the solid solution were exposed to the liquid for extended periods up to 500 seconds; details of this interrupted coring were quantified by EMPA. Similar, smaller, and more gradual variations could also be detected in the (Al) matrix, and these compositional fluctuations are considered to reflect discontinuities in the local eutectic growth rates. Modification and twin-ning, induced by Na, are observed in both Al-Si and Al-Ge but decrease progressively with Ge content; coring in the modified ternary alloys is more continuous.

Effect of te on morphological transitions in Fe-C-Si alloys: Part i. Directional solidification

Directional solidification experiments have been carried out on Fe-3.4 wt pct C-2 wt pct Si alloys with and without the addition of 0.06 wt pct Te. The effect of the Te addition upon the gray-to-white transition velocity and the white-to-gray transition velocity has been determined. Quenched solid/liquid interface studies on mixed structures, white + type D gray and type D gray + type A gray, have determined the temperature differences at the growth front for these structures.

The effects of silicon on the reaction between solid iron and liquid 55 wt pct Al−Zn baths

The effect of various silicon levels on the reaction between iron panels and Al-Zn-Si liquid baths during hot dipping at 610°C was studied. Five different baths were used: 55Al−0.7Si−Zn, 55Al−1.7Si−Zn, 55Al−3.0Si−Zn, 55Al−5.0Si−Zn, and 55Al−6.88Si−Zn (in wt pct). The phases which formed as a result of this reaction were identified as Fe2Al5 and FeAl3 (binary Fe−Al phases with less than 2 wt pct Si and Zn in solution),T1, T2, T4, T8, andT 5H (ternary Fe−Al−Si phases), andT 5C (a quaternary Fe−Al−Si−Zn phase). Compositional variations through the reaction zone were determined. The phase sequence in the reaction zone of the panel dipped for 3600 seconds in the 1.7 wt pct Si bath was iron panel/(Fe2Al5+T 1)/FeAl3/(T 5H+T 5C)/overlay. In the panel dipped for 1800 seconds in the 3.0 wt pct Si bath the reaction zone consisted of iron panel/Fe2Al5/(Fe2Al5+T 1)/T 1/FeAl3/(FeAl3+T 2)/T 5H/overlay. In the panel dipped for 3600 seconds in the 6.88 wt pct Si bath the phase sequence was iron panel/Fe2Al5/(Fe2Al5+T1)/(T1+FeAl3)/(T1+T2)/T2/T8/T4/overlay. The growth kinetics of the reaction zone were also studied. A minimum growth rate for the reaction zone which formed from a reaction between the iron panel and molten Al−Zn−Si bath was found in the 3.0 wt pct Si bath. The growth kinetics of the reaction layers were found to be diffusion controlled in the 0.7, 1.7, and 6.88 wt pct Si baths, and interface controlled in the 3.0 and 5.0 wt pct Si baths. The presence of the interface between theT2/T5H, Fe2Al5/T 1, orT 1/FeAl3 phases is believed responsible for the interface controlled growth kinetics exhibited in the 3.0 and 5.0 wt pct Si baths.

Structure and properties of a β solution treated, quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S2 by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.

Modeling of fixed bed heat storage units utilizing phase change materials

A computer model has been developed for the calculation of the heat exchanged and temperature profiles in a packed bed containing a phase change material. The packed bed is intended as a heat storage unit in which an inert fluid flowing through the bed exchanges heat with an encapsulated spherical shot of the phase change (melting and freezing) material. Examples of predicted bed temperature profiles during heat storage and utilization cycles are given. For A1-12 wt pct Si and Al-30 wt pct Si shot, a sequence of storage and utilization cycles with cyclic cocurrent fluid flow was found to utilize the high latent heat of fusion of the shot efficiently and permit the utilization of the bed as a near isothermal (577°C) heat recuperator.

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ2V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstatten structures on furnace or air cooling, fine Widmanstatten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstatten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.

Low temperature deposition of tribological coatings

The results of microstructural and tribological studies performed on sputterdeposited coatings of ZrO2 20Y2O3, TiB2, and B18 wt pct Si are reported. In each case, deposition parameters were identified that offered good tribological properties while maintaining low deposition temperatures (< 400 °C) to avoid distortion and softening of prior heattreated substrates. Zirconia deposits were prepared with a pronounced columnar microstructure that provided excellent thermal fatigue and wear resistance, making them candidates for heat engine applications. The TiB2 coatings were the hardest coatings prepared. These provided good wear resistance on steel substrates. The BSi coating displayed excellent wear resistance and good adherence to Cu and Al alloys.

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al3Fe and the incommensurate structure Al x Fe. In Al3Fe, stacking faults on {001} were frequently observed. The structure of Al x Fe is probably related to Al6Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.

The liquidus surface for the Al- Li- Si system from 0.20 wt Pct Li and 0.20 Wt pct Si

The liquidus surface for the Al- Li- Si system from 0.20 wt Pct Li and 0.20 Wt pct Si

Modification in the aluminum silicon system

Alloys in the range 0 to 24 wt pct Si have been examined by careful thermal analysis, macroand microscopical study with modification by a mixed alkali fluoride flux, and by separate additions of sodium, potassium, lithium, and strontium. Sodium and strontium exert similar effects and potassium and lithium differing and minor effects; with a mixed alkali flux the influence of sodium is dominant. The normal and modified eutectic arrests in the presence of primary aluminum both show comparable supercoolings and recalescence behavior relative to the horizontal growth temperatures, but this is not observed in hypereutectic alloys containing primary silicon. Both normal and modified eutectics grow radially inward from crucible walls, but the details of the growth fronts are very different: there is no nucleation from the bulk liquid in modified alloys and it is uncertain if this occurs in normal alloys. It is concluded that the structural modification in furnace cooled ingots, as in directionally grown samples, is primarily caused by modified growth of silicon. The mechanism(s) for such modification are briefly discussed.

Deformation processing of an aluminum alloy containing particles: Studies on AI-5 pct Si alloy 4043

Al-5 wt pct Si alloy is processed by upset forging in the temperature range 300 K to 800 K and in the strain rate range 0.02 to 200 s−1. The hardness and tensile properties of the product have been studied. A “safe” window in the strain rate-temperature field has been identified for processing of this alloy to obtain maximum tensile ductility in the product. For the above strain rate range, the temperature range of processing is 550 K to 700 K for obtaining high ductility in the product. On the basis of microstructure and the ductility of the product, the temperature-strain rate regimes of damage due to cavity formation at particles and wedge cracking have been isolated for this alloy. The tensile fracture features recorded on the product specimens are in conformity with the above damage mechanisms. A high temperature treatment above ≈600 K followed by fairly fast cooling gives solid solution strengthening in the alloy at room temperature.

Kinetic and Thermodynamic Aspects of the Bainite Reaction in a Silicon Steel

ABSTRACTThe bainite reaction in an Fe - 3.85 wt pct Si - 0.9 wt pct C steel is studied by several experimental techniques in the range of 250–450°C.The high silicon content prevents the formation of cementite, so that the reaction is separated to two clearly distinct stages. In the primary stage ferrite forms alone, except at temperatures lower than 310°C where some carbides precipitate in it, and austenite becomes enriched in carbon. In the secondary stage occurring only above 400°C, the enriched austenite decomposes to ferrite and an unknown silicon carbide.The microstructure, the enrichment of the austenite and the overall reaction kinetics of the two stages are studied and are found to be consistent with a displacive mechanism of the bainite reaction.A tentative model, accounting for the competition of shear and diffusion, is proposed in order to fit our experimental data.

The effect of nitrogen on stacking fault energy of Fe-Ni-Cr-Mn steels

The effect of nitrogen content on stacking fault energy (SFE) has been measured in a series of Fe-21Cr-6Ni-9Mn alloys. Stacking fault energies were determined from node measurements using weak beam imaging techniques in transmission electron microscopy. Nitrogen additions lower the SFE from 53 mJ/m2 at 0.21 wt pct to 33 mJ/m2 at 0.24 wt pct. Further increases to 0.52 wt pct do not markedly change the SFE. Carbon and silicon had no effect on SFE in the ranges 0.010 to 0.060 wt pct C and 0.17 to 0.25 wt pct Si. The shift in SFE from 0.21 to 0.24 wt pct N is accompanied by a transition to a more planar plastic deformation mode. The sharp transition precludes the use of linear regression analysis for relating SFE to nitrogen content in this class of alloys.

Diffusivity of carbon in austenitic Fe‐Si‐C alloys

Diffusion couple experiments on the carbon diffusion in austenitic Fe‐Si‐C alloys at temperatures between 950 to 1150°C. Influence of the Si‐content in the range 1 to 6 wtpct Si and of the carbon content on the carbon diffusivity. Discussion of the decrease in diffusivity caused by Si. Method for computer evaluation of concentration‐penetration curves from diffusion couple experiments.

Gaseous oxygen absorption by molten iron and some Fe-AI, Fe-Si, Fe-Ti, and Fe-V alloys

An experimental investigation of the initial rates of oxygen dissolution in molten iron and some Fe-(≤9 pct Al), Fe-(≤6 pct Si), Fe-(≤1 pct Ti) and Fe-(≤1 pct V) alloys was carried out in pure oxygen. Two experimental techniques were employed in this study: a modified constant-volume Sieverts method and a falling droplet technique. It was found that the oxidation behavior of liquid iron-based alloys in gaseous pure oxygen as a function of alloy composition was similar under conditions of the falling droplet and modified constant volume Sieverts methods. Marked declines in the oxygen absorption rates were observed for Fe-Al and Fe-Si alloys when the initial alloy compositions reached 6 wt pct Al and 3 wt pct Si in iron, respectively. This behavior indicated a change in mode of oxidation from a burning to a passive type. Fe-Ti and Fe-V alloys initially containing up to 1 wt pct solute in iron exhibited only a burning type behavior. The sudden decline in oxygen absorption rate in molten iron-aluminum and iron-silicon alloys is discussed in terms of changes in the nature of the surface oxide film with increasing amounts of alloying element in the metal.

Effects of silicon on the oxidation, hot-corrosion, and mechanical behavior of two cast nickel-base superalloys

Cast specimens of nickel-base superalloys 713C and Mar-M200 with nominal additions of 0, 0.5, and 1 wt pct Si were evaluated for oxidation and corrosion resistance, tensile and stress-rupture properties, microstructure, and phase relations. Results are com-pared with those of an earlier study of the effects of Si in B-1900. Si had similar effects on all three superalloys. It improves oxidation resistance but the improvement in 713C and Mar-M200 was considerably less than in B-1900. Hot-corrosion resistance is also improved somewhat. Si is, however, detrimental to mechanical properties, in particular, rupture strength and tensile ductility. Si has two obvious microstructural effects. It in-creases the amount of γ precipitated in eutectie nodules and promotes a Mo(Ni,Si)2 Laves phase in the alloys containing Mo. These microstructural effects do not appear responsible for the degradation of mechanical properties, however.

Pulsed laser induced deformation in an Fe-3 Wt Pct Si alloy

The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5×108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.

Unidirectionally solidified Ti−TiB and Ti−Ti5Si3 eutectic composites

Induction melting and electron beam melting techniques were employed in the production of unidirectionally solidified eutectic composites of Ti-1.7 wt pct B and Ti-8.5 wt pct Si. The grown eutectics were reinforced by 7.7 volume pct of TiB fibers and 31 volume pct of Ti5Si3 fibers respectively. Controlled dendritic solidification of a hypereutectic composition of Ti-12 wt pct Si was also accomplished. Tensile, compressive, creep, and stress rupture specimens were cut from the eutectic composites and tested with reinforcing fibers parallel to the load axis. Ti−TiB eutectic was found to have less than the critical volume fraction of fibers necessary for reinforcement, while Ti−Ti5Si3 composite attained a compressive yield strength of 275,000 psi and a compressive Young's modulus of 30×108 psi after heat treatment. The 500 and 4000 hr stress rupture properties of Ti−Si eutectic were superior to commercial titanium alloys at 1000° and 1200°F. The minimum creep rate of Ti−Ti5Si3 eutectic composite was lower than all other titanium alloys at 1000°F. Tensile, compressive, and creep properties of the Ti-8.5 wt pct Si eutectic are discussed in terms of the current theories of composite behavior.