Low Linear Thermal Expansion Coefficient Research Articles

Древесно-композитные плиты с низким коэффициентом линейного теплового расширения

A number of industries require materials with a low coefficient of linear thermal expansion (CLTE), in particular, in the production of satellite spherical antennas. The latter are formed from composites containing carbon fibers and synthetic resins. The composition is cured by heating up to 180 °C. This leads to a thermal expansion of the mold and a change in the geometric characteristics of the product. Therefore, specific requirements are imposed on the materials for making molds. The high cost of special materials used for molds determines the need to search for other materials with a low CLTE. Wood is a possible solution to this problem. Its CLTE along the fibers is less than that of the vast majority of materials, and is approximately 3 ‧ 10-6 K–1, which is comparable to special materials. However, the expansion of wood across the fibers is much higher than the longitudinal one, which excludes the use of solid wood. Anisotropy can be reduced by creating a composite in which the fibers are uniformly oriented in all structural directions, bringing the value of wood expansion across the fibers closer to the value of expansion along the fibers. The traditional approach to producing wood composites, based on the use of synthetic adhesives, fails to achieve a noticeable reduction in thermal expansion due to the high CLTE of adhesives The use of boards made of hydrodynamically activated wood particles without binders is promising. Three series of experiments have been carried out: with varying the density of the boards, preliminary thermal modification of the original wood and the use of alkali during hydrodynamic processing. The thermal expansion study has been carried out using the NETZSCH DIL-402 C induction dilatometer in dynamic mode with a heating rate of 2 K/min. It has been established that thermal expansion increases with increasing density.The average CLTE at a density of 950 kg/m3 is 12 ‧10–6 K–1 and at a density of 1,100 kg/m3 it is 17‧10–6 K–1. At a comparable density, the thermal expansion of boards without binders is significantly lower than that of fiberboards (MDF). Preliminary thermal modification of wood does not significantly affect the CLTE of the boards. The use of alkali in the hydrodynamic treatment also has no effect.

Magnet selection for permanent magnet synchronous machines by decision science

This work investigates magnet selection for permanent magnet synchronous machines (PMSMs) by decision science. 20 multiple-attribute decision-making (MADM) methods ranked a list of 45 industrial-grade permanent magnets based on eight property requirements―high energy density (BHMAX), high coercivity (HC), high magnetic remanence (BR), high electrical resistivity (ρ), high Curie temperature (TC), high maximum service temperature (TS), low linear thermal expansion coefficient (α), and low density (d)―targeted to selecting magnets for PMSMs. The Shannon's entropy method evaluated the objective attribute weights of ρ, BHMAX, and HC, as ∼46%, ∼24%, and ∼23%, respectively, and the rest—BR, TC, TS, d, and α―combined as ∼7%; equal weights (∼12.5% each) to all the attributes were the subjective weights. Principal component analysis (PCA) consolidated ranks from various MADM methods, and hierarchical clustering (HC) grouped magnets based on their similarities. For PMSMs in applications where ρ, BHMAX, and HC, are more critical compared to the rest of the properties, the eight top-ranked magnets are N35 (Neodymium® 35), N35H (Neodymium® 35H), N30 (Neodymium® 30), N30H (Neodymium® 30H), N27H (Neodymium® 27H), N27 (Neodymium® 27), F5–S (Ferrite 5, Sintered Iron Strontium Oxide), and F3–S (Ferrite 3, Sintered Iron Barium Oxide) in that order. On the other hand, for PMSMs in applications where all the attributes are equally important, the eight top-ranked magnets are N35 (Neodymium® 35), N35H (Neodymium® 35H), N30H (Neodymium® 30H), N30 (Neodymium® 30), N27H (Neodymium® 27H), N27 (Neodymium® 27), CS4 (Samarium Cobalt 4), and F5–S (Ferrite 5, Sintered Iron Strontium Oxide) in that order. The consolidated ranks of the magnets obtained by assigning objective and subjective attribute weights showed a strong correlation of ∼0.94. This investigation targeted magnet selection for PMSMs from a comprehensive dataset of industrial-grade permanent magnets, utilizing several property requirements, applying both objective and subjective attribute weights identified Neodymium® 35 as the top-ranked magnet.

Construction of novel Cu-α-diimide interactions for enhancing thermal resistance and dimensional stability of polyimide films

The new-generation polyimide films with simultaneously outstanding thermal stability and low coefficient of linear thermal expansion (CTE) are urgently needed. Herein, coordination bonds were employed to improve polyimide films' heat resistance and dimensional stability. Polyimide-Cu complex films were fabricated by incorporating α-diimide units (bidentate ligands) into the BPDA-ODA (the structure of commercially available Upilex-R® polyimide films) backbones. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Energy dispersive X-ray mapping results showed that the coordination interactions occur between α-diimide and copper ions. Wide-angle X-ray diffraction (WAXD) and two-dimensional small-angle X-ray scattering (SAXS) spectra demonstrated that Cu-α-diimide coordination structures introduced in intermolecular chains leading polyimide films transformed from a semi-crystalline state to amorphous state, and inhibited the long-period formation structures. The plateau modulus and longer characteristic relaxation time (τ∗ = 1996.5 s) revealed by dynamic thermomechanical analysis indicated that crosslink networks were successfully formed by constructing coordination bonds. As a result, polyimide films' thermal resistance and dimensional stability were simultaneously enhanced. The typical PI-Cu50 films exhibited elevated glass transition temperatures of 353 °C (pristine PI-Cu0 ∼ 302 °C) and a lower CTE of 47.4 ppm/K (pristine PI-Cu0 films ∼ 64.4 ppm/K) in ranges of 300–400 °C, showing a promising application in the field of high-temperature environments.

Theoretical Prediction of Structural, Mechanical, and Thermophysical Properties of the Precipitates in 2xxx Series Aluminum Alloy

We presented a theoretical study for the structural, mechanical, and thermophysical properties of the precipitates in 2xxx series aluminum alloy by applying the widely used density functional theory of Perdew-Burke-Ernzerhof (PBE). The results indicated that the most thermodynamically stable structure refers to the Al3Zr phase in regardless of its different polymorphs, while the formation enthalpy of Al5Cu2Mg8Si6 is only -0.02 eV (close to zero) indicating its metastable nature. The universal anisotropy index of AU follows the trend of: Al2Cu > Al2CuMg ≈ Al3Zr_D022 ≈ Al20Cu2Mn3 > Al3Fe ≈ Al6Mn > Al3Zr_D023 ≈ Al3Zr_L12 > Al7Cu2Fe > Al3Fe2Si. The thermal expansion coefficients (TECs) were calculated based on Quasi harmonic approximation (QHA); Al2CuMg shows the highest linear thermal expansion coefficient (LTEC), followed by Al3Fe, Al2Cu, Al3Zr_L12 and others, while Al3Zr_D022 is the lowest one. The calculated data of three Al3Zr polymorphs follow the order of L12 > D023 > D022, all of them show much lower LTEC than Al substance. For multi-phase aluminum alloys, when the expansion coefficient of the precipitates is quite different from the matrix, it may cause a relatively large internal stress, or even produce cracks under actual service conditions. Therefore, it is necessary to discuss the heat misfit degree during the material design. The discrepancy between a-Al and Al2CuMg is the smallest, which may decrease the heat misfit degree between them and improve the thermal shock resistant behaviors.

Cordierite-based refractory ceramics from natural halloysite and peridotite: Insights on technological properties

Cordierite-based ceramics were fabricated from Moroccan natural halloysite clay by using a simple and low-cost manufacturing method. To this end, peridotite and halloysite samples, collected from Beni Bousera and Melilla sites, Morocco, were used as raw materials for ceramics manufacturing. A starting mixture was prepared (76.08 wt% of clay and 23.92 wt% of peridotite), molded and heated to the desired temperature (1250, 1300 and 1350 °C) to fabricate cordierite ceramic specimens. Both raw materials (peridotite and halloysite) and final ceramics were analyzed using routine characterization techniques including chemical analysis by XRF, mineralogical analysis by XRD, thermogravimetric analysis, and morphological characteristics using scanning electron microscopy (SEM). The prepared ceramics were investigated regarding their mineralogical composition, thermal and technological properties, chemical resistance, and microstructural characteristics. Our results indicated that peridotite sample is mainly composed of silica (40.25 wt%) and magnesia (38.05 wt%) while halloysite is consisted essentially of silica (38.00 wt%) and alumina (34.13 wt%). This was confirmed by XRD, TG-DTA and FTIR analyses. The prepared ceramic specimens at different sintering temperatures (i.e., 1250, 1300 and 1350 °C) have regular cylindrical forms, displaying good ceramic properties. This is consolidated with the main technological tests including porosity (4.56–3.11%), bulk density (2.45–2.78 g/cm3), shrinkage (6.51–10.31%), indirect tensile strength (20.35–27.60 MPa), and low linear thermal expansion coefficient (3.05–2.18 × 10−6/°C). Cordierite specimen prepared at 1350 °C provided the best ceramic sample with the highest technological properties, good chemical resistance and thermal properties. Thus, naturally abundant halloysite and peridotite deposits are potential candidates for cordierite-based ceramic manufacture. Therefore, the achieved results have provided cost-effective ceramic bricks with physical, thermal and mechanical properties that are favorable to be used as refractory bricks.

Fabrication, Structure, and Thermal Properties of Mg-Cu Alloys as High Temperature PCM for Thermal Energy Storage.

This work studied the thermophysical properties of Mg-24%Cu, Mg-31%Cu, and Mg-45%Cu (wt.%) alloys to comprehensively consider the possibility of using them as thermal energy storage (TES) phase change materials (PCMs) used at high temperatures. The microstructure, phase composition, phase change temperatures, and enthalpy of these alloys were investigated by an electron probe micro analyzer (EPMA), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The XRD and EPMA results indicated that the binary eutectic phase composed of α-Mg and Mg2Cu exists in the microstructure of the prepared Mg–Cu series alloys. The microstructure of Mg-24%Cu and Mg-31%Cu is composed of α-Mg matrix and binary eutectic phases, and Mg-45%Cu is composed of primary Mg2Cu and binary eutectic phases. The number of eutectic phases is largest in Mg-31%Cu alloy. The DSC curves indicated that the onset melting temperature of Mg-24%Cu, Mg-31%Cu, and Mg-45%Cu alloys were 485, 486, and 485 °C, and the melting enthalpies were 152, 215, and 91 J/g. Thermal expansion and thermal conductivity were also determined, revealing that the Mg–Cu alloys had a low linear thermal expansion coefficient and high thermal conductivity with respect to increasing temperatures. In conclusion, the thermal properties demonstrated that the Mg–Cu alloys can be considered as a potential PCM for TES.

Development of a technology for producing ceramic refractory material in a composition of montmorillonite clays (bentonite-like) and ferrochrome production wastes

The article presents the results of scientific and experimental studies on the creation of ceramic refractory material. Montmorillonite clay (bentonite-like) of the Pogodaevsky deposit and ferro-dust from self-decaying slags of low-carbon ferrochrome of the Aktobe ferroalloy plant were selected as the most effective types of raw materials in terms of their physico-mechanical properties and chemico-mineralogical composition. The composition of ceramic mass in chamotte - ferro dust - binder and rational technological parameters of molding, drying and firing was developed. The basic physical and mechanical properties of the obtained ceramic samples, which have high strength, average density, low linear thermal expansion coefficient and fire shrinkage, are studied. Ceramic samples based on the proposed raw material composition can withstand temperatures up to 1350 С, which is 200-300 С higher than ceramic samples based on clay raw materials. The high strength indices and refractoriness of samples are explained by the fact that in result of heat treatment of the studied ceramic composition, a densely sintered framework with reinforced refractory minerals such as forsterite Mg2 (SiO4) and spinel (MgAl2O4) is formed. The prospects of this research area are associated with the creation of innovative technologies for the production of popular refractory materials, but also with an environmental issue. Processing of technogenic resources of Kazakhstan will not only reduce the consumption of natural resources, but also reduce the human impact on the environment.

The formation of phases with low or negative linear thermal expansion coefficient in porous mullite ceramics

The formation of phases with low or negative linear thermal expansion coefficient in porous mullite ceramics

Permanent disposal of Cs ions in the form of dense pollucite ceramics having low thermal expansion coefficient

Abstract A promising method for removal of Cs ions from water and their incorporation into stable crystal structure ready for safe and permanent disposal was described. Cs-exchanged X zeolite was hot-pressed at temperature ranging from 800 to 950 °C to fabricate dense pollucite ceramics. It was found that the application of external pressure reduced the pollucite formation temperature. The effect of sintering temperature on density, phase composition and mechanical properties was investigated. The highest density of 92.5 %TD and the highest compressive strength of 79 MPa were measured in pollucite hot-pressed at 950 °C for 3 h. Heterogeneity of samples obtained at 950 °C was determined using scanning electron microscopy. The pollucite hot-pressed at 950 °C had low linear thermal expansion coefficient of ∼4.67 × 10−6 K−1 in the temperature range from 100 to 1000 °C.

Effect of Firing Temperature and Duration on Fused Silica Investment Shell Strength at Room Temperature

Fused silica is widely used in investment casting shell molds as flour, stucco and binder for its superior properties, such as low linear thermal expansion coefficient and high thermal shock resistance. Investment shell molds are usually fired after the de-wax process to remove the residue pattern material, and to improve shell strength through sintering. The firing temperatures are generally high enough to affect the phase constituent and microstructure of the shells, which further influences the shell strength. Firing duration at firing temperature is another important factor having an impact on the shell strength due to grain coarsening, but it is often overlooked by investment foundries. In this article, the room temperature moduli of rupture of investment shells fired at different conditions were determined. The crystallinity of fused silica shells was determined using X-ray diffraction. The differences on the shell strength were discussed and correlated with the microstructure and phase constitution in the shells after different firing processes.

Analysis of operating conditions of heavy-loaded and heat-stressed friction units of powerful internal combustion engines and development of copper nanocomposite powder materials for them

The article provides analysis of operating conditions for the «valve – valve guide» heavy-loaded and heat-stressed friction unit of the gas distribution mechanism of powerful internal combustion engines and establishes requirements to materials for valve guides. It describes nanoc omposite materials based on copper powder with aluminum, carbon, iron and oxygen additives developed by the authors of the present article. The reactionary mechanical alloying method and powder metallurgy technologies in use made it possible to develop materials having a dispersion-strengthened structure with nano-disperse level sizes of structural components (dispersoids, subgrains). They have a recrystallization temperature of over 950оС, good therm-conductivity, a low linear thermal expansion coefficient and excellent tribotechnical properties. For example, a copper nanocomposite material containing 0.9 wt % aluminum, 0.3 wt % carbon, 0.4 wt % iron and 0.017 wt % oxygen, by its tribotechnical properties, greatly surpasses CuNi2Si (F65 DIN 17666) bronze widely used by many leading engine-making companies for production of valve guides. By their properties, the developed materials meet requirements that have to be imposed on modern and promising materials for valve guides of powerful internal combustion engines.

Improvement of Durability of Purging Plugs Using MgO Micropowder for Refining Ladles

To modulate the matrix of purging plugs, MgO micropowder was introduced as a replacement to magnesia powder in alumina–magnesia castables, and the effect of MgO micropowder on the properties of alumina–magnesia castables and the possibility of developing chrome‐free castables were investigated. Experimental results showed that the introduction of MgO micropowder resulted in an improvement in the volume stability, strength, and thermal shock resistance of alumina–magnesia castables due to its high surface energy and small particle size. However, excessive amounts of MgO micropowder led to a lower densification, and there was a slight degradation in the performance of the alumina–magnesia castables. The slag resistance of the prepared alumina–magnesia castables was significantly better than that of the alumina–chrome castables. Microstructure and energy spectrum analysis showed that the formation of a solidified reaction layer, mainly consisting of spinel and CaAl12O19, was the major cause of the observed difference in slag resistance. In addition, the alumina–magnesia castables had a lower linear thermal expansion coefficient than that of the alumina–chrome castables at each experimental temperature, which effectively decreased the thermal stress during its service period, thus exhibiting good thermal shock resistance.

(Invited) Flexible Memory Applications Using Oxide Semiconductor Thin-Film Transistors

Flexible nonvolatile memories (NVMs) have emerged as powerful assistants for future electronic systems featured to have wearable, stretchable, and bendable characteristics. There have been many unique flexible applications, such as radio frequency identification tag, electronic sensor arrays, flat-panel displays, and electronic circuit configuration. Furthermore, the suitable choice of NVM can effectively reduce the power consumption of set applications. From these technical backgrounds, flexible NVM devices with various operating origins have been fabricated on flexible plastic substrates. Among them, a charge-trap-type memory device composed of charge-storage layer is one of the most typical device configurations. However, conventional charge-trap NVMs have many problems such as asymmetric on- and off-program conditions, long program time for trap/detrap process, and unstable memory operations. Consequently, for the practical applications of flexible NVM devices, it is very important to realize both functions of a lower voltage operation and a more stable memory retention behavior. However, because it is difficult to simultaneously optimize these two functions owing to their trade-off relationship, device performances including the stable memory window and large on/off ratio should be guaranteed for low-power consumption and excellent device stability. Furthermore, the process temperature should be controlled to be as low as possible with the considerations of thermal stability of the flexible plastic substrate. In this work, we propose the flexible charge-trap memory thin-film transistor (f-MTFT) using all oxide-layers, in which sputter-deposited In-Ga-Zn-O (IGZO) and atomic-layer-deposited (ALD) ZnO thin films were used as an active channel and charge-trap layer (CTL), respectively. Poly(ethylene naphthalate) (PEN) was chosen as a flexible substrate due to its superior properties, such as a low linear thermal expansion coefficient, good chemical resistance, surface smoothness, and optical clarity, compared with the other types of plastic substrates. The main object of this work is to demonstrate the appropriate methodologies for high-performance f-MTFTs fabricated on flexible PEN substrate. Thus, fabrication procedures are carefully optimized for guaranteeing the stable memory device characteristics. As results, wide memory margin and very fast program speed are successfully demonstrated. Furthermore, the effect of mechanical deformation on the memory properties are examined as a flexible memory device. The operation stabilities at high temperature of 80 oC are also investigated. To realize the f-MTFTs with high performance and stability, following three strategies were introduced. First, the PEN substrate was pre-treated with hybrid barrier layer structure, which was composed of 3.6-μm-thick acrylic-based organic and 30-nm-thick inorganic Al2O3 films. These organic and inorganic barrier layers were prepared by spin-coating and ALD to obtain a smooth surface and to reduce a water vapor transmission rate, respectively. The second strategy was to optimize the deposition temperature for ZnO CTL, because the electronic natures of the ALD-grown ZnO sensitively depends on its deposition temperature. We previously fabricated the memory devices on glass substrate using the ZnO CTLs by controlling the deposition temperature to 100, 150, and 200 oC. As results, the 100-oC ZnO device exhibited wider memory window and stable retention properties than others. Thus, the ZnO deposition temperature was set as 100 oC. The third strategy was to optimize the patterning process of gate-stack structure. The double-layered structure of the Al2O3 tunneling layers was designed to prevent the wet-etching damages for the IGZO channel layer during the patterning process of CTL. When a single Al2O3 tunneling layer was prepared without an additional protection layer, the IGZO channel layer was decisively over-etched due to the differences in etch rates of each layer. On the contrary, the IGZO channel could be effectively protected by second tunneling layer by wrapping the side areas of the channel layer when the double-layered tunneling layer was introduced. Alternatively, a top-protection layer protected the surface of ZnO CTL during the lithography process. The obtained device characteristics of the fabricated f-MTFT can be summarized as follows. The memory margin, memory window width, and program time were evaluated as 4.7×107, 25.6 V, and 500 ns, respectively. These device characteristics were not markedly degraded even after the delamination, under the bending state with a curvature radius of 3.3 mm, and after the 104 times repetitive bending cycles at a curvature radius of 4.8 mm. Furthermore, the stable memory characteristics were successfully guaranteed even at a high temperature of 80 oC. Device parameters confirmed for the fabricated f-MTFTs presented remarkable improvements compared with previously reported various flexible memory devices. We can conclude that the proposed f-MTFTs on PEN substrates can be a suitable candidate for the highly-functional and stable flexible memory device embedded into the new-era flexible electronic systems.

Sinterability and thermal properties of cordierite ceramics prepared from Algerian kaolinite and magnesium hydroxide

ABSTRACTIn this paper, we investigate the effect of MgO additions on the formation and densification behaviour of the cordierite obtained from some mixtures of Algerian kaolin and magnesium hydroxide. The sintering properties of these compositions have been evaluated by X-ray diffraction and bulk density. XRD analysis revealed that the major phase of the synthesized ceramics was cordierite along with a trace of spinel. Firing the pressed specimens at 1400°C for 1 hour yielded a dense cordierite ceramics with a relative density higher than 96%, a negligible open porosity and a lower linear thermal expansion coefficient of 2.73×10−6 K−1 between 200 and 800°C. Cordierite due to its very low coefficient of thermal expansion is considered as promising candidate for advanced applications.

Rubber‐toughened polyamide‐6 with a low thermal expansion coefficient: effect of preferential distribution of rubber and inorganic filler

AbstractThe effects of morphological changes on the thermal expansion, toughness and heat resistance of polyamide‐6 (PA)/styrene–ethylene–butylene–styrene (SEBS)/polyphenylene ether (PPE) blends were investigated. Compared with the typical ‘sea (PA matrix)–island (PPE domain)–lake (SEBS in PPE domain)’ morphology, an injection‐molded ternary blend with a preferential distribution of SEBS component at the interface between PA and PPE exhibited a low coefficient of linear thermal expansion (CLTE) in the flow direction. This low CLTE was ascribed to the deformation of SEBS and PA into a co‐continuous microlayer network structure during injection molding. Consequently, the expansion preferentially occurred towards the thickness direction. Further CLTE reduction either by a change in PA viscosity or by the selective location of an inorganic filler was examined, and its influences on impact strength and heat resistance are discussed based on transmission electron microscopy observations. © 2015 Society of Chemical Industry

Thermal and spectroscopic properties of Nd3+-doped novel fluorogallate glass

An Nd3+-doped fluorogallate glass with better thermal stability (Tg=650.5°C, ΔT=190°C), and lower linear thermal expansion coefficient (8.7×10−6/°C) compared with other commercialized laser glasses is reported in this paper. Judd–Ofelt theory is applied to the absorption spectrum of the glass to derive the intensity parameters (Ωt), which are in turn used to calculate the radiative properties of Nd3+ luminescent levels. The developed glass possesses the widest effective emission bandwidth (Δλeff=49nm) ever reported at around 1.07μm, large gain bandwidth parameter (σem×Δλeff=102.9×10−27cm3), and high fluorescence quantum efficiency (η=66%). These results indicate that this new kind of material is an interesting candidate for laser applications.

Low-Melting Glass-Ceramic Composites with Low Linear Thermal Expansion Coefficient for Radio-Electronics

Glass-ceramic composite materials based on lead-borate glass and eucryptite – a compound with a negative coefficient of linear thermal expansion (CTE), along with the conditions for their production are studied in this paper. Effects of the amount and granulometric composition of the eucryptite as well as time/temperature processing conditions on the change of the linear thermal expansion coefficient of the sintered samples are also examined.

Thermal Expansion Mismatch Analysis of Nano‐bonded Refractory Castables

A mathematical model to predict the thermomechanical stresses was used to understand the expansion mismatch damage in castables, evaluated by hot elastic modulus measurements. The results indicated that via elastic modulus evaluation and the tensile stress calculation, the critical interactions among the particles leading to the formation of flaws were identified. To assure the materials' integrity, it is fundamental to keep a low Δα between the components and not only a low overall linear thermal expansion coefficient. For multicomponent compositions, a second‐phase raw material with different properties should be added as finer particles to minimize the thermomechanical stresses generated.

Heat-Resistant Wrought Weldable Alloy for GTE Components with Low Linear Thermal Expansion Coefficient

Features of the effect of alloying elements on linear thermal expansion coefficient of wrought heat-resistant alloys based on the Ni–Fe system are studied. Short-term and long-term strength at 600°C is simulated and parameters are used successfully as equivalent alloy chemical composition, calculated by a system of unpolarized ionic radii equations. A new deformable weldable alloy based on Ni–Fe–Co is developed with working temperature up to 650°C, with $ \upsigma_u^{20 } $ = 1400 MPa, $ \upsigma_u^{600 } $ = 1200 MPa, and $ \upsigma_{100}^{600 } $ = 950 MPa, with low linear thermal expansion coefficient (α = 11.8 · 10–6 K–1 in the temperature range 20–600°C). The alloy is structurally stable, efficient in forming and welding, and with respect to set of properties it is better than standard alloys of similar designation. New alloy structure, phase composition, and properties are described.

Effect of Stress-strain State during Helical Rolling on Metal and Alloy Structure and Ductility

A working hypothesis is proposed for the mechanism of the effect of deformation regimes on granular structure and phase fineness governing an increase in ductility for silumins and low-alloy steels. In particular, an alternating nature of deformation during helical rolling (HR) provides a reduction in grain size by a factor of 5-6 compared with longitudinal rolling. For hypereutectic silumins with HR it is possible to obtain a polygonized subgranular structure, to increase α-solid solution grain and silicon crystal fineness, giving them a globular shape, and thereby provide a capacity for treatment under pressure. An important role in increasing deformation capacity of silumins is played by the new technology of melt modifying treatment and optimum temperature-rate deformation regimes. strength ( σ u = 190-320 MPa; σ 0.2 = 155-260 MPa, E = 75-80 GPa; σ -1 = 90-130 MPa). In a cold condition, relative elon- gation δ is not more than 1%, and relative reduction of area ψ is up to 4%. Strength properties of silumins are maintained to T = -70°C. Silumins have typically good production and user properties: high fluidity, low shrinkage during solidification; good weldability, high corrosion resistance, heat and wear resistance; low linear thermal expansion coefficient; nonmagnetic nature (1). Technology for preparing high silicon master alloys, based on a carbothermal reduction method of cheap and widespread raw material (nepheline, alunite, kaolin, etc.) makes it possible to reduce silumin cost by 15-20% compared with the currently used technology for synthesizing aluminum with silicon. Hypereutectic silumins may be considered as natural composite materials, having a two-phase structure consisting of ductile eutectic matrix and brittle primary silicon crystals with a size of not more than 150 μm, and also crystals of iron, titanium, and zirconium aluminides (intermetallic phases). An increase in phase fineness, playing the role of a filler, promotes an increase in both strength and ductility. Silumin alloys after processing SiC and Al 2 O 3 powders have high strength (σ u = = 650-700 MPa). Objects of high-silicon hypereutectic silumins because of their low ductility are prepared by molding and centrifu- gal casting (pipes) and liquid stamping, for example, production of internal combustion engine pistons. Further expansion of the range of these applications is connected with the solution of the problem of increasing alloy workability, development, and introduction of forming technology.