Recent developments in hypoeutectic Al-Si A-S4G alloy

The effect of adding Sr in the form of the Al-5 Sr master alloy to commercial A-S4G and high purity Al-4Si alloys on the modification process has been investigated. The volume fraction of the eutectic matrix decreased by modification due to the movement of the eutectic point to the higher Si content side. The tensile properties, especially elongation, have been increased by modification. The elongation of the A-S4G alloy is increased from a value of 0.9 to 15% by modification. Additionally, the elongation of the modified high purity alloy reached a value of 34%. The fracture path of the modified alloys circumvents the -phase while it is not yet known if it propagates intergranularly or transgranularly through the eutectic matrix. The fracture surface revealed dimple and smooth ripple patterns reflecting the high ductility of the modified alloys.     

Effect of phase composition and microstructure on the thermal diffusivity of silicon nitride

The effects of compositional and microstructural variables and processing conditions on the room temperature thermal diffusivity of hot-pressed and reaction-sintered silicon nitride were determined. The thermal diffusivity for hot-pressed silicon nitride increases with-content. Maximum thermal diffusivity is reached at about 3 wt % MgO. The higher thermal diffusivity of the -phase is attributed to its higher purity level and the less distorted crystal structure compared to the-phase. In reaction-sintered nitride the thermal diffusivity is strongly influenced by the relative amount and needle-like morphology of the-phase. Correlations of the thermal diffusivity with mechanical properties are discussed.     

The effect of silicon on the eutectic grain size of gravity cast Zn-12% Al

For gravity castings made from Zn-12% Al alloys wide variations of impact strength have been observed that could not be explained in terms of casting soundness. Previous unpublished research isolated trace impurities of silicon as the cause of a large increase in eutectic grain size and subsequent decrease in impact strength. In the present study the effect of silicon on the as-cast eutectic grain size and impact strength of Zn-12Al is confirmed. Thermal analysis and interface equilibration experiments indicate that silicon impedes nucleation of the eutectic phase. Finding that the presence of silicon lowers the primary/eutectic liquid interfacial energy, it is concluded that silicon, rejected by the pro-eutecticphase during primary solidification, poisons the interface causing the difficulty with the nucleation of eutectic on the primary surface leading to the increase in eutectic grain size.     

EPMA mapping of small particles of α-AlFeSi and β-AlFeSi in AA6063 alloy billets

It is well known that the spatial distribution and the spatial density of the particles o-AlFeSi and -AlFeSi in the billets of Al-Mg-Si alloys, such as AA6063 alloys affect the quality of anodizing performance of their extrusions. For this reason it is very important to control the spatial distribution and the spatial density of both AlFeSi particles at extrusion plants. The X-ray diffraction method (XRD) has been used for discrimination between -AlFeSi and -ALFeSi particles. However it is not an appropriate method for determining the spatial distributions of particles in the alloys. As an alternative method an electron probe microanalyzer (EPMA) has been used for determining the spatial distributions of each element in the microstructures. However, unfortunately it is difficult to discriminate between the particles composed of the same elements like -AlFeSi and -AlFeSi particles. Thus, we tried to develop a convenient method to discriminate between -AlFeSi and -AlFeSi particles in the microstructure of AA6063 alloys and developed the EPMA mapping of -AlFeSi and -AlFeSi particles. First, in order to discriminate between the two particles, we tried to use the relative X-ray intensity ratio, the I _Fe/I _Si ratio instead of the Fe/Si mass ratio. Then, we calculated the value of the I _Fe/I _Si ratio from -AlFeSi and -AlFeSi by using Monte Carlo calculations and obtained the critical value of the I _Fe/I _Si ratio, to distinguish between -AlFeSi and -AlFeSi. After that, using the discrimination value, we developed the EPMA mapping program (EPMA method) to observe the distributions of -AlFeSi and -AlFeSi, and to calculate the areas (%) of -AlFeSi and -AlFeSi. Finally, we checked the correlation between the EPMA and the XRD methods. Consequently, the two methods were in good agreement. Today, this EPMA method instead of the XRD method is successfully used in the quality control of 6063 aluminum alloy billets after heat treatment at our aluminum extrusion works.     

Influence of yttrium on microstructure and point defects in α-Al2O3 in relation to oxidation

Although the influence of yttrium on transport properties of alumina has been the object of many studies, the mechanisms by which this element acts have not yet been elucidated. The method of modification by yttrium of the microstructure of polycrystalline alumina and the nature of the point defects created by this doping element were studied. The results obtained are discussed in relation to alumina transport properties and especially in relation with the effect of yttrium on the oxidation mechanism of alumina former alloys, taking into account the doping amount.     

Mechanical properties of alkoxy-derived cordierite ceramics

Mechanical properties of cordierite ceramics prepared by the controlled hydrolysis and following polycondensation of aluminium, magnesium and silicon alkoxides were investigated in great detail. Flexural strengths of- and-cordierite ceramics are about 60 and 100 MPa, respectively. The flexural strengths of these ceramics are mainly influenced by cracks arising from thermal mismatch between - and -cordierite precipitated during sintering. High fracture toughness of-cordierite ceramics prepared by this method is ascribed to the fine microstructure of the ceramics. The high-temperature flexural strength of-cordierite ceramics is little reduced below 1000° C because of high purity of the ceramics.[/p]     

Effect of solution heat treatment and additives on the hardness, tensile properties and fracture behaviour of Al-Si (A413.1) automotive alloys

A study was carried out to determine the role of Mg, Cu, Be, Ag, Ni, and Zn additives during the solution heat treatment of grain refined, Sr-modified eutectic A413.1 (Al-11.7% Si) alloy, and their consequent effect on mechanical properties. For comparison purposes, some of the alloys were also studied in the non-modified condition. The alloys were cast in the form of test bars using a steel permanent mold preheated at 425°C that provided a microstructure with an average dendrite arm spacing (DAS) of 22 m. The test bars were solution heat treated at 500 ± 2°C for times up to 24 h, followed by artificial aging at 155°C for 5 h (T6 treatment). Tensile and hardness tests were carried out on the heat-treated test bars. Details of the microstructural evaluation are reported in a previous article [1].With respect to the mechanical properties, it is found that the hardness and strength (YS, UTS) of Mg-containing alloys decrease with the addition of Sr due to the sluggish dissolution of the Al₅Cu₂Mg₈Si₆ phase during solution treatment, and a delay in the precipitation of Mg₂Si or Al₂MgCu phases during artificial aging thereafter. The properties of the Cu-containing alloys, however, remain unaffected by the addition of Sr. With the exception of Ni, all alloying elements used improve hardness and strength, particularly after heat treatment. In the case of Ni, addition of up to 1.41% Ni is observed to decrease the mechanical properties in the T6 condition.Fracture of non-modified alloys takes place through crack initiation within the brittle acicular Si particles without the crack passing through the ductile Al matrix. In the Sr-modified alloys, the fracture is of ductile type, as evidenced by the pinpoint nature of the -Al dendrites on the fracture surface. The number of cracked Si particles and intermetallics beneath the fracture surface increases in proportion to the increase in alloy strength.     

Effect of Al-5Ti-1B on the microstructure of near-eutectic Al-13.0%Si alloys modified with Sr

The addition of Sr in Al-Si alloys as a modifier causes a transition of the morphology of eutectic silicon from coarse plate-like or acicular to fine fibrous. However, it may also lead to the formation of long columnar dendritic -Al phase. Al-5Ti-1B master alloy is often used as a grain refiner to achieve the fine equaixed grains in aluminum and aluminum alloys. The aim of the present study is to highlight the effect of Al-5Ti-1B master alloy additions on the microstructure of near-eutectic Al-Si alloys modified with Sr. When the addition of Al-5Ti-1B master alloy was below 0.82 mass%, the dendritic -Al phase changed from long columnar to equiaxed, and there were no noticeable changes of the morphology and size of eutectic silicon, while the size of eutectic cells decreased slightly. However, when the addition was above 0.82 mass%, the deleterious influence of Al-5Ti-1B master alloy on the modification effect of Sr emerged and with further increases in addition level a fully unmodified microstructure was finally produced. The results indicate that the effective Sr in the melt decreases with increased addition of Al-5Ti-1B. The poisoning event of Al-5Ti-1B master alloy on the modification of Sr is supposed to be related with the interaction between Sr and Ti.     

Effect of selected impurities on the high temperature mechanical properties of hot-pressed silicon nitride

The effect of impurities on the high temperature mechanical properties of hot-pressed silicon nitride has been determined. Selected impurity additions were made to both relatively pure -phase and -phase silicon nitride starting powders. These powders were hot-pressed to full density using 5 wt % MgO as the pressive additive. The silicon nitride hot-pressed from the -phase powder exhibited higher strength at both 25 and 1400 C than that fabricated from the -phase powder. The impurity additions had no effect on the room temperature mechanical properties. The CaO additions had the most significant effect on the high temperature mechanical properties. In both the material hot-pressed from the -phase and -phase powders, increasing CaO additions severely reduced the high temperature strength and increased the amount of non-elastic deformation observed prior to fracture. Although alkali additions (Na₂CO₃, Li₂CO₃, K₂CO₃) also tended to have the same effects as the CaO, the high volatility of these compounds resulted in a much reduced concentration in the hot-pressed material, thus minimizing somewhat their tendency to enhance the high temperature strength degradation. The Fe₂O₃ and Al₂O₃ had no apparent effect on the high temperature mechanical properties.     

Influence of additions on the solidification behaviour of Ni-B alloys — crystallography of Ni-Ni3B eutectic

The influence of small additions (1 to 2 at.%) of some elements (chromium, copper, silicon, aluminium, iron, titanium, vanadium) on the solidification behaviour and crystallography of Ni()-Ni₃B eutectic composition is examined by differential thermal analysis, X-ray and electron diffractions, scanning and transmission electron microscopy. With the exception of Fe-doped eutectic alloys, all the alloys whether doped or undoped exhibit a very large undercooling for the nucleation of Ni₃B. We give an interpretation of the controversial hypothesis of this undercooling. We propose a crystallographic orientation relationship between the cubic phase Ni() and the orthorhombic phase Ni₃B in the lamellar eutectic Ni-Ni₃B. We also examine a complex transformation which occurs in Ni₃B during slow cooling.     

Structural modification of Al-Si eutectic alloy by Sr and its effect on tensile and fracture characteristics

Modification of the commercial A-S13 alloy and high purity Al-Si eutectic alloy has been successfully achieved using Al-5 Sr master alloy. The refined structure showed more fine and uniformly distributed eutectic silicon in the high purity alloy. The incidence of Al-dendrites due to modification was related to the movement of the eutectic point to the higher-silicon side. The chill-cast alloys showed better refined structure and higher mechanical properties in comparison with the sand-cast alloys. The modified chill-cast alloy exhibited the best mechanical properties. The ultimate. tensile strength of which reached 280 M Pa and the elongation was 9.2%. The fracture patterns changed from well faceted brittle appearance to smooth silky and dimple-like for the normal and modified alloys respectively. This reflects the change from brittle to ductile behaviour after modification. A model for the fracture mechanism was proposed to explain the observed higher ductility for the modified alloy.     

Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90m were produced by warm-working and annealing the alloy in the single--phase field. A uniform distribution of the coherent ₂ particles in the matrix was produced by ageing the alloy in the two-phase ( + ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the ₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with the ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.     

Effects of the variation in α-phase volume fraction on the thermal stability of TiAl alloys with a lamellar microstructure

The thermal stability of two-phase ( + ) lamellar microstructure in Ti-Al-Mo PST (polysynthetically twinned that has single colony) crystals, containing C or Si, was investigated. In addition, the variation of -phase volume fraction in Ti-Al-Mo-(C,Si) systems was investigated at several temperatures. Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys did not recrystallized (stable in this paper) during heat treatments at various heating rates and temperatures. Moreover, the -phase volume fractions of Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys which were stable compositions, changed less than those of Ti-47Al and Ti-46Al-1.5Mo alloys which were unstable compositions. The instability of the latter alloys was caused by their relatively higher variation of -phase volume fraction during heating. Therefore, it is suggested that the variation of -phase volume fraction is an important factor in controlling the thermal stability of lamellar microstructure.     

Crack formation in iron-nickel alloys under stress in alkaline media

The corrosion resistance and the character of crack formation in iron, nickel, and iron-nickel alloys under stress in a boiling NaOH solution were studied. The character of fracture of two-phase alloys was found to be related to the presence of an -phase at the grain boundaries. In the absence of the ferrite constituent, Fe-Ni alloys fail by intercrystalline fracture and the time-to-rupture decreases with increasing stacking fault density.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

Fracture toughness of eutectic Al-Si casting alloy with different microstructural features

The static fracture toughness of a series of eutectic Al-Si casting alloy with different microstructural features has been evaluated. The dominant influence of eutectic silicon in controlling the fracture toughness is thus clarified. The relationship between the fracture toughness and the microstructure was established. Fracture toughness was found to be strongly associated with the size and morphology of silicon particles. The other feature which greatly influences the fracture toughness is the ratio (/DE)_Si, i.e. the silicon particle spacing divided by the equivalent particle diameter, rather than the silicon particle spacing, _Si. Fracture toughness also correlates well with the void growth parameter, VGP (=_y (/DE)_Si), proposed by the authors. The results of the present work can be used to develop an understanding of the variation of fracture toughness with the microstructural features of eutectic Al-Si alloys.     

Grain refinement and microstructural effects on mechanical and tribological behaviours of Ti and B modified aluminium bronze

Microstructural effects on mechanical and tribological behaviours have been studied for a series of aluminium bronzes with different microstructures. ASTM 1045 and 52100 steels were used as the counterparts in the friction and wear tests. Experimental data show that the coefficients of friction, wear rate and mechanical properties strongly depend on the volume fraction of -phase present in the alloy and, to a lesser extent, on the average -grain size. The minimum coefficients of friction and the wear rate correspond to a yield strength of 370 N mm^–2 and a bulk hardness of HB 168. Except for extreme low average -grain size (corresponding to low volume fraction), both the coefficients of friction and the wear rate show linear relations with the reciprocal of the yield strength of the alloy, but not to the reciprocal of the hardness as expected. Based on these results, a design principle for high strength wear-resistant aluminium bronze has been developed.Dr T. Leo Ngai is also a part-time Research Fellow in the Materials Research Center, City Polytechnic of Hong Kong, Hong Kong.     

Liquid phase sintering of an AA2014-based composite prepared from an elemental powder mixture

The AA2014 alloy reinforced with 10 vol% SiC particles has been prepared from elemental powders blended with the ceramic particles, instead of using a pre-alloyed atomized powder as the matrix material. Of critical importance in taking this uncommon processing route is liquid-phase sintering to transform the initial free aluminum and copper powder particles into the -Al and the -phase, thereby producing an integrated structure. The present investigation looked into the effect of sintering parameter on the microstructure and mechanical properties of the composite. It is shown that a sufficient time of sintering in the presence of a small volume fraction of the eutectic liquid must be given to dissolve the free copper particles in the -Al solid solution. However, a prolonged sintering time will lead to the thickening of grain boundaries. A fast cooling after sintering is necessary to avoid the formation of a layered structure at grain boundaries as a result of local sequential solidification. With an adequate sintering scheme, the composite can be successfully fabricated to attain a strength of over 440MPa after the standard T6 heat treatment.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.