The grain refinement of high-purity aluminum by aluminum-transition metal alloys

In view of the continuing interest in the solidification characteristics of dilute Al-Ti alloys,¹ a recent study² of the mechanism(s) of grain refinement induced in high purity aluminum by various additions of master alloys containing Ti, B, Cr, Mo, V and Zr is reported. For alloys containing more than 0.2 wt pct Ti, TiAl₃ was shown to be commonly a nucleant, in both Al-Ti and Al-Ti-B systems. In addition, it was found that the nature of the master alloys is important in determining the degree of grain refinement a given alloying addition will produce. A “saturation” effect is reported for additions of Ti, and ti-B,i.e., further additions of titanium beyond a given level do not provide any further reduction in grain size.     

Influence of Mg on Grain Refinement of Near Eutectic Al-Si Alloys

Although the grain-refinement practice is well established for wrought Al alloys, in the case of foundry alloys such as near eutectic Al-Si alloys, the underlying mechanisms and the use of grain refiners need better understanding. Conventional grain refiners such as Al-5Ti-1B are not effective in grain refining the Al-Si alloys due to the poisoning effect of Si. In this work, we report the results of a newly developed grain refiner, which can effectively grain refine as well as modify eutectic and primary Si in near eutectic Al-Si alloys. Among the material choices, the grain refining response with Al-1Ti-3B master alloy is found to be superior compared to the conventional Al-5Ti-1B master alloy. It was also found that magnesium additions of 0.2 wt pct along with the Al-1Ti-3B master alloy further enhance the near eutectic Al-Si alloy’s grain refining efficiency, thus leading to improved bulk mechanical properties. We have found that magnesium essentially scavenges the oxygen present on the surface of nucleant particles, improves wettability, and reduces the agglomeration tendency of boride particles, thereby enhancing grain refining efficiency. It allows the nucleant particles to act as potent and active nucleation sites even at levels as low as 0.2 pct in the Al-1Ti-3B master alloy.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Effect of grain refinement on the fluidity of two commercial Al-Si foundry alloys

The effect of grain refinement on the fluidity of AlSi7Mg and AlSi11Mg has been investigated by spiral tests. Two different types of grain refiners have been evaluated. An AlTi5Bl master alloy was added to different Ti contents. Since the commercial alloys had a high initial content of titanium, model alloys were made to investigate the fluidity at low grain refiner additions. Commercial alloys grain refined only by boron additions have also been investigated. The results from the fluidity measurements have been verified by measuring the dendrite coherency point of the different cast alloys. Although different, the two methods show similar trends. The spirals from each fraction grain refiner cast were subsequently investigated metallographically at the tip of the spirals and at a reference point a distance behind, but no obvious difference in structure was observed. For both alloys, an increase in fluidity is observed as the content of grain refiner increases above 0.12 pct Ti, while the fluidity is impaired with increased grain refinement below 0.12 pct Ti. The alloys grain refined with ~0.015 pct B show the highest fraction solid at dendrite coherency, the smallest grain size, and the best fluidity.     

Modification of Cast Al-Mg<Subscript>2</Subscript>Si Metal Matrix Composite by Li

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in-situ prepared Al-15 pct Mg₂Si composite were investigated. Adding 0.3 pct Li reduced the average size of Mg₂Si primary particles from ~30 to ~6 μm. The effect of cooling rate was investigated by the use of a mold with different section thicknesses from 3 to 9 mm. The results show a refinement of primary particle size as a result of both Li additions and cooling rate increases, and their effects are additive. Similarly, both effects increased ultimate tensile stress (UTS) and elongation values. The thin sections show somewhat unexpectedly low and scattered tensile results attributed to the casting defects observed in fracture surfaces. The Li-modified alloy displays serrated yielding behavior that is not fully explained here. The refinement by Li and enhanced cooling rate is explained in terms of an analogy with the effect of Sr and cooling rate in Al-Si alloys, and is ultimately attributed to the effect of the alkali and alkaline earth metals deactivating oxide double films (bifilms) suspended in Al melts as favored substrates for intermetallics.     

Influence of chromium and impurities on the grain- refining behavior of aluminum

The grain-refining behavior of high purity aluminum (HPA1) and commercial purity aluminum (CPA1) containing Fe and Si as impurities (<0.2 wt pct each) has been studied with and without the presence of Cr in small and large quantities (0.2 and 2 wt pct). The Al-5Ti-lB master alloy ingot (0.2 wt pct) was used as a grain refiner. The emphasis was on the influence of individual elements and their interactions with the other elements on the grain-refining behavior of Al. Good grain refinement with insignificant fading in CPA1 was observed in comparison to HPA1. Similar results were obtained with a small concentration of Cr in HPA1 in HPA1-0.2 wt pct Cr alloy. The CPA1 and HPA1-0.2 wt pct Cr alloy have given the best grain-refining results among all the cases studied. A combination of small quantities of Fe, Si, and Cr (CPA1-0.2 wt pct Cr) has shown early and significant fading. A large concentration of Cr (2 wt pct) has shown a poisoning effect irrespective of the presence or absence of impurities such as Fe and Si in Al. Thus, Cr was found to be beneficial for grain refinement at smaller concentrations in the absence of impurities. But at higher concentrations of Cr, it had an adverse effect,i.e., led to coarser grain sizes both in the presence and absence of impurities.     

Influence of Extrusion on the Microstructure and Mechanical Behavior of Mg-9Li-3Al-xSr Alloys

Mg-9Li-3Al-xSr (LA93-xSr, x = 0, 1.5, 2.5, and 3.5 wt pct) alloys were cast and extruded at 533 K (260 °C) with an extrusion ratio of 28. The microstructure and mechanical response are reported and discussed paying particular attention to the influence of extrusion and Sr content on phase composition, strength, and ductility. The results of the current study show that LA93-xSr alloys contain both α-Mg (hcp) and β-Li (bcc) matrix phases. Moreover, the addition of Sr refines the grain size in the as-cast alloys and leads to the formation of the intermetallic compound (Al₄Sr). Our results show significant grain refinement during extrusion and almost no influence of Sr content on the grain size of the extruded alloys. The microstructure evolution during extrusion is governed by continuous dynamic recrystallization (CDRX) in the α-Mg phase, whereas discontinuous dynamic recrystallization (DDRX) occurs in the β-Li phase. The mechanical behavior of the extruded LA93-xSr alloy is discussed in terms of grain refinement and dislocation strengthening. The tensile strength of the extruded alloys first increases and then decreases, whereas the elongation decreases monotonically with increasing Sr; in contrast, hardness increases for all Sr compositions studied herein. Specifically, when Sr content is 2.5 wt pct, the extruded Mg-9Li-3Al-2.5Sr (LAJ932) alloy exhibits a favorable combination of strength and ductility with an ultimate tensile strength of 235 MPa, yield strength of 221 MPa, and an elongation of 19.4 pct.     

Effect of Compositional Variation on the Microstructural Evolution and the Castability of Al–Mg–Si Ternary Alloys

This study examined the microstructural evolution and castability of Al–Mg–Si ternary alloys with varying Si contents. Al–6Mg–xSi alloys (where x = 0, 1, 3, 5, and 7; all compositions in mass pct) were examined, with Al–6 mass pct Mg as a base alloy. The results showed that in the ternary alloys with Si ≤ 3 pct, the solidification process ended with the formation of eutectic α-Al–Mg₂Si phases generated by a univariant reaction. However, in the case of ternary alloys with Si > 3 pct, solidification was completed with the formation of α-Al–Mg₂Si–Si ternary eutectic phases generated by a three-phase invariant reaction. In addition to the eutectic Mg₂Si phases, the primary Mg₂Si phases formed in each of the ternary alloys, and the size of both sets of phases increased with increasing Si content. The two-phase eutectic α-Al–Mg₂Si nucleated from the primary Mg₂Si phases. The inoculated Al–6Mg–1Si alloy had the smallest grain size. Moreover, the grain-refining efficacy of the Al–5Ti–B master alloy in the ternary alloys decreased with increasing Si content in the alloys. Despite the poisoning effect of Si on the potency of TiB₂ compounds in the inoculated Al–6Mg–1Si alloy, the grain size of the alloy was slightly smaller than that of the Al–6Mg binary alloy. This resulted from the increasing growth restriction factor (induced by Si addition) of the Al–6Mg–1Si alloy. In terms of the castability, the examined alloys showed different levels of susceptibility to hot tearing. Among the alloys, the ternary Al–6Mg–5Si alloy exhibited the highest susceptibility to hot tearing, whereas the Al–6Mg–7Si exhibited the lowest. The severity of hot tearing initiated by the unraveling of the bifilm was determined by the freezing range, grain size, and the amount of eutectic phases at the end of the solidification process.

     

Effect of strontium on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy

The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases—blocky-shaped Mg₁₇Al₁₂, acicular Mg₂₀Al₂₀Mn₅Sr, and insular Mg₁₆(Al,Zn)₂Sr—are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped Mg₁₇Al₁₂ phase increases, the acicular Mg₂₀Al₂₀Mn₅Sr phase diminishes, and the insular Mg₁₆(Al,Zn)₂Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σ _y ) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ _y =35.88+279.13d ^−1/2 according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture.     

Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy

Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C, the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉_Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency of the 〈100〉_Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing Cu additions also increase the homogeneity and cause refinement of the 〈100〉_Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al₂Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉_Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute supersaturation at the 〈100〉_Al GeSi/α-Al interfaces.     

Refinement of Eutectic Si Phase in Al-5Si Alloys with Yb Additions

A series of Al-5 wt pct Si alloys with Yb additions (up to 6100 ppm) have been investigated using thermal analysis and multiscale microstructure characterization techniques. The addition of Yb was found to cause no modification effect to a fibrous morphology involving Si twinning; however, a refined plate-like eutectic structure was observed. The Al₂Si₂Yb phase was observed with Yb addition level of more than 1000 ppm. Within the eutectic Al and Si phases, the Al₂Si₂Yb phase was also found as a precipitation from the remained liquid. No Yb was detected in the α-Al matrix or plate-like Si particle, even with Yb addition up to 6100 ppm. The absence of Yb inside the eutectic Si particle may partly explain why no significant Si twinning was observed along {111}_Si planes in the eutectic Si particle. In addition, the formation of the thermodynamic stable YbP phases is also proposed to deteriorate the potency of AlP phase in Al alloys. This investigation highlights to distinguish the modification associated with the ever present P in Al alloys. We define modification as a transition from faceted to fibrous morphology, while a reduction of the Si size is termed refinement.     

Study of Heterogeneous Nucleation of Eutectic Si in High-Purity Al-Si Alloys with Sr Addition

Al-5 wt pct Si master-alloys with controlled Sr and/or P addition/s were produced using super purity Al 99.99 wt pct and Si 99.999 wt pct materials in an arc melter. The master-alloy was melt-spun resulting in the production of thin ribbons. The Al matrix of the ribbons contained entrained Al-Si eutectic droplets that were subsequently investigated. Differential scanning calorimetry, thermodynamic calculations, and transmission electron microscopy techniques were employed to examine the effect of the Sr and P additions on eutectic undercoolings and nucleation phenomenon. Results indicate that, unlike P, Sr does not promote nucleation. Increasing Sr additions depressed the eutectic nucleation temperature. This may be a result of the formation of a Sr phase that could consume or detrimentally affect potent AlP nucleation sites.     

Microstructure and Mechanical Properties of Extruded Magnesium-Aluminum-Cerium Alloy Tubes

Current commercial magnesium extrusion alloys do not offer desirable combinations of strength, ductility, and extrusion speed for automotive structural applications. The effect of small additions of cerium (Ce) to pure magnesium (Mg) and Mg-3 pct Al alloy extruded tubes has been studied. The results suggest that 0.2 pct Ce addition can significantly improve the extrudability and mechanical properties of the Mg extrusions. The improvement in mechanical properties is due to grain refinement and dispersion strengthening provided by the Mg₁₂Ce particles and the beneficial texture obtained. Higher Ce contents further increase strength, but significantly reduce ductility and cause excessive surface oxidation during extrusion. The beneficial effect of 0.2 pct Ce on mechanical properties of pure Mg is not observed when it is added to Mg-3 pct Al alloy, due to the higher affinity of Ce to Al to form the Al₁₁Ce₃ phase in the Mg-Al-Ce ternary alloys. The Mg-0.2 pct Ce alloy is a promising base alloy for further development in automotive applications; however, Al should be avoided in Mg-Ce–based extrusion alloys.     

The electrical conductivity of cast Al−Si alloys in the range 2 to 12.6 wt pct silicon

The changes in electrical conductivity of cast Al−Si alloys, in the range of 2 to 12.6 wt pct silicon due to strontium additions (0.03 wt pct) have been investigated and explained in terms of variations in microstructure. The strontium-containing alloys exhibited a higher conductivity than alloys with no strontium, and this conductivity difference increased as the silicon and magnesium contents were increased and the solidification rate was decreased. It has been demonstrated that this difference is due to changes in microstructural features of the eutectic silicon upon modification.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

含钡合金在钢液中的脱氧行为研究

在MoSi2炉上采用MgO质坩蜗对钢管进行了含钡合金脱氧行为的研究。实验中钡系合金主要选取SiAUBa、SiCaBa、SiAlBaCa、SiAlBeCaSr及用于钙处理的SiCa包芯线，选用Al作为对比脱氧剂，考察了钡系合金脱氧的全氧含量，脱氧产物的分布、尺寸和形貌，探讨了钡系合金脱氧和对夹杂物变性作用的机理。     

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pct Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.