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.     

The nucleation of aluminum grains in alloys of aluminum with titanium and boron

The microstructure of a ternary alloy, Al-5 wt pct Ti, 1 wt pct B, has been examined by optical and electron transmission microscopy, by selected area diffraction, and electron probe microscopy, by selected area diffraction, and electron probe microanalysis. Particles of Al₃Ti are found at the center of grains and there exist preferred epitaxial orientations between this compound and the surrounding aluminum. Particles containing titanium and boron occur at aluminum grain boundaries and have no preferred configurations with respect to the aluminum or to one another. It is concluded that the active heterogeneous nuclei are therefore Al₃Ti and that particles of TiB₂, AlB₂, or a ternary compound are not active in this alloy. Grain size measurements in binary Al-Ti alloys suggest that particles of a nucleating phase must be present at concentrations as low as 0.01 wt pct Ti, and it is suggested that these could be Al₃Ti if the existing binary phase diagram Al-Ti is in error.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The role of boron in the grain

It has been known for nearly forty years that the presence of boron in Al-Ti grain refiners has a beneficial effect. Three major theories have been proposed to explain the phenomenon: 1. Boron alters important phase relationships in the Al-Ti-B system. 2. Boron forms TiB₂, which is nearly insoluble and acts as an effective nucleus for grain refining. 3. A metastable phase, (Al, Ti)B₂, forms and either acts directly as a grain refining agent, or somehow alters the phase relationships as in (1) above. A close examination of these theories, however, shows that none is wholly satisfactory in explaining the experimental evidence. Hence, experimental work was carried out to elucidate further the important role of boron. The (Al, Ti)B₂ phase was found to be an effective nucleant in some alloys, in agreement with prior work. In addition, this study suggests that boron may at times improve the grain refining response of Al-Ti alloys by altering the surface structure of TiAl₃ nuclei. The mechanisms of grain refinement, therefore, appear to be more complex than has generally been realized. Formerly Research Engineer, Cabot R & D Laboratories, Reading, PA Formerly Senior Research Engineer, KB Alloys, Reading, PA     

Evaluation of Al-Ti-C Master Alloys as Grain Refiner for Aluminum and Its Alloys

Al-Ti-C master alloys have a great potential as efficient grain refiners for aluminum and its alloys. In the present work, the Al-Ti-C master alloys are synthesized via a relatively novel technique through the reaction of a compacted mixture of K₂TiF₆ and graphite with molten aluminum. The obtained alloys are examined using scanning electron microscopy (SEM), energy-dispersive spectroscopy, and X-ray diffraction (XRD) methods. The results indicate that the produced Al-Ti-C master alloys contain TiC and TiAl₃ particles within the aluminum matrix. Also, these alloys were evaluated using the KBI test mold. The results indicate that the produced Al-Ti-C master alloy is an efficient grain refiner for pure aluminum and its alloys compared with the Al-Ti-B one. The factors affecting the grain refinement of aluminum and its alloys are studied. The proper conditions for evaluating the efficiency of the produced Al-Ti-C master alloy to obtain a minimum grain size are as follows: temperature, 993 K (720 °C); holding time, 2 minutes; and (Ti/Al) weight ratio, 0.01 pct.     

Effect of Hot Rolling on Grain Refining Performance of Al–5Ti–1B Master Alloy on Hot Tearing on Al–7Si–3Cu Alloy

It has been known experimentally that TiAl₃ acts as a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To improve microscopic understanding, hot rolling technique has been performed to the Al–5Ti–1B alloy and the effect of shape and size of the particles on grain refinement has been studied. The effect of hot rolling of Al–5Ti–1B master alloy on its grain refining performance and hot tearing have been studied by OM, XRD, and SEM. Hot rolling improves the grain refining performance of this master alloy, which is required to reduce hot tearing in Al–7Si–3Cu alloy. The improvement in grain refining performance of Al–5Ti–1B master alloy on rolling is due to the fracture of larger TiAl₃ particles into fine particles during rolling. The presented results illustrate that the morphology of TiAl₃ particles alter from the plate-like structure in the as-cast condition Al–5Ti–1B master alloy to the blocky type after rolling due to the fragmentation of plate-like structures. The grain refining response and effect on hot tearing of Al–7Si–3Cu alloy have been studied with as-cast and rolled Al–5Ti–1B master alloys. The results display hot-rolled master alloys revealing enhanced grain refining performance and minimizing hot tear tendency of the alloy at much lower addition level as compared to as-cast master alloys.     

Thermodynamics of calcium and oxygen in molten titanium and titanium-aluminum alloy

The deoxidation equilibrium of molten titanium and titanium-aluminum alloys saturated with solid CaO has been measured in the temperature range from 1823 to 2023 K. The equilibrium constant of reaction CaO (s)=Ca (mass pct in Ti,Ti-Al)+O (mass pct in Ti,Ti-Al) and the interaction parameter between calcium and oxygen were determined for Ti, TiAl, and TiAl₃. The standard Gibbs energy of reaction for TiAl was obtained as follows: $$\Delta G^\circ = 279,000 - 103TJ/mol$$ The possibilities for the deoxidation of titanium and titanium-aluminum alloys by using calcium-based fluxes are discussed.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB₂, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB₂ particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.     

The mechanisms of grain refinement in dilute aluminum alloys

The contribution of the peritectic reaction in producing grain refinement in aluminum alloys has been studied in three binary aluminum systems. It appears that titanium has a unique effect compared with zirconium or chromium; there is evidence of a refinement mechanism associated with the peritectic reaction in Al?Ti alloys which was not observed in the other two systems. Additions of boron to Al?Ti and Al?Zr alloys emphasizes the difference in effect of these two transition elements as grain refiners. The significance of the present work is discussed with reference to conflicts apparent in earlier experimental data.     

Evolution of microstructures in the nickel modified titanium trialuminides near the L12 phase field

This study focuses upon the evolution of microstructures during solidification processing of several intermetallic alloys around the Ll₂ phase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent homogenization followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron microscopy with energy-dispersive spectroscopy (EDS) analysis and X-ray diffraction. The microstructural evolution in homogenized alloys was dependent on both nickel and titanium content. Very fine precipitates of Al₂Ti were observed within the Ll₂ phase in alloys containing 62 to 65 at. pct Al and at least 25 at. pct Ti. The Al₂Ti precipitates are stable at least up to 1000 °C and undergo complete dissolution at 1200 °C. In alloys containing around 66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al₃Ti, Al₅Ti₂, and Al₁₁Ti₅ were observed. A modified room temperature isotherm in the Al-Ti-Ni ternary system is proposed, taking into account the existence of Al₂Ti, Al₁₁Ti₅, Al₅Ti₂, and Al₃Ti in equilibrium with the Ll₂ phase. It seems that at room temperature, the Ll₂ phase field for homogenized alloys is extremely small. It will be practically impossible to obtain a single-phase microstructure at room temperature in the Al-Ti-Ni ternary alloys after homogenization at 1000 °C followed by furnace cooling. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

Effect of scandium on the phase composition and mechanical properties of ABM alloys

The effect of scandium on the composition and mechanical properties of ABM-1 alloys (Al-30% Be-5% Mg) is studied. The scandium content is varied from 0.1 to 0.5 wt %. It is established that, in the studied part of the Al-Be-Mg-Sc system, an aluminum solid solution (Al) and the ScBe₁₃ compound are in equilibrium with a beryllium solid solution (Be). Magnesium dissolves in both the aluminum component and the ScBe₁₃ compound. The strengthening effect related to the decomposition of the solid solution and the precipitation of Al₃Sc cannot be extended to the strengthening of ABM-type alloys. Additions of 0.1–0.15 wt % Sc only weakly improve the mechanical properties of the alloys due to the refinement of beryllium-component grains. At high scandium contents, the strength increases insignificantly due to primary precipitation of ScBe₁₃ and the plasticity decreases simultaneously.     

The role of solute in grain refinement of magnesium

The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of grain refinement of magnesium alloys is included in this article to provide an update on research in this field.     

Development of In Situ As Cast Al–Mg2Si Particulate Composite: Microstructure Refinement and Modification Studies

In situ processed hypereutectic Al?CMg₂Si composites are reported as potential replacements for Al?CSi?CMg heat-treated alloys in specific automotive applications like brake discs, cylinder heads and piston heads. Experiments were conducted for preparing as cast hypereutectic Al?C15?%Mg₂Si alloy, with 8?% extra silicon. The alloy microstructure revealed a dendritic morphology of ??-Al, primary (P)-Mg₂Si particles and Fe rich inclusions embedded in the ternary eutectic matrix. Addition of conventional grain refiner Al?C5Ti?C1B (1?%wt) lead to refinement of P-Mg₂Si dendrites to faceted hopper-like polyhedral particles of ~30???m in size. Addition of combination of B-rich grain refiner Al?C1Ti?C2B and phosphorous, resulted in finer (~20???m), dense and fairly uniformly distributed P-Mg₂Si particles in the microstructure. Electron probe microanalysis revealed the presence of free boron on all P-Mg₂Si sites. Coupled compounds consisting of Ti, B and P were found to be associated with P-Mg₂Si which may be responsible for enhanced refinement and modification of P-Mg₂Si particles.     

The effect of molybdenum on γ′ coarsening and on elevated-temperature hardness in some experimental nickel-base superalloys

The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing 3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testingin vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly. The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and titanium contents is discussed.     

Activities of boron in the binary Ni-B and the ternary Co-Fe-B melts

Activities of boron in the binary Ni-B and the ternary Co-Fe-B melts have been directly determined by the electromotive force (emf) measurement. A boron crystal was mainly used as the reference electrode, and a ternary 29 wt pct B₂O₃-42 wt pct CaO-29 wt pct Al₂O₃ oxide melt was used as the electrolyte. Large negative deviations from Raoult’s law have been found in both the binary and the ternary systems. The activity of nickel has been calculated from the measured boron activity by means of the Gibbs-Duhem equation. Standard Gibbs energies of formation of Ni₃B (s), Fe₂B (s), and Co₃B (s) were estimated and compared with available data. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo.     

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.

     

Reaction Scheme and Liquidus Surface of the Ternary System Aluminum-Chromium-Titanium

The constitution of the ternary system Al-Cr-Ti is investigated over the entire composition range using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), differential thermal analysis (DTA) up to 1500 °C, and metallography. Solid-state phase equilibria at 900 °C are determined for alloys containing ≤75 at. pct aluminum and at 600 °C for alloys containing >75 at. pct Al. A reaction scheme linking these solid-state equilibria with the liquidus surface is presented. The liquidus surface for ≤50 at. pct aluminum is dominated by the primary crystallization field of bcc β(Ti,Cr,Al). In the region >50 at. pct Al, the ternary L1₂-type phase τ forms in a peritectic reaction p _max at 1393 °C from L + TiAl. Furthermore, with the addition of chromium, the binary peritectic L + α(Ti,Al) = TiAl changes into an eutectic L = α(Ti,Al) + TiAl. This eutectic trough descends monotonously through a series of transition reactions and ternary peritectics to end in the binary eutectic L = Cr₇Al₄₅ + (Al).     

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.     

Influence of Reaction Temperature and Reaction Time for the Manufacturing of Al–Ti–B (Ti:B?=?5:1, 1:3) Master Alloys and Their Grain Refining Efficiency on Al–7Si Alloys

In the present work, ternary Al?CTi?CB master alloys have been prepared in an induction furnace by the reaction between preheated halide salts (K₂TiF₆ and KBF₄) and liquid molten Al. A number of process parameters such as reaction temperature (800, 900, 1,000?°C), reaction time (45, 60, 75?min.) and compositions (Ti/B ratio: 5/1, 1/3) have been studied. The indigenously prepared master alloys were characterised by chemical analysis, particles size analysis, XRD and SEM/EDX microanalysis. Results of particle size analysis suggest that the sizes of the intermetallic particles [Al₃Ti and TiB₂ in Al?C5Ti?C1B and (Al, Ti)B₂ in Al?C1Ti?C3B] present in various Al?CTi?CB master alloys increases with increase in reaction temperature (800?C1,000?°C) and reaction time (45?C75?min.). The population of the particles decreases with increase in reaction time and temperature. Further, SEM/EDX studies revealed that different morphologies of the intermetallic particles were observed at different reaction temperatures and reaction times. Further, the performances of the above-prepared master alloys were assessed for their grain refining efficiency on Al?C7Si alloy by macroscopy, DAS analysis. Grain refinement studies suggest that, B-rich Al?C1Ti?C3B master alloy shows better grain refinement performance on Al?C7Si alloy when compared to Ti-rich Al?C5Ti?C1B master alloy.