Effect of heating rate on the grain refinement of a TiAl alloy by cyclic heat treatment

This letter reports the effect of heating rate on the grain refinement of a Ti–46Al–2Cr–2Nb (at.%) alloy by cyclic heat treatment. Results show that microstructures with two different sizes were developed at two different heating rates. A coarse fully lamellar structure can be refined to a fine one with a size of 15 μm if the heating rate, together with the heating temperature, is appropriately controlled.     

Grain refinement of biomedical Co-27Cr-5Mo-0.16N alloy by reverse transformation

Advanced grain refinement of a biomedical Ni-free Co-27Cr-5Mo-0.16N alloy without hot or cold plastic deformation was successfully achieved by a reverse transformation from a lamellar (hcp + Cr₂N) phase to an fcc phase. The technique consisted of a two-step heat treatment. First, the solution-treated specimen was subjected to isothermal aging at 1073 K for 90 ks, forming a lamellar structure of hcp and Cr₂N phases. Then, the aged specimen having a completely lamellar microstructure was reverse-treated at temperatures from 1273 to 1473 K, where the fcc phase is stable. The resultant grains were approximately 1/10 of their initial size. Moreover, tensile testing after reverse transformation showed excellent strength with good ductility compared to samples examined before the reverse transformation. Our results will contribute to the development of biomedical Ni-free Co-Cr-Mo-N alloys with refined grain size and good mechanical properties, without requiring any hot workings.     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

High-temperature tensile properties and fracture characteristics in a monolithic gamma TiAl alloy and a TiB2 particle-reinforced TiAl alloy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Nontraditional grain refinement through phase transformation in an Al‐Zn alloy

Abstract

Grain size is an important factor affecting the mechanical properties of polycrystalline engineering materials. By conventional methods grains can be refined only by recrystallization following large amounts of plastic deformation. In the present study it will be shown how thermal cycling through the discontinuous precipitation reaction (discontinuous precipitation followed by continuous dissolution) can be utilized to obtain grain refinement in an Al‐Zn alloy without any plastic deformation.     

Grain refinement of AM60B magnesium alloy by SiC particles

AM60B alloy has been refined by SiC particles and the corresponding refining mechanism has been mainly discussed. The results indicate that the addition of 0.2 wt% SiC particles in form of mixture with Mg powder decreases the grain size from 317 μm of the not refined alloy to 46 μm. The decrease of β phase and formation of Mg₂Si and Al₄C₃ phases well demonstrate that the reactions of 3SiC + 4Al = Al₄C₃ + 3Si and 2Si + Mg = Mg₂Si occur during refining treatment. In addition, the crystal nuclei are composed of two kinds of elements, Al and C. All of these imply that the formed Al₄C₃ particles are the actual heterogeneous nucleation substrates.     

Evaporation of Al and microstructural variation in TiAl alloy during heat treatment

Abstract

A TiAl alloy of Ti-47Al-2Cr-2Nb at.- was used to study phase transformation and microstructure control by heat treatment and a glow discharge spectrometer was applied to analyse the compositional variation from the specimen surface to the interior. It was found that Al tended to evaporate at elevated temperatures. As a result, Al was significantly depleted and a quite different microstructure developed in the outer layer compared with that in the interior. Results also showed that the outer layer appeared thicker with increasing heating temperature, time, and cooling rate, and with changing the environment from vacuum to air. Since the evaporation of Al leads to microstructure variation and perhaps property deterioration as well, it is suggested that this effect should be considered for alloy design and processing.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Grain refinement of AZ91 alloy by introducing ultrasonic vibration during solidification

Ultrasonic vibration was introduced into the solidification of AZ91 alloy. Various microstructures were produced in this alloy using ultrasonic vibrations at different temperatures of the melt. The coarse dendrite microstructures were obtained with ultrasonic vibrations at temperatures below the liquidus temperature. The fine uniform grains were achieved under ultrasonic vibrations during the nucleation stage, which was mainly attributed to the cavitation and the acoustic flow induced by the ultrasonic vibration.     

Grain refinement of AZ31 magnesium alloy by electromagnetic stirring under effect of grain-refiner

The effects of electromagnetic stirring and Al₄C₃ grain refiner on the grain refinement of semi-continuously cast AZ31 magnesium alloy were discussed in this investigation. The results indicate that electromagnetic stirring has an effective refining effect on the grain size of AZ31 magnesium alloy under the effect of Al₄C₃ grain refiner. Electromagnetic stirring can ‘activate’ the Al₄C₃ particles, resulting in more heterogeneous nucleation sites for the primary α-Mg grains. But, longer holding time can ‘deactivate’ the Al₄C₃ particles and poison the grain refining effect.     

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract

Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.     

Grain refinement in solidification of highly undercooled eutectic Ni–Si alloy

Substantial undercooling up to 550 K (0.386T_E, with T_E as the melting point) was achieved in eutectic Ni_78.6Si_21.4 alloy melt using glass fluxing combined with cyclic superheating. Accordingly, a particular refined microcrystalline morphology is obtained in the as-solidified structure. The physical mechanism of the grain refinement subjected to high undercooling is interpreted in terms of the classical nucleation theory and LKT/BCT model. It was concluded that the above refinement can be ascribed to the substantially increased nucleation rate under high undercooling.     

Bioactive coating on a cobalt base alloy by heat treatment

A method to promote a bioactive surface on the cobalt base alloy ASTM F-75 was tested. A set of cylindrical samples was obtained using the investment casting technique and packed in a mixture composed of 70% β-tricalcium phosphate and 30% bioactive glass and then heat treated for 1 h at 1220 °C. To characterize the in vitro bioactivity, a set of heat treated metallic specimens was immersed in a simulated body fluid with an ionic concentration nearly equal to that of human blood plasma (SBF) for 7, 10 and 21 days and in a more concentrated solution (1.5 SBF). After heat treatment, fine agglomerates homogeneously distributed, containing Ca, P, O and Na were observed on the metallic surface. After immersion of the samples in simulated body fluids, a thicker layer identified as apatite was formed on the samples immersed in SBF for 21 days and on all samples immersed in 1.5 SBF.     

Grain refinement and mechanical properties enhancement of AZ91E alloy by addition of ceramic particles

Improved mechanical properties and structural uniformity of Mg-based alloys can be achieved by use of grain-refining additives prior to casting. Ceramic particles of α-Al₂O₃ and SiC can serve as such additives to refine the microstructure of Mg–Al-based alloys. However, direct introduction of ceramic particles into Mg matrix is limited by the poor wetting of those particles by liquid Mg and their massive agglomeration. Mg/α-Al₂O₃ and Mg/SiC master alloys were prepared using a method based on the insertion of the ceramic particles into a molten Mg bath through a Mg-nitride layer formed on the surface of the molten bath. The mixture of Mg/ceramic particles was cooled to room temperature under a nitrogen atmosphere. Mg-15%Al₂O₃ and AZ91E + 10%SiC master alloys were obtained. These master alloys were used to refine AZ91E alloys by introducing various amounts of ceramic particles to manufacture AZ91E + 1%Al₂O₃, AZ91E + 1%SiC, and AZ91E + 3%SiC alloys. These were cast using high-pressure die casting and gravity die casting. The alloy AZ91E + 1%Al₂O₃ was grain refined to ~20 μm and the alloys AZ91E + SiC were grain refined to ~50 μm as against 110 μm in non-refined counterparts. The mechanical properties of the modified alloys are substantially better than those of a non-refined AZ91E alloy which is the result of a combination of grain refinement and reinforcement of the matrix by ceramic particles. Alloy AZ91E + 1%Al₂O₃ exhibited the best mechanical properties.     

Grain refinement in as-cast AZ80 Mg alloy under large strain deformation

Grain refinement in as-cast AZ80 magnesium alloy under large strain deformation was studied by hot multiple forging (MF). The results show that during the deformation there exists a critical strain controlling the degree of the homogeneity, which is in the range of 2–2.4. A homogeneous microstructure with fine dynamic recrystallized grains can be attained when the applied strain exceeds the critical strain and after that, it is difficult to get more grain refinement further. A main characteristic of microstructure evolution is directly associated with grain splitting due to the formation of microbands that develop in various directions. Such microbands intersect each other during hot MF, resulting in continuous subdivision of coarse grains into misoriented fine domains. Further deformation leads to increase in the number and misorientation of these boundaries and finally almost full development of fine equiaxed grains in high strain. New grains are concluded to be evolved by a kind of continuous reaction, that is essentially similar to continuous dynamic recrystallization.     

Plasticity induced transformation in a metastable β Ti-1023 alloy by controlled heat treatments

Abstract

Investigations into the possibility of improving the strength–ductility relation in a metastable β-titanium alloy (Ti–10V–2Fe–3Al) through plasticity induced transformation (PiTTi) have been carried out. Various heat treatments in the β and/or α+β condition were performed to study their influence on both the microstructure and solute partitioning, which eventually control the PiTTi effect. Stress induced martensite formation promoting such effect has been observed upon compression testing for β and β+(α+β) microstructures. The stress–strain curves exhibiting stress induced martensite show a ～20% increase in strength, while still retaining a reasonable ductility level. Microstructural parameters such as grain size and solute concentration (especially V) in β have been related to the alloy's ability to exhibit PiTTi.