G.C. Yang

Structural evolution of undercooled Ni-Cu alloys

The structural evolution of the whole Ni–Cu system with undercooling was systematically investigated. For pure Ni and Cu only one grain refinement was found when they were undercooled higher than the critical value ΔT*. For the alloys, however, another grain refinement could occur in a certain range of the lower undercooling. Based on current dendrite growth theory, a thermodynamic concept, dimensionless superheating, was developed to evaluate the tendency of the dendrite remelting. With undercooling increasing, the dimensionless superheating of the alloy increased first and then decreased, which suggested that the dendrite remelting should only be responsible for the grain refinement at low undercoolings although the effect of the remelting during the recalescence on the eventual structural morphology deserved special attention. The decrease of the grain size above ΔT* was attributed to the stress that originated from the impediment of the static liquid to the solidification contraction or expansion of the rapidly growing dendrite skeleton, and led to the distortion and break-up of the dendrites. The recrystallization happening in cooling could make the grain size decrease further. The measurement of the refined grain size indicated that it was a strong function of the composition.

Solidification structure formation in undercooled Fe-Ni alloy

Abstract The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.

Solidification behavior of undercooled Cu70Ni30 alloy melt

The solidification behavior of undercooled Cu70Ni30 alloy in cylindrical crucible was investigated. By controlling the nucleation point at the top of the specimen, a directionally solidified dendrite structure in the undercooling range of 90–185 K has been obtained even if no longitudinal temperature gradient existed in the melt prior to nucleation. The dendrite growth velocity as a function of undercooling was measured by high speed cinematography. The results indicate that the dendrite growth velocity increases slowly with increasing undercooling when the directional solidification occurs. However, as the directional solidification breaks down above 185 K, the dendrite growth velocity rises up quickly. It is demonstrated that the breakdown of the directional dendrite growth at high undercoolings results from the destabilization of the dendrite growth, while this destabilization is caused by the solidification contraction stress occurring in the rapid dendrite growth. At higher undercoolings the destabilization even leads to the formation of a number of new crystals, and the recrystallization of the deformed solid during or immediately after solidification can make the grain size decrease further. On the assumption that only the thermal diffusion fields around the dendrites overlap, and that the dendrite tip can be represented by a paraboloid of revolution, a semi-quantitative theoretical model has been established.

Structure evolution in undercooled DD3 single crystal superalloy

Abstract A substantial undercooling up to 210 K was produced in the DD3 single crystal superalloy melt by employing the method of molten salt denucleating combined with thermal cycle, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–210 K, the solidification structure could be classified into four categories. At the lower range of undercooling, 0–25 K, the dendrite growth is dominantly controlled by solute diffusion, so the formed dendritic morphologies are similar to those of the conventional as-cast structure. With the increase of undercooling, the alloy undergoes two grain refinements. The first grain refinement occurs in a certain range of undercooling, 30–70 K, because of the dendrite break-up or ripening owing to remelting. At a higher range of undercooling, 78–150 K, however, the severe solute trapping that results from high dendrite growth velocity weakens the effect of solutal diffusion on the dendrite growth. In this case, highly developed fine dendrite is formed as a result of the restrained ripening process. The decrease of the grain size, i.e. the formation of fine equiaxed structure above the critical undercooling (ΔT*=180 K), can be attributed to the stress that originates from the extremely rapid solidification process, which results in the dendrite distortion, disintegration and recrystallization finally.

Coupled growth behavior in the rapidly solidified Ti–Al peritectic alloys

Abstract Laser melting technology, an ultra-high-temperature gradient direction solidification process, has been adopted on near equal atomic percent Ti–Al peritectic alloys with an effort to achieve the two-phase coupled growth structure. Both the alloy composition range and its local solidification parameters were determined by means of mathematical calculation of local solidification parameters in the melt pool. X-ray, SEM, TEM and optical microscopy techniques were carried out to investigate the microstructure and identify the phase composition. The two-phase (α and γ) coupled growth morphology under conditions of high growth velocity and high-temperature gradient was first detected in the laser resolidified Ti–Al peritectic alloys. The aluminum composition range appearing in the coupled growth of α and γ phases, lies in Ti–(51.0–54.0)xa0at% Al, a little shift towards the left direction of the hypoperitectic plateau. Microstructural analysis showed that the coupled growth morphology changed from regular lamellar, irregular blocks and equiaxed structures in sequence, with the temperature gradient decreasing during growth. Energy spectrum analysis results showed that not only the solute diffusion but also the dissolution of phase γ played an important role in the coupled structure evolution. Rapid eutectic growth KT model (Kurz, Trivedi, Metall Trans. A 22 (1991) 3051) could be used effectively to predict the characteristic lamellar spacing of two-phase coupled structures in Ti–Al peritectic alloys. The transformation from a peritectic, L+α→γ, to metastable eutectic reaction, L→α+γ, of near equal atomic percent Ti–Al peritectic alloys, induced the formation of two-phase coupled growth morphologies.

Direct observation of phase transformation layers in the undercooled hypoperitectic Ti47Al53 alloy

Abstract The hypoperitectic Ti 47 Al 53 alloy was cyclically superheated in a containerless electromagnetic levitation apparatus in order to reach the state of supercooling. The maximum undercooling of the alloy melt was up to 250xa0K. Critical undercooling ranges for the formation of various competing phases were determined by transmission electron microscopy. The primary phase β in 0 K T K , the primary phase α in 36 K T K , and the primary phase γ in 120 K T K were determined by microstructure analysis. The transient nucleation theory was adopted to explain the phase evolution relationship in the undercooled melt on the consideration of the competing phase with the shortest incubation time having separated from the undercooled melt. Due to different structures of primary and secondary phases, clear fault ribbons from β to α (or α to γ) were detected on the boundary layer between primary and secondary phases. The reason for the formation of fault layers on the boundary layer was given with a view of the crystalline structure evolution.

Effect of solidification time on the structural evolution of undercooled single phase alloys

Abstract The Cu 70 Ni 30 alloy melt was bulk undercooled by combining the glass fluxing technique with superheating–cooling cycles. Different solidification times, ranging from several seconds to several hundred seconds, were designed to investigate the structural evolution of the alloy in the undercooling range 0–250xa0K. The grain refinements at low and high undercoolings were all observed for all values of solidification time. The results indicate that the grain refinement at low undercoolings mainly originates from the dendrite remelting driven by the chemical superheating rather than by solid–liquid interface energy, and that at high undercoolings the refinement is not related to the dendrite remelting at all.

Morphological evolution of banding structures at high solidification velocity

Abstract Laser rapid solidification experiments were performed on a Ti–53at.%Al hypoperitectic alloy to investigate the dendritic growth behavior near the limit of high velocity absolute stability. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly. High velocity banding structures was firstly detected in Ti–Al peritectic alloys. The high velocity banding structures are parallel to the solid–liquid interface and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In the light bands with cellular growth mode, all dislocation assembles parallel to the growth direction and forms cell boundaries; while all dislocation distributes randomly in the dark bands. The determined growth velocity range for the appearance of banding structures is 0.5–1.1 m/s and the origin of the banding structure agrees well with the prediction of the CGZK phenomenological model.

Decagonal quasicrystal growth in the undercooled Al72Ni12Co16 alloy

Abstract High undercooling of the single-phase decagonal quasicrystal alloy Al 72 Ni 12 Co 16 was achieved by cyclically superheating the melt in a containerless electromagnetic levitation apparatus. The maximum undercooling of it was up to 368xa0K. XRD, TEM, SEM and optical microscopy techniques were adopted to investigate the microstructure and identify the phase composition, respectively. A distinct roughening transition was detected in the undercooled samples with the undercooling increasing. The decagonal quasicrystal phase in samples with small undercoolings showed a facet morphology and grew in a lateral growth mode below the roughening transition undercooling point, Δ T =64xa0K. Then the decagonal phase took on an equiaxed structure and indicated a continuous growth pattern. The roughening transition was caused by a sudden change of the growth direction from the two-fold axis normal to plane (0xa00xa00xa00xa02) to the ten-fold axis in the undercooled melt. Toners surface dynamics models of growing three-dimensional quasicrystal and BCT growth model in the undercooled melt (proposed by Boettinger, Coriel and Trivedi) were adopted to explain the roughening transition phenomenon.

Mechanism for metastable phase formation in undercooled Ti–Al peritectic alloys

Abstract Melt undercooling technique has been proved to be a powerful tool to investigate the formation of metastable phase in rapid solidification processing. Here, the recently observed phase evolution behaviour of the near equal atomic percent Ti–Al alloys was analyzed, as a function of melt undercooling, by thermodynamic and kinetic calculation. It was found that the formation of metastable phase is lightly predicted by the chemical Gibbs energy difference among competing phase but strongly controlled by kinetic effects arising from the competition of nucleation. The transient nucleation theory, with a consideration of incubation time, was proved to be an effective tool to explain the metastable phase formation in the undercooled near equal atomic percent Ti–Al alloys.