Formation of ■ phase in Cu-Ge peritectic alloys

Rapid growth behavior of ■ phase has been investigated in the undercooling experiments of Cu-14%Ge, Cu-15%Ge, Cu-18.5%Ge and Cu-22%Ge alloys. Alloys of the four compositions obtain the maximum undercoolings of 202 K(0.17TL), 245 K(0.20TL), 223 K(0.20TL) and 176 K(0.17TL), respectively. As the content of Ge increases, the microstructural transition of "α (Cu) dendrite ■ peritectic phase →■ peritectic phase →■ dendrite (ε ■ ) eutectic" takes place in the alloy at small undercooling, while the microstructural transition of "fragmented α (Cu) dendrite ■ peritectic phase →■peritectic phase →■ dendrite ε phase" happens in the alloy at large undercooling. EDS analysis of the Ge content in ■ peritectic phase indicates that undercooling enlarges the solid solubility of α dendrite, which leads to a decrease in the Ge content in ■ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ■ peritectic phase, the Ge content in ■ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.     

Peritectic solidification under high undercooling conditions

The solidification characteristics of highly undercooled Cu-7.77%Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 TL). It is found that the α(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases , which is lower than the thermodynamic calculation result owing to the actual non-adia-batic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite growth velocities of primary α(Co) phase are subsequently calculated as a function of undercooling on the basis of this model.     

Primary dendrite growth of Ni<Subscript>3</Subscript>Sn intermetallic compound during rapid solidification of undercooled Ni-Sn-Ge alloy

Liquid Ni-31.7%Sn-2.5%Ge alloy was highly undercooled by up to 238 K(0.17TL) with glass fluxing and drop tube techniques.The dendritic growth velocity of primary Ni3Sn compound shows a power-law relation to undercooling and achieves a maximum velocity of 380 mm/s.The addition of Ge reduces its growth velocity as compared with the binary Ni75Sn25 alloy.A structural transition from coarse dendrites into equiaxed grains occurs once undercooling exceeds a critical value of about 125 K,which is accompanied by both grain refinement and solute trapping.The Ni3Sn intermetallic compound behaves like a normal solid solution phase showing nonfaceted growth during rapid solidification.     

Rapid dendritic growth of Al-Ge hypoeutectic alloy in a drop tube

Abstract Dendritic growth in Al-45% Ge hypoeutectic alloy has been investigated during free fall in a 3 m drop tube. Calculationsindicate that the undercooling obtained for the falling Al-45% Ge droplets ranges from 13 K to 201 K. The maximum undercooling attains0. 27 T_L. With the increase of undercooling, the primary (Al) phase undergoes a "columnar dendrite to equiaxed dendrite" structural tran-sition. According to the current rapid dendritic growth theory, the growth of primary (Al) phase is always controlled by solute diffusion.     

Rapid growth of primary dendrite in highly undercooled copper-antimany alloy

The droplets of Cu-11wt.%Sb hypoeutectic alloy have been rapidly solidified during containerless processing in a 3 m drop tube. The undercooling and cooling rates are estimated, and both play a dominant role in the dendritic growth of primary Cu phase. Undercoolings up to 200 K (0.16TL, where TL is the liquidus temperature) have been obtained in the experiment. With the increase of undercooling, the microstructural evolution of primary Cu phase proceeds from remelted dendrites to the equiaxed grains. A coarse dendritic grain microstructure can form in the undercooling range of 61～102 K and at cooling rates of 1.35×102～2.66×103 K/s. The segregationless solidification of Cu-11wt.%Sb hypoeutectic alloy occurs when undercooling is more than 176 K. The growth of primary Cu phase is mainly controlled by solute diffusion.     

Phase separation and microstructural characteristics of undercooled Cu-Pb immiscible alloy

Bulk samples of Cu-80%Pb hypermonotectic alloy were undercooled by up to 270 K (0.21 TL) with glass fluxing technique. The undercooling behavior and the final microstructure were investigated experimentally. It was found that the macrosegregation decreased with the increase of undercooling exponentially. When undercooling reached 270 K, the volume fraction of macrosegregation was reduced by one order of magnitude. Meanwhile, high undercooling brought about significant changes to the microstructural morphology of S(Cu) phase. At small undercoolings, S(Cu) phase grew in dendritic manner. As undercooling increased, S(Cu) dendrite transformed gradually to spherical shell. This morphology transition was ascribed to the concurrent action of the phase separation within miscibility gap and the subsequent solidification process of L2 (Pb) matrix. As an essential step to model the final microstructure, theoretical calculations related to the nucleation of L1 (Cu) droplets were carried out.     

Rapid growth of FeAl intermetallic compound under high undercooling conditions

Fe-58at%Al alloy is undercooled up to 222K (0.15TL) with the drop tube technique.It is found that there exists a critical undercooling about 185 K,beyond which a “dendrite-equiaxed” growth morphology transition occurs in FeAl intermetallic compound. This transition is characterized by sharp decrease of its grain size.Once the undercooling exceeds 215 K, the peritectic transformation is suppressed completely and a fibrous structure is formed,which results from the cooperative growth of FeAl and FeAl2 compounds.     

Dendrite growth and micromechanical properties of rapidly solidified ternary Ni-Fe-Ti alloy

The rapid solidification of undercooled liquid Ni_(45)Fe_(40)Ti_(15)alloy was realized by glass fluxing technique.The microstructure of this alloy consists of primaryγ-(Fe,Ni)phase and a small amount of interdendritic pseudobinary eutectic.The primaryγ-(Fe,Ni)phase transferred from coarse dendrite to fragmented dendrite and the lamellar eutectic became fractured with the increase of undercooling.The growth velocity ofγ-(Fe,Ni)dendrite increased following a power relation with the rise of undercooling.The addition of solute Ti suppressed the rapid growth ofγ-(Fe,Ni)dendrite,as compared with the calculation results of Fe-Ni alloy based on LKT model.The microhardness values of the alloy and the primaryγ-(Fe,Ni)phase increased by 1.5 times owing to the microstructural refinement caused by the rapid dendrite growth.The difference was enlarged as undercooling increases,resulting from the enhanced hardening effects on the alloy from the increased grain boundaries and the second phase.     

Rapid monotecticsolidincation during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and a-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.     

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tin) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.     

Microstructural evolution of ternary Ag33Cu42Ge25 eutectic alloy inside ultrasonic field

Ultrasonic field with a frequency of 20 k Hz is introduced into the solidification process of ternary Ag33Cu42Ge25 eutectic alloy from the sample bottom to its top. The ultrasound stimulates the nucleation of alloy melt and prevents its bulk undercooling. At low ultrasound power of 250 W,the primary ε2phase in the whole alloy sample grows into non-faceted equiaxed grains, which differs to its faceted morphology of long strip under static condition. The pseudobinary(Ag t ε2) eutectic transits from dendrite shape grain composed of rod type eutectic to equiaxed chrysanthemus shape formed by lamellar structure. By contrast, the ultrasound produces no obvious variation in the morphology of ternary(Ag t Ge t ε2) eutectic except a coarsening effect. When ultrasound power rises to 500 W, divorced ternary(Ag t Ge t ε2) eutectic forms at the sample bottom. However, in the upper part, the ultrasonic energy weakens, and it only brings about prominent refining effect to primary ε2phase.The microstructural evolution mechanism is investigated on the cavitation, acoustic streaming and acoustic attenuation.     

A first-principles study of displacive β to ω transition in Ti-V alloys

The ground state properties of β and ω phases in Ti-(0–30 at%)V alloys were calculated, and subsequently thermodynamics and energy barriers of the displacive β to ω transition were investigated by first-principles. The results show that the lattice parameters of β and ω phases decreases with increasing V content in Ti-V alloys. The principal lattice strains for the β to ω transition are highly compositional dependent, and the volume variation decreases with increasing V content. The mechanical stability of the ω phase increases initially at the V content around 10 at% and then decreases with increasing V content. Based on the quasiharmonic Debye model, a metastable diffusionless phase diagram has been established, showing that the ω phase is thermodynamically more stable than the β phase at room temperature, anticipating a spontaneous transition from β to ω phases in Ti-V alloys. The calculations of energy pathways indicate that there is an energy barrier during the displacive βto ω transition in Ti-V alloys at temperatures from 100 to 500 K, but not at 0 K.     

Effect of Ca modification on the elemental composition,microstructure and tensile properties of Al-7Si-0.3Mg alloy

The effect of Ca addition on the elemental composition, microstructure, Brinell hardness and tensile properties of Al-7Si-0.3Mg alloy were investigated. The residual content of Ca in the alloy linearly increased with the amount of Ca added to the melt. The optimal microstructure and properties were obtained by adding 0.06wt% Ca to Al-7Si-0.3Mg alloy. The secondary dendrite arm spacing (SDAS) of the primary α phase decreased from 44.41 μm to 19.4 μm, and the eutectic Si changed from coarse plates to fine coral. The length of the Fe-rich phase (β-Al₅FeSi) decreased from 30.2 μm to 3.8 μm, and the Brinell hardness can reach to 66.9. The ultimate tensile strength, yield strength, and elongation of the resulting alloy increased from 159.5 MPa, 79 MPa, and 2.5% to 212 MPa, 86.5 MPa, and 4.5%, respectively. The addition of Ca can effectively refine the primary α phase and modify the eutectic Si phase, likely because Ca enrichment at the front of the solid-liquid interface led to undercooling of the alloy, reduced the growth rate of the primary α phase, and refined the grain size. Also, it could increase the latent heat of crystallization, undercooling, and the nucleation rate of eutectic Si, which was beneficial to the improvement of the morphology of eutectic Si.     

Alloying elements characterization in a Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-1Nd titanium alloy by carbon addition

The effects of carbon addition (0.01wt%-0.43wt%) on a Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-1Nd (wt%) alloy with a bimodal microstructure were investigated. Electron probe microanalysis was carried out to examine the partitioning behavior of carbon and the relation of carbon content to the distributions of Al and Mo in the primary α phase (α_p) and β transformed structure (β). It was found that interstitial carbon is enriched in the α_p phase and its content slightly reduces with the increase of the volume fraction of α_p.The measurements of carbon content in the present alloy with an α_p of 15vol% showed that the carbon content in the α_p phase increases with the increment of carbon addition until a maximum but keeps almost constant in the β phase. The addition of carbon reduces the solubility of Al and Mo in the α_p phase and leads to the increment of Mo partitioning to the β phase. When the carbon content is over 0.17wt% (0.67at%), carbide precipitation occurs in the matrix and its volume fraction is related to the volume fraction of α_p which can be explained in term of the difference of carbon solubility in the α_p and β phases.     

Effects of aging on the microstructure of a Cu-Al-Ni-Mn shape memory alloy

The influence of aging on the microstructure and mechanical properties of Cu-11.6wt%Al-3.9wt%Ni-2.5wt%Mn shape memory alloy (SMA) was studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometer, and differential scanning calorimeter (DSC). Experimental results show that bainite, γ_2, and α phase precipitates occur with the aging effect in the alloy. After aging at 300dgC, the bainitic precipitates appear at the early stages of aging, while the precipitates of γ₂ phase are observed for a longer aging time. When the aging temperature increases, the bainite gradually evolves into γ₂ phase and equilibrium α phase (bcc) precipitates from the remaining parent phase. Thus, the bainite, γ₂, and α phases appear, while the martensite phase disappears progressively in the alloy. The bainitic precipitates decrease the reverse transformation temperature while the γ₂ phase precipitates increase these temperatures with a decrease of solute content in the retained parent phase. On the other hand, these precipitations cause an increasing in hardness of the alloy.     

The giant magnetocaloric effect in Gd<Subscript>5</Subscript>Si<Subscript>2</Subscript>Ge<Subscript>2</Subscript> with low purity gadolinium

The giant magnetocaloric effect Gd5Si2Ge2 alloy was prepared with 99wt% low purity commercial Gd. Powder XRD and magnetic measurements showed that the Gd5Si2Ge2 alloy annealed at 1200℃ for 1h had a significant magnetic- crystallographic first order phase transition at about 270 K. The maximal magnetic entropy change is 17.55 J· kg^-1· K^-1 under a magnetic field change of 0-5 T. The distinct increase of magnetic entropy change belongs to the first-order phase transition from the orthorhombic Gd5Si4-type to the monoclinic Gd5Si2Ge2-type after high temperature heat-treatment.     

Solidification characteristics of Pb-Sb hypereutectic alloy within ultrasonic field

The solidification of Pb-16%Sb hypereutectic alloy is investigated within ultrasonic field with a fre-quency of 15 kHz. It is found that the ultrasonic field promotes crystal nucleation and terminates the further bulk undercooling of the alloy melt. Theoretical analysis shows that the cavitation effect and the forced bulk vibration are the main factors that reduce the undercooling level. With the increase of ul-trasound intensity, the primary (Sb) phase experiences a growth mode transition from faceted to non-faceted branched growth, and the macrosegregation of primary (Sb) phase is gradually sup-pressed. In addition, the microstructures of Pb-Sb eutectic exhibit a conspicuous coarsening with in-creasing ultrasound intensity, and a structural transition of “lamellar eutectic—anomalous eutectic” occurs when ultrasound intensity rises up to 1.6 W/cm2. The ultrasonic field also changes the solute distribution adjacent to the solidification front, which lowers the Pb contents in primary (Sb) phase.     

Dynamic solidification of Sn-38.1% Pb eutectic alloy within ultrasonic field

The dynamic solidification of Sn-38.1% Pb eutectic alloy within an ultrasonic field is investigated at a frequency of 35 kHz.As the sample height H is reduced,the effect of ultrasound on macrosegregation becomes more prominent,and the volume fraction of spherical eutectic cells increases correspondingly.When H equals the wavelength λ in liquid alloy,the introduction of ultrasound enlarges the distribution region of the primary (Sn) phase,but reduces the domains of the Sn-Pb eutectic and primary (Pb) phases.Meanwhile,a "dendritic-equiaxed" structural transition occurs in the primary (Sn) phase,and its grain size is significantly reduced within the ultrasonic field.Once H decreases to λ/2 and λ/4,the ultrasonic field promotes crystal nucleation and suppresses further undercooling of the bulk liquid alloy.Theoretical analyses indicate that the local high pressure induced by the cavitation effect and the stirring effect due to acoustic streaming are the main factors dominating the eutectic growth mechanism during dynamic solidification.     

Phase-selection and metastable phase formation in highly undercooled eutectic Ni_(78.6)Si_(21.4)alloys

A large undercooling (250 K) was achieved in eutectic Ni78.6 Si21.4 melt by the combination of molten-glass denucleation and cyclic superheating. The metastable phase formation process in the bulk undercooled eutectic Ni78.6 Si21.4 melts was investigated. With the increase of undercooling, different metastable phases form in eutectic Ni78.6 Si21.4 melts and part of these metastable phases can be kept at room temperature through slow post-solidification. Under large undercooling, the metastable phases β2-Ni3Si, Ni31Si12 and Ni3Si2 were identified. Especially, the Ni3Si2 phase was obtained in eutectic Ni78.6 Si21.4 alloy for the first time. Based on the principle of free energy minimum and transient nucleation theory, the solidification behavior of melts was analyzed with regard to the metastable phase formation when the melts were in highly undercooled state.     

Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu (wt%, signed as 3C), fabricated by traditional cast (TC) and spray forming (SF) processes, were investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that Al₇Cu₂Fe phase can be formed and transformed in TC- and SF-3C alloys between 802–813 K and 800–815 K, respectively. The transformation from β-Al₅FeSi to δ-Al₄FeSi₂ phase via peritectic reaction can occur at around 858–870 K and 876–890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature of δ-Al₄FeSi₂ phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation of δ-Al₄FeSi₂→β-Al₅FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.