Phase-selection and metastable phase formation in highly undercooled eutectic Ni78.6Si21.4 alloys

A large undercooling (250 K) was achieved in eutectic Ni_78.6Si_21.4 melt by the combination of molten-glass denucleation and cyclic superheating. The metastable phase formation process in the bulk undercooled eutectic Ni_78.6Si_21.4 melts was investigated. With the increase of undercooling, different metastable phases form in eutectic Ni_78.6Si_21.4 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 Ni_78.6Si_21.4 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.     

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.     

Rapid solidification of undercooled Al-Cu-Si eutectic alloys

Under the conventional solidification condition, a liquid aluminium alloy can be hardly undercooled because of oxidation. In this work, rapid solidification of an undercooled liquid Al_80.4Cu_13.6Si₆ ternary eutectic alloy was realized by the glass fluxing method combined with recycled superheating. The relationship between superheating and undercooling was investigated at a certain cooling rate of the alloy melt. The maximum undercooling is 147 K (0.18T _E). The undercooled ternary eutectic is composed of α(Al) solid solution, (Si) semiconductor and θ(CuAl₂) intermetallic compound. In the (Al+Si+θ) ternary eutectic, (Si) faceted phase grows independently, while (Al) and θ non-faceted phases grow cooperatively in the lamellar mode. When undercooling is small, only (Al) solid solution forms as the leading phase. Once undercooling exceeds 73 K, (Si) phase nucleates firstly and grows as the primary phase. The alloy microstructure consists of primary (Al) dendrite, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic at small undercooling, while at large undercooling primary (Si) block, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic coexist. As undercooling increases, the volume fraction of primary (Al) dendrite decreases and that of primary (Si) block increases. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101, 50395105) and the Doctorate Foundation of Northwestern Polytechnical University (Grant No. CX200419)     

Recalescence behavior and solidification structure of the undercooled Fe82B17Sii eutectic alloy

By cyclic superheating incorporated with glass fluxing denucleation method the Fe^B^Si, eutectic alloy was undercooled up to A T = 342 K. The relations between recalescence behavior and solidification structures were systematically studied in the undercooling range of 6—342 K. Two critical undercoolings were observed: mixed eutectic was the unique growth morphology when the undercooling was less than A Ti =63 K; but the microstructure transformed to complete undercooled anomalous eutectic when the undercooling was greater than AT2 = 164 K. The two eutectic phases a(Fe,Si) and Fe^B conformed to the non-reciprocal nucleation effect. The boundary of the coupled zone of a (Fe,Si)-Fe2B system shifted toward the Fe^B side, and intersected the eutectic composition line at A71 = 154 K and A71 = 264 K, whose valley was at about AT = 207 K.     

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 dendrite growth in quaternary Ni-based alloys

Dendritic growth is one of the most common micro-structural formation mechanisms during crystal growth. Its morphology provides the kinetics information of crystal growth. Therefore, it is valuable to perform the research on rapid dendrite growth in order…     

Formation of ζ phase in Cu-Ge peritectic alloys

Rapid growth behavior of ζ 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 ＂a（Cu） dendrite ＋ ζ＂ peritectic phase → ζ＂ peritectic phase →, ζ dendrite ＋ （ε＋ζ） eutectic＂ takes place in the alloy at small undercooling, while the microstructural transition of ＂fragmented α （Cu）dendrite ＋ ζ peritectic phase →, ζ peritectic phase →ζ 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 ζ rdendrite, which leads to a decrease in the Ge content in ζ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ζ peritectic phase, the Ge content in ζ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.     

Forming mechanism of refined granular crystalline of Cu70Ni30 alloy by undercooled solidification

Moth glass fluxing and cyclic superheating techniques were adopted to effectively uudercool the Cu₇₀Ni₃₀ alloy in vacuum. Within the undercooling range of 21 K to 270 K, the microstructure evolution ol the alloy was investigated. When the inell was undercooled to △T > △T', (210 K) , the grain refinement took place abruptly, liascd on the observation of the solidified microstructure, the rnierocheinieal-analysis and the calculated results with UCT model, it is found that the secondary grain refinement mechanism consists of two stages. The dendrite is, firstly, broken into hag-ments owing to the stress caused by uneven shrinking during rapid solidilication, then the fragments, under the driving force of surface and strain energies, merge through the migration of l>ouinlaries, i. c. recryslallization, thus leading to the formation of secondary granular-crystalline .     

放电等离子烧结快速凝固Al75Si25合金的组织及力学性能

高硅铝合金粗大的初晶硅严重影响其力学性能与机械加工性能.本文利用熔融纺丝快速凝固技术、球磨与放电等离子烧结相结合的方法制备了Al₇₅Si₂₅合金.研究发现,放电等离子烧结Al₇₅Si₂₅能够遗传其快速凝固组织的特点.500 MPa,320℃烧结条件可获得密度达到98%以上的块体,其初晶硅弥散分布,组织尺寸细小,具有超细晶粒特征.此外,硅元素过饱和固溶于α（Al）基体.维氏硬度值和抗压强度分别达到298 Hv和674 MPa,具有优异的力学性能.      

铸态和沉积态Al-25Si-5Fe-3Cu合金的显微组织与相形成

采用常规铸造和喷射成形工艺制备了含硅达25%(质量分数)的过共晶Al-Si合金,利用SEM(EDS)、XRD和DSC等分析方法对合金的显微组织和相熔解析出进行了分析研究.结果表明,铸态合金含有粗大块状初晶Si相和粗大针片状含铁相,而喷射成形工艺能够使二者的尺寸、形貌发生改变而有利于合金性能的提高.同时,铸态和沉积态合金中均含有基体Al、初晶Si和Al2Cu相,不同的是铸态合金中含铁相主要为δ-Al4FeSi2相,而沉积态合金中以β-Al5FeSi相为主.分析其原因主要是糊状层的存在引起沉积坯冷却速度降低而导致沉积坯中发生δ-Al4FeSi2相的转变及共晶组织增加,致使沉积态合金中β-Al5FeSi相为主要含铁相.采用DSC实验对沉积态合金在熔化和凝固过程中发生的反应进行了讨论.     

Catalytic nucleation of a meta-stabled phase in phase-seeded undercooled Fe75Ni25 melts

Undercooled Fe75Ni25 melts were phase-seeded by a high purity iron. It was found that above a critical undercooling,△Tc = 135 K, a metastable δ phase (b.c.c) solidifies from the iron-seeded melts instead of a thermodynamically stable γ phase (f.c.c). While undercooling of the melt (△T ) is below △Tc, solidification of the γ phase prevails in the iron-seeded melts. For the undercooled melts subjected to spontaneous nucleation, the γ phase always solidifies. After solidification, the as-solidified γ phase transforms completely to martensite; whereas the as-solidified metastable δ phase partially transforms to the γ phase, and then to martensite. The untransformed δ phase retains as a -ferrite particles in the microstructures. Based on the classical nucleation theory, catalytic nucleation of the metastable δ phase in the phase-seeded undercooledFe75Ni25 melts was analyzed. It was quantitatively demonstrated that when△T>△Tc, the formation of the δ phase can be ascribed to a better catalytic effect of the iron on its nucleation than that on the nucleation of the γphase.     

Solidification microstructure formation in HK40 and HH40 alloys

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatPro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L → L + γ → L + γ + M₇C₃ → γ + M₇C₃ → γ + M₇C₃ + M₂₃C₆→ γ + M₂₃C₆ and L → L + δ → L + δ + γ→ L + δ + γ + M₂₃C₆ δ + γ + M₂₃C₆, respectively. The solidification mode was determined to be the austenitic mode (A mode) in HK40 alloy and the ferritic–austenitic solidification mode (FA mode) in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary γ dendrites grow at the 60° angle to each other. On the other hand, in HH40 alloy, residual δ forms because of the incomplete transformation from δ to γ. Cr₂₃C₆ carbide is produced in solid delta ferrite δ but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.     

A new synthetic route to MgO-MgAl2O4-ZrO2 highly dispersed composite material through formation of Mg5Al2.4Zr1.7O12 metastable phase:synthesis and physical properties

Mg₅Al_2.4Zr_1.7O₁₂ metastable phase was successfully synthesized from analytical-grade MgO, α-Al₂O₃, MgAl₂O₄, and ZrO₂ under an N₂ atmosphere. The sintering temperature was varied from 1650 to 1780℃, and the highest amount of Mg₅Al_2.4Zr_1.7O₁₂ appeared in the composite material when the sintering temperature was 1760℃. According to our research of the formation mechanism of Mg₅Al_2.4Zr_1.7O₁₂, the formation and growth of MgAl₂O₄ dominated when the temperature was not higher than 1650℃. When the temperature was higher than 1650℃, MgO and ZrO₂ tended to diffuse into MgAl₂O₄ and the Mg₅Al_2.4Zr_1.7O₁₂ solid solution was formed. When the temperature reached 1760℃, the formation of Mg₅Al_2.4Zr_1.7O₁₂ was completed. The effect of MgAl₂O₄ spinel crystals was also studied, and their introduction into the composite material promoted the formation and growth of Mg₅Al_2.4Zr_1.7O₁₂. A highly dispersed MgO-MgAl₂O₄-ZrO₂ composite material was prepared through the decomposition of the Mg₅Al_2.4Zr_1.7O₁₂ metastable phase. The as-prepared composite material showed improved overall physical properties because of the good dispersion of MgO, MgAl₂O₄, and ZrO₂ phases.     

High undercooling and rapid dendritic growth of Cu-Sb alloy in drop tube

Droplets of Cu-20%Sb hypoeutectic alloy has been rapidly solidified in drop tube within the containerless condition. With the decrease of droplet diameter, undercooling increases and the microstructures of primary copper dendrite refines. Undercooling up to 207 K (0.17 T _L) is obtained in experiment. Theoretic analysis indicated that because of the broad temperature range of solidification, the rapid growth of primary copper dendrite is controlled by the solutal diffusion. Judging from the calculation of T₀ curve in the phase diagram, it is shown that the critical undercooling of segregationless solidification is δT ₀ = 474 K. At the maximum undercooling of 207 K, the growth velocity of primary copper phase exceeds to 37 mm/s, and the distinct solute trapping occurs.     

Phase transition , structure and shape memory effect of Ni47Ti43Hf10 alloy

A Ni₄₇Ti₄₃Hf₁₀ high temperature shape memory alloy is fabricated. The martensitic transformation temperature (TT) is obtained by differential scanning calorimetry and four-probe electrical resistivity measurements. The effect of thermal cycling is investigated and it is found that the TT tends to be stable quickly, which is of benefit to practical applications. The martensite structure is determined to be B19' monoclinic by X-ray diffraction and transmission electron microscopy. One-way and two-way (which is seldom reported before) shape memory properties are studied by tensile and bending tests. The cycling number of two-way shape memory effect is tested for more than 20000 times.     

ZrAl4 C4、Zr2 Al4 C5和Zr3 Al4 C6结构、弹性和电子性质对比

采用第一性原理方法,对比研究了Zr-Al-C体系纳米层状化合物ZrAl₄C₄、Zr₂Al₄C₅和Zr₃Al₄C₆的结构、弹性和电子性质,并探寻其规律性。结果表明：从晶体结构角度分析,三种材料均可看作由(ZrC)和(Al₄C₃)两个独立单元以不同比例组合而成,可统一表示为(ZrC)_nAl₄C₃(n=1,2,3),且三者的晶格常数a值近似相等,晶格常数c随着ZrC含量的增大而变大。三种化合物弹性性质中,体模量、剪切模量、杨氏模量的大小关系满足：ZrAl₄C₄< Zr₂Al₄C₅< Zr₃Al₄C₆。通过对三者电子态密度和Mulliken布局分析得到,ZrAl₄C₄、Zr₂Al₄C₅和Zr₃Al₄C₆均具有金属键、离子键和共价键的特征,且Zr-C键强于Al-C键,从微观电子角度解释了在(ZrC)_nAl₄C₃(n=1,2,3)体系中Zr-C键含量越高则对应材料的体模量、剪切模量、杨氏模量等弹性性质越大。本文计算结果与已有实验值和理论值吻合较好。     

定向凝固包晶合金相和微观组织选择研究进展

综述了定向凝固包晶合金相和微观组织选择的理论模型和实验研究进展,分析相和微观组织的选择规律,同时讨论了对流对凝固微观组织的影响。依据国内外对包晶合金凝固的研究现状,提出进一步研究的方向。     

机械合金化-热处理制备V5Si3材料

[目的]为了获得V/Si类化合物基本结构和性能,拟合成V/Si系金属间化合物V_5Si_3。[方法]采用V粉和Si粉作原料,按照原子比V∶Si=5∶3进行称量,通过机械合金化与热处理制备V_5Si_3,再利用XRD,SEM/EDS等方法对球磨粉体试样和热处理试样的物相组成、微观形貌和微区成分进行分析与表征,并测量其抗压性能和硬度。[结果]V/Si粉体经过球磨后颗粒度减小,逐渐非晶化,最终得到以原子比V∶Si=5∶3结合的非晶态物质;经过热处理,非晶态结构转变为V_5Si_3晶体。[结论]随着热处理温度的升高,晶体结晶度提高,材料的抗压强度和显微硬度增加,抗压强度达到640 MPa,显微硬度最高为656 MPa。     

等离子喷涂Al2O3-TiO2系涂层的相组成定量分析

采用大气等离子喷涂制备了TiO2质量百分比分别为0%、3%、13%、20%、40%5种Al2O3-TiO2系涂层.利用Rietveld法以及添加标样的办法对喷涂前后的材料物相进行了定量分析,探讨了材料的相变过程.经喷涂,大部分α-Al2O3转变为亚稳相γ-Al2O3,喷涂粉末中存在的TiAl2O5保留在涂层中,有四种涂层中还形成了非晶相.涂层中的非晶相的含量先随着TiO2增加而增加,在TiO2的质量百分比为13%时最多,而后下降.这个趋势是喷涂粉末中Al2O3含量以及喷涂过程中材料的冷却速率共同作用的结果.