铝合金中孪生枝晶的研究进展

羽状晶是铸造组织中与柱状晶、等轴晶并列的第三类组织,在凝固理论研究和工业应用中有重要价值。近年来研究表明孪生枝晶是羽状晶最基本的微观组织特征。综述了孪生枝晶尖端形貌、增殖机制、与常规枝晶的竞争生长规律以及羽状晶力学性能等方面的研究进展,讨论了不同因素对孪生枝晶形成和生长的影响,总结了研究中尚未解决的问题。     

Dendrite growth morphologies in aluminium alloys

Different aluminium alloys, in particular Al–Cu, Al–Mg and Al–Si, have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, particular growth morphologies were observed. The most common is linked with the formation of feathery grains: these are constituted by series of twinned lamellae, in which the dendrites have 110 trunks cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular 100 dendritic grains. The dendritic growth along 110 crystallographic directions is supposed to be due to a change in the anisotropy of certain properties of the alloy, such as the solid/liquid interfacial energy and/or the atom attachment kinetics. When solidification conditions become less favourable to 110 growth morphologies, a mixed dendritic form containing 110 trunks and 100 arms may be obtained. In the case of the 5182 Al–Mg type alloy, 110 columnar grains which were not twinned could be observed together with feathery crystals. The possibility of such changes in the growth direction of aluminium alloys was further demonstrated by the observation of dendrites of Al–Cu–Mg solidified in a Bridgman device. In this case, 112 dendrites grow and progressively change their growth direction, thus showing curved shapes.     

Feathery grain growth during solidification under forced flow conditions

The grain morphology developed during solidification of an Al–4.5% Cu alloy is represented generally by columnar or equiaxed dendrites. Twinned feathery grains are found in the structure formed under certain heat and flow conditions during solidification. In this work, these conditions were achieved during solidification in a cavity under forced flow. Feathery grain formation is studied by means of fluid dynamics simulations with solidification included and by experiments. In order to determine the crystallographic orientation of feathery grains, electron backscattered diffraction measurements were performed. The growth features of feathery grains were analyzed by observations made normal and parallel to the growth direction. Some correlations between twinned feathery morphology, flow and solidification parameters were obtained.     

Twinned dendrite growth in binary aluminum alloys

The formation of twinned dendrites (feathery grains) in binary Al–Zn, Al–Mg, Al–Cu and Al–Ni alloys has been studied in specimens directionally solidified under identical thermal conditions, i.e. G ≈ 100 K cm⁻¹, v ≈ 1 mm s⁻¹, and with slight natural convection in the melt. The influence of the solute element nature and content has been found to be of less importance than previously reported since feathery grains were formed in all four alloys, regardless whether the alloying elements are hexagonal close packed (Zn and Mg) or face-centered cubic with a high (Ni) or low (Cu) stacking fault energy. A detailed analysis confirmed that twinned dendrites grow along 〈1 1 0〉 directions in all four cases, with a complex branch morphology made of up to six to nine arms. Surprisingly, at high Zn or Mg compositions for which regular dendrites grow along 〈1 1 0〉 instead of 〈1 0 0〉, [Gonzales F, Rappaz M. Metall Trans A 2006; 37: 2797. [1]] no twinned dendrites could be formed. In terms of both the growth kinetics advantage of twinned dendrites over regular ones and the associated tip shape, some experimental evidence seems to contradict the doublon conjecture suggested by Henry [Henry S. PhD thesis, Ecole Polytechnique Fédéral de Lausanne, 1999. [21]], at least for the solute compositions studied in the present work.     

铝锂合金焊缝凝固组织特征

８０９０铝锂合金焊缝中存在的一种细小的等轴光滑晶，其形态和形成机制与等轴树枝晶不同，细等轴晶沿凝固停顿线生长，主要在熔合线附近形成，在焊缝内部呈带状分布。研究表明细等轴晶的形成要求特殊的化学成分（含Ｚｒ，Ｌｉ元素）和一定的凝固条件（小的成分过冷度），在细晶区由于晶界含较多的第二相，导致穿区该区的破坏呈现沿晶特征。     

Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al–Zn–Cr alloys: Can icosahedral solid clusters play a role?

The equiaxed solidification of Al–20 wt.% Zn alloys revealed an unexpectedly large number of fine grains which are in a twin, or near-twin, relationship with their nearest neighbors when minute amounts of Cr (1000 ppm) are added to the melt. Several occurrences of neighboring grains sharing a nearly common 〈1 1 0〉 direction with a fivefold symmetry multi-twinning relationship have been found. These findings are a very strong indication that the primary face-centered cubic Al phase forms on either icosahedron quasicrystals or nuclei of the parent stable Al₄₅Cr₇ phase, which exhibits several fivefold symmetry building blocks in its large monoclinic unit cell. They are further supported by thermodynamic calculations and by grains sometimes exhibiting orientations compatible with the so-called interlocked icosahedron. These results are important, not only because they provide an explanation of the nucleation of twinned dendrites in Al alloys, a topic that has remained unclear over the past 60 years despite several recent investigations, but also because they identify a so far neglected nucleation mechanism in aluminum alloys, which could also apply to other metallic systems.     

Formation of Face-Centered Cubic Phase in Ti35 Alloy Under In Situ Heating Transmission Electron Microscopy

A thermally induced hexagonal close-packed (HCP) to face-centered cubic (FCC) phase transition was investigated in an α-type Ti35 alloy with twinned structure by in situ heating transmission electron microscopy (TEM) and ab initio calculations. TEM observations indicated that the HCP to FCC phase transition occurred both within matrix/twin and at the twin boundaries in the thinner region of the TEM film, and the FCC-Ti precipitated as plates within the matrix/twin, while as equiaxed cells at twin boundaries. The crystallographic orientation relationship between HCP-Ti and FCC-Ti can be described as: $\left\{ {111} \right\}_{{{\text{FCC}}}} //\left\{ {0002} \right\}_{{{\text{HCP}}}} \;{\text{and}}\; < 110 >\,_{{{\text{FCC}}}} //\, <1\overline{2} 10>\,_{{{\text{HCP}}}}$. The HCP to FCC phase transition was accomplished by forming an intermediate state with a BB stacking sequence through the slip of partial dislocations. The formation of such FCC-Ti may be related to the thermal stress and temperature. Ab initio calculations showed that the formation of FCC-Ti may also be related to the contamination of interstitial atoms such as oxygen.     

{112} 〈111〉 Twins or Twinned Variants Induced by Martensitic Transformation?

Twinned substructure in lath martensite was induced in the interstitial free (IF) steel via a high pressure thermal cycle (heating up to 1100 ℃ and holding for 30 min, cooling at 10 ℃/s to room temperature under a pressure of 4 GPa). Experimental observations and theoretical simulation confirm that the twinned substructure has the origin related to the twinned variants rather than the bcc {112} 〈111〉 twins, while extra diffraction spots were caused by crystal overlapping rather than any extra phase. The differences in crystallography and electron diffraction behavior between twinned variants and {112} 〈111〉 twins were discussed in detail.     

Tailoring the Texture and Mechanical Properties of Textured-AZ31 Mg Alloy Sheets by Twinning and Recrystallization

In this study, a method of corrugated width limit alignment processes (CWLA) was applied to pre-twin in hot-rolled Mg alloy sheets. The microstructure evolution and mechanical properties of the CWLA sheets were studied. The results demonstrated that the grain orientation was transformed from strong basal texture to RD-rotated texture after CWLA treatment and up to 22.2% of {10-12} tensile twin boundaries were produced in the microstructure. The 60 ± 5° < 10-10 > twin variant pairs were dominant in the twinned microstructure. Two kinds of dynamic recrystallization (DRX) mechanism, i.e., continuous DRX (CDRX) at low-temperature CWLA and the discontinuous DRX (DDRX) at high-temperature condition were observed. After post-annealing, the twin-induced static recrystallization (SRX) and abnormal grain growth were observed in the CWLA samples, and most twin boundaries were retained. After CAWL for 60 min at 300 °C and post annealing, an improvement on plasticity and strength were achieved with 55% increment for uniform elongation (UE) and 10% increment for ultimate tensile strength (UTS).     

Solidification of high-refractory ruthenium-containing superalloys

The effect of alloy chemistry on single crystal solidification has been investigated in a series of model high refractory Ni-base superalloys with variations in ruthenium, rhenium and aluminum. Over the range of composition investigated, the initial phase to form during solidification was either the γ phase (FCC Ni-solid solution) or the δ phase (HCP Re-rich phase). In a quaternary Ni-Al-Ta-Re alloy containing 1.7 at.% (5.6 wt.%) Re, δ-Re nucleated at a temperature well above the liquidus temperature of pure Ni and grew unconstrained into six-fold Re-rich dendrites. These dendrites served as potent nucleation sites for γ grains as temperature decreased during directional solidification. Ruthenium additions in the range of 2.5∽9.0 at.% (4.1∽14.1 wt.%) lowered the temperature at which the δ nucleated and eventually suppressed the formation of this phase during solidification. Ru additions also increased the liquidus temperatures of the multicomponent superalloys. The implications for the design of Ru-containing superalloys are discussed.     

In situ TEM study of deformation twinning in Ni–Mn–Ga non-modulated martensite

The straining of non-modulated (NM) Ni–Mn–Ga martensite was studied by in situ transmission electron microscopy (TEM). Initially, the self-accommodated NM martensitic structure consists of internally twinned domains. During straining, the detwinning process starts within these domains. The internal twin variant more favorably oriented to the stress grows at the expense of the other one. In the detwinned, single-variant domain, a new twin variant can form, gradually replacing the existing variant via the twinning process. Both processes—detwinning and new twinning—proceed by the same mechanism, namely by the movement of twinning dislocations along the twin boundary. Lattice dislocations are also created in the detwinning process. While the boundaries between the internal twins are coherent and mobile, the boundaries between the internally twinned domains are incoherent, strained and not mobile. The planes of the coherent twin boundary are {2 0 2) planes and the Burgers vectors of the twinning dislocations are parallel to the 〈1 0 1] direction. The magnitude of the Burgers vector determined from the TEM observations disagrees with the calculation from the lattice constant measurement by X-ray diffraction. Possible reasons for this discrepancy are discussed.     

In situ observation of hot tearing formation in succinonitrile-acetone

Hot tears have been induced during the solidification of a succinonitrile-acetone alloy by pulling the columnar dendrites in the transverse direction with a pulling stick. The opening of the mushy zone (hot tears) always occurred at grain boundaries. At low volume fraction of solid, the opening can be compensated by leaner-solute interdendritic liquid (i.e., “healed” hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores induced by solidification shrinkage. Their surface (edge) is made of secondary dendrite arms, which have not yet bridged, but a few spikes have also been observed. These later spikes formed either by the necking of solid bridges established across the grain boundaries prior to pulling, or by the sudden break-up of the liquid film during pulling. Similar spikes have been found by SEM on the hot tear surface of an aluminium–copper alloy.     

Formation mechanism of laser-clad gradient thermal barrier coatings

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Mechanism of competitive grain growth in directional solidification of a nickel-base superalloy

The structure evolution of bicrystal (BC) samples during directional solidification (DS) was explored in an attempt to understand the mechanism of competitive grain growth. It was found that in the case of diverging dendrites the favorably oriented grain overgrows the misaligned grain. However, in the case of converging dendrites the result differs from the prediction of the generally accepted model for competitive grain growth. First, the unfavorably oriented dendrites are able to overgrow the favorably oriented dendrites. Second, the misaligned grain overgrows the favorably oriented grain by blocking the dendrites of the favorably oriented grain at the grain boundary. Based on the experimental results, the process by which a favored 〈0 0 1〉 texture is developed during DS process of a nickel-base superalloy is illustrated.     

Al,Ti和Nb对Fe—Ni—Cr—Co—Nb—Ti—Al合金铸态组织的影响

对低Ａｌ,高Ｔｉ,Ｎｂ（合金Ａ）和高Ａｌ、低Ｔｉ,Ｎｂ（合金Ｂ）两种成分特征的Ｆｅ－Ｎｉ－Ｃｒ－Ｃｏ－Ｎｂ－Ｔｉ－Ａｌ合金宏观和微观铸态组织的观察分析表明,合金Ａ凝固速度较快,柱状晶区较大;两者枝晶间及铸态晶界上都分布着较多块（厚片）状富含Ｎｂ和Ｔｉ的ＭＣ和Ｌａｖｅｓ相多晶体;合金Ｂ柱状晶区晶界Ｌａｖｅｓ相较多;合金Ａ晶界ＭＣ和Ｌａｖｅｓ相附近析 体状γ′和η相及胞状η相;合金ＢＫγ′的尺寸仅约为     

Twinning and detwinning of 〈0 1 1〉 type II twin in shape memory alloy

〈0 1 1〉 type II twin is a major twinning mode frequently observed in NiTi shape memory alloy. Its response to applied stresses is responsible for various inelastic deformation processes. In order to predict the shape recovery characteristic under various conditions, it is of primary importance to predict the martensite detwinning characteristic prior to the reverse transformation. However, due to its irrational nature, the type II twin plane is not as easily comprehensible as that of other types of twins and a strong disagreement exists over the nature of the twin boundary. In the present research, the 〈0 1 1〉 type II twin boundary before and after deformation was investigated. A complete model of 〈0 1 1〉 type II twin plane and its role in the detwinning process are proposed which can be used to account for the various experimental observations.     

Phase field modeling of dendritic coarsening during isothermal solidification

Dendritic coarsening in Al-2mol%Si alloy during isothermal solidification at 880K was investigated by phase field modeling. Three coarsening mechanisms operate in the alloy: (a) melting of small dendrite arms; (b) coalescence of dendrites near the tips leading to the entrapment of liquid droplets; (c) smoothing of dendrites. Dendrite melting is found to be dominant in the stage of dendritic growth, whereas coalescence of dendrites and smoothing of dendrites are dominant during isothermal holding. The simulated results provide a better understanding of dendrite coarsening during isothermal solidification.     

Phase-field simulation of formation of cellular dendrites and fine cellular structures at high growth velocities during directional solidification of Ti56Al44 alloy

A phase-field model whose free energy of the solidification system derived from the Calphad thermodynamic modeling of phase diagram was used to simulate formation of cellular dendrites and fine cellular structures of Ti56Al44 alloy during directional solidification at high growth velocities. The liquid-solid phase transition of L→β was chosen. The dynamics of breakdown of initially planar interfaces into cellular dendrites and fine cellular structures were shown firstly at two growth velocities. Then the unidirectional free growths of two initial nucleations evolving to fine cellular dendrites were investigated. The tip splitting phenomenon is observed and the negative temperature gradient in the liquid represents its supercooling directional solidification. The simulation results show the realistic evolution of interfaces and microstructures and they agree with experimental one.     

Mechanical deformation of dendrites by fluid flow during the solidification of undercooled melts

Mechanical interactions between growing dendrites and their parent melt are normally considered to be of little significance. During conventional solidification processing this is undoubtedly true. However, during the solidification of undercooled melts the twin conditions required to produce mechanical damage to dendrites, high flow velocities and very fine dendrites, may exist. This is most likely in strongly partitioning alloy systems where the tip radius experiences a local minimum at undercoolings in the range of 50–100 K. In this paper we present a model for the skin stress resulting from fluid flow around a family of realistically shaped dendrites. We find that within a narrow undercooling range about the minimum in the tip radius, mechanical deformation of the growing dendrite is likely. Experimental evidence is presented from the Cu–3wt%Sn and Cu–O alloy systems that appear to show evidence of extensively deformed dendritic structures consistent with mechanical damage. Other mechanisms for causing dendritic bending during growth are considered and shown to be unlikely in this case.     

Evolution of Annealing Twins and Recrystallization Texture in Thin-Walled Copper Tube During Heat Treatment

Thin-walled copper tubes are usually produced by multi-pass float-plug drawing deformation. In general, the annealing treatment subsequently is necessary to release the stored energy and adjusts the microstructure. In this study, an investigation on the evolution of annealing twins as well as textures in the thin-walled (Ф6 mm×0.3 mm) copper tube underwent holding time-free heat treatment was reported. Electron backscattered diffraction analysis reveals that a large number of Σ3 boundaries (60° 〈111〉 twin relationship) are produced at the early stage of heat treatment, which is due to the lower boundary energy. With the recrystallization proceeding, the migration rate of grain boundaries decreases on account of the grain growth; meanwhile, the unique Σ9 boundaries (38.9° 〈110〉 relationship) are formed due to the interaction of the Σ3 boundaries. As a result, the number fractions of Σ3 boundaries and high-angle grain boundaries decrease rapidly. During the grain growth stage, a strong recrystallization texture was formed due to the fact that the grains of Goss orientation have a growth advantage over the others. As a result, the initial copper texture was transferred into the Goss texture in domination.