抽拉速率对Ti-48Al-6Nb合金籽晶法定向凝固组织的影响

以Ti-44Al-3Nb-2Si为籽晶,研究了不同抽拉速率对Ti-48Al-6Nb合金籽晶法定向全片层组织的影响。结果表明,6μm/s抽拉速率下,固/液界面为平界面生长,凝固组织中片层方向能够与生长方向保持一致。当抽拉速率增加到15μm/s时,固/液界面以枝晶方式生长,凝固组织中出现非择优<112軈0>取向的α相与择优取向β相共生生长组织。当抽拉速率增加到25μm/s时,α相树枝晶生长将在枝晶间析出β相,破坏定向全片层组织。用Ti-44Al-3Nb-2Si籽晶,低抽拉速率保证高温α相以平界面生长,可以实现Ti-48Al-6Nb合金的引晶。     

Ti-47Al合金籽晶法定向凝固过程中的组织演化

以Ti-43Al-3Si为籽晶,对Ti-47Al合金籽晶法定向凝固过程中的相选择和组织演化规律进行了研究.凝固初期,液相中富含Si,促使α相为其初生相,获得了与生长方向平行的全片层组织.随着凝固的进行,液相成分逐渐接近Ti-47Al.生长速率为90 mm/h时,凝固组织以胞状树枝晶形态生长,由于晶体生长形态对界面前沿溶质分布的影响,在稳定的溶质边界层建立前,Al和Si含量不能保证α相的连续生长,β相在α枝晶间形核长大,出现与凝固方向成45°夹角的片层,破坏了片层取向的一致;生长速率增至720 mm/h时,凝固组织中枝晶生长发达,枝晶间距减小,基本转变为以择优取向生长的α相.     

TiAl合金定向全片层组织的籽晶法制备

以Ti-43Al-3Si为籽晶,在Ti-47Al合金中成功的制备出了与生长方向平行的定向全片层组织.结合Ti-Al二元相图,分析了亚包晶和过包晶成分合金的凝固路径,确定了籽晶法控制TiAl合金片层方向对母料合金成分的要求.通过对不同生长速率下Ti-47Al合金籽晶法定向凝固组织的观察发现,成功的引晶生长应保证高温α相沿非择优取向<1120>以平界面生长,即采用足够高的温度梯度和足够低的生长速率. Α相胞状树枝晶生长过程中将在枝晶间析出β相,破坏定向全片层组织.     

生长速度对Ti-45%Al包晶合金定向凝固组织的影响

通过电磁感应加热和液态金属冷却相结合的实验方法制备了Ti-45%Al(at%)包晶合金的定向凝固试样,并观察在不同生长速度下的微观组织和界面形态。结果发现,抽拉速度为10μm/s时,界面以胞晶形式生长,最终组织的α2 γ片层与生长方向垂直;抽拉速度为50μm/s时,观察到明显的枝晶生长界面,α2 γ片层与生长方向成45°夹角。低速下的胞状间距明显大于高速下的枝晶间距,说明快速凝固有利于减小枝晶间距。     

Pb-Bi包晶合金定向凝固微观组织演化

对Pb-(26,28,30,34)Bi(质量分数,%,下同)包晶合金进行平界面生长的低速定向凝固到枝晶状生长的高速定向凝固实验,研究了Pb-Bi包晶合金的微观组织形成及其演化。实验结果表明,在温度梯度G=30K/mm条件下,当凝固速度V=0.25μm/s时,初生α相和包晶β相均以平界面生长,凝固组织的演化过程为:单相初生α相→两相竞争组织→β单相。V=0.5μm/s时,定向凝固组织的演化过程为:单相初生α相→胞状α相+胞间包晶β相→α+β两相竞争组织→β单相。在G=20K/mm条件下,当凝固速度V=1μm/s时,初生α相以胞状领先生长,包晶β相则在胞状α间形核生长,并包裹住α胞。当凝固速度增加至V≥2μm/s时,初生α相由胞状转变为枝晶状,包晶β相则在枝晶间包围α枝晶。     

生长速度对定向凝固Ti-50Al-6Nb合金微观组织演变及其硬度的影响

对Ti-50Al-6Nb(at%)合金进行不同生长速度下的定向凝固实验,研究了生长速度对微观组织演变、相变路径、微观偏析和硬度值的影响。结果表明,在较低的生长速度下(≤10μm/s),凝固过程为完全β相凝固,固液界面随生长速度的增加发生平胞枝转变,最终得到α2/γ层片组织(在10μm/s时为α2/γ+B2相的组织),其中层片结构与生长方向呈45°或0°夹角。在较高速度(≥15μm/s)时,初生相为α相,固液界面为规则枝晶生长。最终组织为α2/γ层片组织,并与生长方向呈90°夹角。该成分合金的硬度值随着生长速度的增加而增加。与其它合金相比,该成分合金的硬度值较小。     

抽拉速率对Ti-47.5Al-6Nb-2Cr-0.5Si合金定向凝固组织的影响

采用感应加热定向凝固炉对Ti-47.5Al-6Nb-2Cr-0.5Si(at%)合金在抽拉速率6、15、25、100μm/s下进行定向凝固实验。结果表明:抽拉速率为6μm/s时,固/液界面是胞枝状,片层取向与生长方向夹角为45°,初生相为β相;抽拉速率为15~100μm/s时,固/液界面以树枝状向前推进,且随着抽拉速率的增加,一次枝晶间距减小,片层取向与生长方向的夹角由45°向90°转变,初生相由β相转变为α相;抽拉速率大于6μm/s时,组织中出现硅化物析出相,且随着抽拉速率的增大,析出相越来越多,分布变密,沿生长方向逐渐成线条状。     

抽拉速率对定向凝固Ni-45Ti-5Al合金微观组织的影响

采用Bridgman型液态金属冷却定向凝固方法,研究Ni-45Ti-5Al(摩尔分数,%)合金在不同抽拉速率(20、100和200μm/s)下定向凝固后的相组成及其形态特征。结果表明:Ni-45Ti-5Al合金定向凝固生长区呈现明显的柱状晶生长形态,定向效果良好,NiTi基体以[100]方向为择优取向,Ti2Ni析出相沿[111]晶向择优生长。随着抽拉速率的提高,Ti2Ni相更加细小、分散,由在胞晶界上几乎连续分布改变为断续分布。在20～200μm/s的宽生长速率范围内,均以胞状晶形态生长,固/液界面形态没有发生显著变化;随着抽拉速率从20μm/s增加到200μm/s,定向胞晶组织明显细化,平均胞晶间距由85μm减小到25μm。     

Mg-14.61Gd合金的定向凝固组织及生长取向

采用电子背散射衍射(EBSD)和元胞自动机有限元(CAFE)方法研究了Mg-14.61Gd合金在温度梯度G=30 K/mm和抽拉速率v=10～200μm/s条件下的定向凝固组织和生长取向。研究发现,Mg-14.61Gd合金纵向凝固组织呈单一方向的α-Mg枝晶生长形貌,随着v的增加,枝晶界面生长方式由凸前生长向平齐生长转变,枝晶间距减小。当v从10μm/s增至100μm/s时,α-Mg枝晶的生长取向由1120和1010转变为1120,其与凝固热流的偏离角(θ)由11.0°减小至5.7°,热流是影响生长取向的主导因素;当v从100μm/s增至200μm/s时,α-Mg枝晶的生长取向仍为1120,但θ却逐渐增大至10.6°,此时,晶体的各向异性占主导。研究表明,CAFE模型可以合理预测定向凝固镁合金的晶粒组织和生长取向。     

抽拉速率对Ti-47Al-3Nb-0.3Si定向凝固组织的影响

采用电阻加热定向凝固炉对Ti-47Al-3Nb-0.3Si合金在抽拉速率为5、15、50、100 μm/s下进行定向凝固试验,并观察和分析不同抽拉速率下的固液界面形貌、过渡区和定向生长区凝固组织.结果表明,抽拉速率为5 μm/s时,固/液界面以胞状界面向前推进,抽拉速率在15～100 μm/s时,固/液界面以树枝状向前推进,且随着抽拉速率的增大,一次枝晶间距减小.当抽拉速率为5～50 μm/s时,片层组织与生长方向夹角为45°,当抽拉速率增加到100μm/s时,片层组织与生长方向基本垂直.利用扫描电镜背散射观察定向生长区形貌发现,抽拉速率等于或大于15 μm/s时,在柱状晶内部或晶界处形成胞状Al偏析,且出现共晶γ相及Ti5(Si,Al)3相.     

High temperature phase equilibria near Ti–50 at% Al composition in Ti–Al system studied by directional solidification

High temperature phase diagram near the stoichiometric composition of TiAl has been established by the directional solidification and quenching technique. The quenched dendrite morphologies showed that the first solidified phase was the β phase in Ti–44, 46, 48 at% Al alloys and the α phase in Ti–50, 52 at% Al alloys. From the EDS analysis of the quenched dendrite tips and measurement of the temperature gradient directly recorded during directional solidification in Ti–(44–52 at%)Al alloys, the solid-liquid phase equilibria could be determined. The phase transformation temperatures were also confirmed by DTA. The phase equilibria established in this study agreed with the phase diagram that Okamoto proposed, while the β+γ two phase region, which Murray suggested, was not found near the TiAl composition. The lamellar orientation in TiAl alloys has been reported to be controlled in the growth direction in the presence of the primary β phase from a liquid. A composition in which the liquid phase was fully transformed to the β phase was selected, Ti–44 at% Al alloy, and directional solidification was performed at the growth rate of 45 mm/h. It was found that the lamellar orientation was aligned at nearly 0° and 45° to the growth direction. It is thought that the success in controlling the lamellar orientation is due to β solidification of the Ti–44 at% Al alloy at the beginning of liquid/solid transformation.     

定向凝固Ti-46Al-8Nb合金的微观组织演变与微观偏析(英文)

在3～70μm/s的生长速度范围内对Ti-46Al-8Nb(摩尔分数,%)合金进行定向凝固实验,研究其微观组织演变和微观偏析形式。在该生长速度范围内,固-液界面表现为规则的枝晶生长,一次枝晶间距随着生长速度的加快而逐渐减小。在定向凝固过程中观察到典型的L+β→α包晶反应,最终得到具有α2/γ层片和B2相的微观组织。在各个晶粒中,层片与β相枝晶初始生长方向呈0°或45°。包晶反应导致严重的成分偏析,在凝固过程中,铝富集在枝晶间,铌富集在枝晶心部。随着α相的形核和生长,铌的偏析程度逐渐增大,从而促进B2相的析出,而富集在枝晶间的铝在包晶反应发生后逐渐变得均匀、一致。     

定向凝固Ti-45Al-7Nb合金的组织特征

研究了液态金属冷却定向凝固条件下Ti-45Al-7Nb(at%)合金的组织特征。结果表明:定向凝固过程中合金的凝固路径及稳态区片层取向均受到抽拉速率的影响。当抽拉速率为0.36mm/min时,合金的初生相为α相,与生长方向垂直的片层组织生长占优;抽拉速率介于1～10mm/min时,合金初生相变为β相,凝固过程中会经历β单相区,片层取向多为与生长方向成0°和45°;当抽拉速率增加至20mm/min时,合金初生相仍为β相,随后发生L+β→α包晶反应,与抽拉方向之间的夹角介于45°～75°之间的片层组织生长占优。     

Microstructure selection in the interdendritic region during directional solidification of a Ni-23at.%Al alloy

Directional solidification experiments of a binary Ni-23at.%Al alloy were carried out to examine the effects of growth velocity on the microstructure selection in the interdendritic region. Only the growth velocity was changed from 5 μm/s to 60 μm/s under a given thermal gradient. As a result, the noticeable change in the microstructure during solidification occurred between the γ dendrites. The γ interdendritic microstructure was varied as a function of growth velocity from rod γ-γ′ coupled peritectic structure to planar γ′ structure and then to eutectic structures consisting of stable γ′-β eutectic and metastable γ-β, eutectic structures. The microstructure selected preferentially among the γ dendrites was considered by calculating the interface temperature of a phase growing into its parent melt. It is shown that the microstructure selection in the γ interdendritic region is determined by a phase or a structure kinetically leading at the highest interface temperature under a given growth condition.     

Lamellar orientation control of Ti–47Al–0.5W–0.5Si by directional solidification using β seeding technique

In this paper, we report a β seeding technique for the lamellar orientation controlling in TiAl alloy, which is a novelty and effective method for aligning the lamellar orientation of Ti–47Al–0.5W–0.5Si with primary α phase. The shorter composition transition zone and simpler process procedure can improve the deficiency of α seeding technique. The proper temperature gradient and normal growth rate are necessary for aligning the lamellar orientation in TiAl alloy with primary α phase using β seeding technique.     

Microstructure evolution in directionally solidified Ti–(50, 52) at%Al alloys

Ti–(50, 52) at%Al alloys were directionally solidified over a wide range of growth velocities. Detailed microstructure evolution was investigated. For Ti–50Al alloys, growths of primary α phase dominated. Only a little peritectic γ phase was found in intercellular or interdendritic regions. For Ti–52Al alloys, corporative growths of both phases were observed. Primary α transformed from cells to dendrites with increase of growth velocities, and γ showed a faceted-growth pattern. The peritectic reaction became dominated when α grew as dendrites at a relatively high velocity. The lamellar orientation evolution was also investigated, which was history-dependant on that in non-melted as-cast crystals.     

TiAl合金全片层组织的热稳定性研究

研究了Al含量、冷却速率和添加硼元素对TiAl合金全片层组织在1150℃的热稳定性的影响。研究表明：Al含量在46％～48％（原子分数,下同）范围的二元TiAl合金的Al含量越高,γ偏析程度越严重,铸造片层组织的热稳定性越差;Ti-48AI合金α单相区固溶处理后炉冷的粗片层组织的稳定性远远优于空冷的细片层组织,空冷细片层组织容易在晶界处发生不连续粗化转变,并且空冷片层晶粒内的魏氏片层（LW）与基体的界面往往与晶界一同成为片层组织发生分解的起始部位;Ti-48A1合金中添加0．8％B因晶界TiB2相的存在能有效抑制细片层组织的晶界不连续粗化,但γ相从TiB2／基体界面和晶界重新形核生长可使片层组织转变为均匀的细晶近γ组织。     

非平衡凝固条件下Zn-5Al合金的组织形成

对Zn-5Al合金采用液淬凝固和定量金相统计试验,考察了合金在非平衡冷却凝固过程中组织的形成和转变。研究表明:合金在非平衡凝固下析出初生富铝相α,它随着冷却速度的提高是逐渐减少的;初生相逐渐从胞状向树枝状转变。Zn-5Al合金在非平衡凝固条件下因析出初生相而获得亚共晶组织,冷速较大时共晶组织皆无明显的共晶核心,并且共晶层片组织逐渐变细,有的甚至无法看到,只有在共晶团搭界处才可看出明显的层片,共晶团内部的共晶层片基本呈无序状态。通过对本试验结果探讨,定性得到了锌铝合金共晶共生区相图,为以后研究Zn-5Al合金在非平衡凝固条件下凝固结晶提供了一些参考。     

生长速度与试样直径对定向凝固Ti-46Al-8Nb合金显微组织和硬度的影响

对Ti-46Al-8Nb（摩尔分数,%）合金进行布里奇曼型定向凝固实验,考察生长速度和试样直径对合金显微组织、相变路径和硬度的影响。结果表明,随着生长速度的增加和试样直径的减小,凝固过程由完全β相凝固转变为具有包晶反应的凝固过程,其最终显微组织为α2/γ层片及有α2/γ层片和B2相组成的多相组织。以上结果是由扩散和对流的减弱造成的溶质富集而引起的。包晶反应的发生往往导致严重的溶质偏析,其中溶质Al和Nb的偏析使得层片组织较为粗大。因此,在较高生长速度下发生包晶反应时,合金硬度值急剧下降,硬度曲线随着生长速度的增加呈不连续变化。严重溶质偏析导致的粗大层片组织也使得该成分合金的硬度低于其他Ti Al基合金的硬度。     

Stability of lamellar microstructures in a Ti–48Al–2Nb–2Cr alloy during heat treatment and its application to lamellae alignment as a quasi-seed

The stability of lamellar microstructures in the Ti–48Al–2Nb–2Cr alloy during heat treatment depends more on heating rate and less on holding time and cooling rate. When the heating rate is higher than 61 °C/min, the original lamellae orientation at room temperature can still maintain unchanged even though the lamellae are transformed to single α grains at a temperature close to melting point. Therefore, the high-temperature α grains, which are transformed from the α₂/γ lamellae within a quasi-seed, are successfully employed to align the lamellar microstructure by directional solidification.