Al-Y包晶合金非平衡凝固过程及组织特征

包晶凝固过程中小平面-小平面两相复相生长方式是凝固领域研究的一个热点。以初生相和包晶相都是严格计量比金属间化合物的Al-Y包晶合金作为研究对象,利用DSC热分析技术,严格控制冷却速度,获得不同凝固条件的非平衡凝固试样,研究了两相为小平面相的Al-Y包晶合金的凝固行为。发现小平面-小平面包晶系合金包晶凝固过程中,非平衡凝固特性及宏观偏析特点比非小平面包晶系更加明显。凝固特征温度与平衡相图偏差明显,包晶反应温度和包晶相直接凝固温度都远高于平衡相图给定的值,相对于相图是在"过热"条件下发生的,而固溶体型包晶合金一般是在"过冷"条件下发生的。包晶转变过程非常微弱,致使初生相残留量远高于平衡相图。即使对于过包晶成分的合金,其凝固组织中仍存在大量的共晶凝固组织,最终得到的凝固组织与平衡相图存在显著差异。     

重力对Sn-20%Ni合金的初生相形态和包晶反应的影响

使用高度为50 m的落管研究了Sn-20%(质量分数) Ni包晶合金在重力和微重力作用下的凝固行为。用金相显微镜(OM)观察了合金的凝固组织并使用图像处理软件IPP(Image Pro Plus)统计了样品中的初生相、包晶相以及终凝相的含量,使用能谱仪(EDS)和X射线衍射(XRD)仪分析了样品凝固组织中的成分分布和组成相。结果表明,Sn-20%Ni包晶合金的凝固以初生相在固液界面前沿形核、枝晶生长和包晶反应的形式进行,重力对初生相的生成和包晶反应都有显著的影响,在微重力作用下的微观组织配比、分布以及合金元素的分布都与在重力作用下明显不同。在重力的作用下残余初生相的含量和残余初生相和包晶相的总量总是比在重力作用下的低,而包晶相的含量则总是比在微重力作用下的高。同时,样品中溶质元素的分布与残余初生相和包晶相的总量的分布趋势基本一致。结果表明,微重力环境有利于Sn-20%Ni合金初生相的形核和长大,而重力环境则促进包晶反应,其原因与重力导致的浮力对流和晶核沉积有关。     

电场对定向凝固类包晶合金凝固组织的影响

为研究直流电流对亚包晶合金凝固过程的影响,选用与亚包晶合金都存在相似凝固过程的AMPD-4.1%SCN透明亚包晶模拟物为研究对象。使用显微镜感光器件(CCD)和智能通讯测温仪表对实验过程进行实时拍照和温度记录,研究了亚包晶透明模拟物在电场作用下的结晶过程和晶体生长规律。结果表明：在电场的作用下,由电迁移效应使定向结晶的亚包晶模拟物的初生β相颗粒逐渐向正极方向迁移,使凝固界面前沿的液相成分与包晶点的成分(0.05%SCN,原子分数)接近,从而促进包晶反应的进行;电场的作用使电流偏聚产生的焦耳热效应和溶质富集引起的成分过冷,使定向凝固的枝晶尖端产生特殊分裂的生长形貌,使枝晶尖端分裂,枝晶间距减小。     

定向凝固包晶合金低速共生生长与带状组织

包晶合金定向凝固过程中的相和组织选择是一个复杂的过程.综述了包晶合金定向凝固中的低速带状与共生生长组织的实验和理论研究情况及其进展.讨论了低速带状与共生生长组织的关系,说明两者并非孤立存在,带状向耦合生长组织的转变可能源于某一临界成分.     

快速凝固Fe-Co合金中的亚稳相

采用熔融玻璃净化和循环过热相结合的方法,研究了Fe-Co合金在深过冷快速凝固中的亚稳相。结果表明,当熔体的过冷度达到某一临界数值(△T_(crit))时,亚稳相(b.c.c)在竞争形核中优先形核生长,并在随后的冷却过程中保留在凝固组织中。在γ单相区内等温退火3 h后,亚稳相完全转变为稳定相;在过冷Fe-Co合金的凝固组织中亚稳相是竞争形核-重熔-γ相外延生长-不完全固态相变的产物。     

二元包晶合金定向凝固研究进展

针对二元包晶合金定向凝固,本文评述了其近年来的理论和实验研究进展,内容包括包晶合金的相选择、两相形核、带状组织以及耦合生长等方面.讨论了定向凝固过程中相和微观组织选择机制,实验中所观察到的带状组织及其形成原因,以及由岛屿带状组织转变成初生相和包晶相的耦合生长组织.     

过冷包晶合金的相选择分析

分析了过冷包晶铁基合金、铜基合金、稀土超导乳化物、稀土木磁化合物非平衡凝固过程中的相选择现象。根据对实验结果的分析，对过冷包晶合金凝固的相选择进行了分类。探讨了过冷包晶合金凝固今后的研究方向和应用前景。     

常规凝固条件下Mg-Zn-Y三元稳定准晶的研究

对常规凝固条件下Mg-Zn-Y系中稳定二十面体准晶相Mg3Zn6Y1的形成进行了研究,分析了按照严格化学计量比配制的大体积单相准晶的组织形态、凝固过程中的相变点、以及准晶在稀溶液中自由生长时的形貌变化.实验发现,按照严格化学计量比配制的准晶合金组织中准晶相占有较大的体积分数,在自由生长的情况下该准晶形貌为花瓣状,此外还有部分拉长的六边形及五边形组织.该准晶是在624.95℃通过包晶反应形成的.     

Nd-Fe-B包晶合金定向凝固组织的研究

利用亚快速定向凝固设备制备Nd-Fe-B合金,并研究了其定向凝固组织.设计合金成分为:亚包晶成分(Nd10.8Fe83.8B5.4)、包晶成分(Nd11.76Fe82.36B5.88)和过包晶成分(Nd13.5Fe79.75B6.75).对抽拉速率在5～500μm/s范围内合金的定向凝固及微观组织的研究表明:三种成分Nd-Fe-B合金的最终凝固组织均由α-Fe枝晶相、包晶Nd2Fe14B相和富Nd相组成;随着抽拉速率的提高,α-Fe相的一次枝晶间距呈减小的趋势;三种成分凝固组织中的α-Fe相的体积分数随着抽拉速率的提高都表现出了先减后增的趋势,而Nd2Fe14B相的体积分数都表现出了先增后减的趋势.     

Zn-8%Al变形锌合金组织演化规律研究

通过对Zn-8%Al合金的熔铸、挤压和拉拔加工,采用金相显微镜、扫描电镜等分析手段,研究了该合金的组织演化和相转变规律。实验结果表明,铸锭凝固初期发生包晶反应L+α→β,初生晶α′边缘包围着一层更富Zn的η相,随着温度的不断下降,进而发生共晶反应L→η+β与共析反应β→η+α。在热挤压过程中,大变形量加工使得枝晶破碎,合金的微观组织得到明显细化,但在横截面上仍可以看到小的枝晶,包晶相β在挤压的过程中已经发生共析转变:β→α+η。由于挤压和拉拔过程中都产生大量的变形热,Zn为层错能高的金属,发生动态再结晶,但由于冷却速度很快,只能发生不完全动态再结晶。Zn在冷加工过程中,出现软化现象,除了可能发生动态回复与动态再结晶外,还有可能会发生η→ηm′+η′→η+α的相变。     

高Nb-TiAl合金的凝固组织与力学性能研究

研究了Al含量变化对高Nb-TiAl合金的凝固组织与力学性能的影响.结果表明:随着Al含量的增加,TiAl合金晶粒尺寸呈增加趋势;当Al含量为45.7%时,凝固过程中局部区域发生包晶转变,使晶粒尺寸显著增大;室温及700℃高温拉伸强度随着Al含量的增加而呈增加的趋势,但发生包晶转变致使室温及700℃高温拉伸强度下降约200MPa;Al含量对延伸率不敏感,持久性能随Al含量的增加呈增加趋势.为控制铸锭凝固后的组织与力学性能,尽量避开包晶转变区,合金中Al含量应低于45.7%.     

Metastable phase diagrams of Cu-based alloy systems with a miscibility gap in undercooled state

Some Cu-based alloy systems with a large positive enthalpy of mixing display a eutectic or peritectic phase diagram under equilibrium conditions, but show a metastable liquid miscibility gap in the undercooled state. When the melt is undercooled below certain temperature beyond the critical liquid-phase separation temperature, it separates into two liquids with different compositions. The compositions of the two liquids change successively upon the metastable phase diagram before solidification occurs. The shape and position of the metastable miscibility gap are dependent of the alloy components and their interaction features. This study reviews the metastable phase diagrams of Cu-based alloy systems, which are derived from experiments and thermodynamic calculations.     

Pr对Sn-Cd包晶合金定向凝固组织的影响

为了解Pr对Sn-Cd包晶合金凝固组织的作用,在不同抽拉速率下对Sn-Cd-Pr合金进行定向凝固实验.采用光学显微镜、扫描电子显微镜(SEM)和能谱分析(EDS)研究了Pr对Sn-Cd包晶合金凝固组织的影响.研究发现:对于Sn-1.8%Cd过包晶合金,在2μm/s时0.5%的Pr增加了包晶相β的体积分数;在4μm/s时凝固组织明显细化.对于Sn-0.65%Cd亚包晶合金,在4μm/s时Pr的加入促进了平→胞界面的转变;在16μm/s时随着Pr含量的增大,凝固组织逐渐细化;在100μm/s时Pr含量的增大使凝固组织仅有微弱的细化.研究表明,Pr的加入可以改变组成相的体积分数、细化组织以及减小界面稳定性.     

强磁场下Zn-2 wt.%Cu合金定向凝固的初步研究

本文进行了10T强磁场下Zn-2wt.%Cu合金的定向凝固的初步研究.结果发现下拉速度较低时,无磁场时晶体以平界面方式生长,而施加磁场则产生带状组织,并且随着磁场的增加带状组织越来越明显,带状组织间距越来越小;当定向凝固速度较高,晶体以枝晶方式生长时,磁场促进枝晶的分枝,并扰乱枝晶规则生长;随定向凝固速度提高,磁场的作用逐渐减弱.     

Comments on ‘Solidification modes and microstructure of Fe–Cr alloys solidified at different undercoolings’

The critical growth velocity for a planar solidification front in undercooled alloy melts is discussed on the basis of the absolute stability theory to reexamine the interpretation of a current analysis of the solidification modes in undercooled bulk Fe–Cr alloys contributed by Xuezhi Zhang. Theoretically, it is possible to produce a planar front in the solidification of undercooled bulk melts. But practically, it is imopssible for the undercooled bulk Fe–Cr melts to produce a planar front in the solidification. The dendritic growth theories as well as the calculations due to Zhang have been analyzed.     

The effect of microstructural evolution on hardening behavior of type 17-4PH stainless steel in long-term aging at 350 °C

The analysed Al₃Ni₂–Al₃Ni multi-layer on a Ni-substrate was solidified in an apparatus for diffusion soldering under vacuum. The δ–ζ multi-layer on the (Γ1 + Fe)-substrate was solidified in an apparatus for hot dip galvanizing.

Isothermal solidification was applied to produce both coatings. The devices constructed allowed the solidification process to be arrested after a predetermined time. The period of time from the beginning of solidification to the applied arrest was introduced into the definition of a back-diffusion parameter, . The back-diffusion parameter was employed to work out a model for solute redistribution. Experimental solute redistribution resulted from undercooled peritectic reactions accompanying the solidification of a multi-layer on a given substrate. The Al-solute and Zn-solute redistributions after the back-diffusion occurring during solidification with the presence of undercooled peritectic reactions were measured to identify the phases appearing. A thermodynamic theorem of the maximum driving force was applied to determine the sequence of phase appearance in contact with the liquid.

The experimental sequence of phases appearing was tested with the thermodynamic predictions. The model for solute redistribution was verified through a fitting of the theoretical solute redistribution profile with experimental data. These data were obtained by means of electron microscopy techniques in both the multi-layers coated onto metallic Ni and (Γ1 + Fe) substrates. The experimental sequences of the peritectic phases appearance in both multi-layers were also confirmed by the model of solute redistribution.     

Metastable phase formation in undercooled Fe-Co melts under terrestrial and parabolic flight conditions

Soft magnetic Fe-Co alloys display primary fcc phase solidification for>19,5 at% Co in conventional near-equilibrium solidification processes. Undercooled Fe-Co melt drops within the composition range of 30 to 50 at% Co have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolution Siphotodiode. Melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterize the solidification sequence and microstructure. The transition from stable fcc phase to metastable bcc primary phase solidification has been observed after reaching a critical undercooling level. The critical undercooling increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. Under microgravity conditions, a much smaller critical undercooling and an increased life time of the metastable bcc phase were obtained. This result was validated with TEM investigations. The appearance of Fe-O particles gives an indirect hint for an intermediate fcc phase formation from the metastable bcc phase at elevated temperature.