扩散条件下抽拉速率对Fe-4.2Ni合金定向凝固组织演化的影响
扩散条件下抽拉速率对Fe-4.2Ni合金定向凝固组织演化的影响
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DOI：
10.3724/sp.j.1037.2009.00684
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发表时间：
2010-03
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相选择和组织演变对包晶反应材料的加工和性能控制具有重要作用。在定向凝固包晶合金中观察到许多有趣的组织，如包晶耦合生长（PCG）、离散带状、岛状带状和振荡树枝状组织。近年来，人们发展了许多关于相选择的理论模型。例如，据信，周晶耦合生长是由岛状条带引发的，并且类似于共晶耦合生长，离散条带是由一个相在另一个相上的成核和横向扩展形成的，并且当一个相在另一个相生长界面上的成核速率高于临界值时形成岛状条带，等等。但这些模型都是在相的生长仅受扩散控制的假设下建立的，而实际的实验研究主要是在有对流存在的大块金属样品中进行的，虽然人们一直认为对流对微观组织演化的影响是非常重要的，不可忽略，但在扩散区的研究却很少。将Fe-4.2Ni亚包埋合金置于内径为1 mm的薄壁管中，在Bridgman炉中进行扩散区定向凝固，实验结果表明，在15和35μm/s的拉速下，定向凝固薄试样获得了不同的组织和相选择，树枝状组织，当拉速为15μm/s时，形成了两相分离生长的稳定凝固组织，其组织演变为：单相δ→树枝状组织→γ和PCG→ δ/γ分离生长;当拉速为35μm/s时，主要组织为γ-IB，其组织演变为：δ→树枝状结构→中心γ-IB和边缘PCG →中心δ和边缘γ-IB.随着提拉速率从15 μm/s增加到35μm/s，形核的γ相与母相δ相竞争生长形成的树枝状组织的形貌发现拉速对各种显微组织的形成和演化起着重要的作用。
Phase selection and microstructure evolution play important roles on processing and property control of the materials with peritectic reactions.Many interesting microstructures,such as peritectic coupled growth(PCG),discrete banding,island banding(IB) and oscillatory tree-like structure were observed in directionally solidified peritectic alloys.In recent years,a number of theoretical models have been developed for the phase selection.For example,it was believed that peritectic coupled growth is initiated by island banding and is similar to eutectic coupled growth, discrete banding is shaped by nucleation and lateral spreading of one phase onto the other phase,and island banding is formed when the nucleation rate of one phase on the other phase growth interface is higher than a critical value,and etc..But these models were developed under the assumption that the growth of the phases are controlled by diffusion only,whereas the actual experimental studies were mainly carried out in bulk metallic samples with the presence of convection.Although it has been believed that the convection effects on microstructure evolution are very important and could not be neglected,studies were seldom carried out in diffusive regime.In this paper, directional solidification of Fe-4.2Ni hypo-peritectic alloy in diffusive regime was carried out in a Bridgman furnace by placing the samples in 1 mm inner diameter thin tubes.The experimental results show that various microstructures and phase selections are obtained in the directionally solidified thin samples at the pulling rates of 15 and 35μm/s.Tree-like structure,peritectic coupled growth and the steadily solidified structure of two phases separated growth are formed at the pulling rate of 15μm/s,and the microstructures evolution is:single phaseδ→tree-like structure→γand PCG→separately growingδ/γ.At the pulling rate of 35μm/s,the main microstructure isγ-IB,and the microstructures evolution is:δ→tree-like structure→γ-IB in the center and PCG at the edge→δin the center andγ-IB at the edge.As the pulling rate increases from 15 to 35μm/s,the morphology of tree-like structure formed by the competitive growth between the nucleatedγ-phase and the parentδ-phase becomes complicated.It is found that the pulling rate plays an important role in both the formation and evolution of various microstructures.