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γ' precipitate shape evolution and splitting in Ni-base alloys

机译：γ'沉淀形状逸出和分裂在Ni碱基合金中

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The shapes of γ'-type (Ni_3X, X = Al, Ga, Ge, Si, Ti) precipitates in aged Ni-X alloys change as their sizes increase. In this paper we consider two types of shape transformations. One is the transformation from the nearly perfect cube shape to the concave cuboidal shape. The other is the splitting transition into either pairs of parallel plates (doublets) or groups of 8 cuboids (octets). From published observations in the literature it is evident that doublets and octets are seen in Ni-base alloys that are continuously cooled from the solution-treatment temperature to just below the solvus, followed by long term aging. There are no reports in the literature of the formation of doublets or octets in alloys that were first solution-treated and quenched prior to aging, which implies that the splitting transitions that putatively occur during continuous cooling never occur during isothermal aging. Neither have doublets and octets ever been observed in any of our solution treated, quenched and aged binary alloys. However, we do find concave cuboidal γ' particles in all 5 binary alloys. This morphology evolves during coarsening when the volume fraction is small (<0.04). The concave cuboidal and splitting shape transformations are discussed in light of the predictions of theoretical calculations and computer modeling experiments. These theoretical efforts indicate that the concave-cuboidal shape cannot be an equilibrium shape, and that either a symmetry-breaking transition to a plate shape or splitting into doublets or octets is energetically favorable. The experimentally measured sizes of the nearly 4-fold symmetric, concave-cuboidal γ' precipitates in our alloys far exceed the theoretically predicted sizes for the transformation to the more energetically stable plate-shape, or for fissioning into doublets or octets. However, some of the theories correctly predict the size range at which nearly perfect cubes of Ni_2Al, Ni_3Ti and Ni_3Si particles transform to the concave cuboidal shape. We show that the morphological changes of the γ' precipitates investigated, as well as other aspects of their coarsening behavior, do not correlate simply with the magnitude of the lattice mismatch, ε. Instead, these tendencies correlate better, though by no means perfectly, with a length scale parameter, l_o = σ/ε~2μ-bar, where σ is the γ'/matrix interfacial free energy and μ-bar is an elastic constant.
机译：随着尺寸的增加，γ'型（Ni_3x，X = Al，Ga，Ge，Si，Ti）沉淀的γ'型（Ni_3x，X = Al，Ga，Ge，Si，Ti）沉淀出来。在本文中，我们考虑了两种类型的形状转换。一个是从几乎完美的立方体形状转变为凹形立方体形状。另一个是将切割过渡到两对平行板（双重）或8间立方体（八位字节）中的转变。从文献中发表的观察结果明显看出，在Ni-碱基合金中，从溶液处理温度达到溶解的低于溶解的Ni-碱基合金中看到双峰和八位字节，然后是长期老化。在老化之前首先在致老化之前形成双峰或八位字节的文献中没有报告，这意味着在等温老化期间，在连续冷却期间，分裂过渡即可在连续冷却期间发生。在任何溶液处理，淬火和老化的二元合金中，既没有观察到双重和八位术。然而，我们确实在所有5个二元合金中找到凹形的立方体γ'颗粒。当体积分数小（<0.04）时，这种形态在粗化时演变。鉴于理论计算和计算机建模实验的预测，讨论了凹形立方体和分裂形状变换。这些理论努力表明，凹形 - 立方体形状不能是平衡形状，并且将对称性转变为板状或分成双峰或八位字节的对称转换是能量有利的。实验测量的近4倍对称的凹形 - 立方体γ'沉淀物在我们的合金中的沉淀物远远超过理论上预测的尺寸，以使变换更高的板状，或用于裂变成双重或八位字节。然而，一些理论正确预测了NI_2AL，NI_3TI和NI_3SI颗粒的几乎完美立方体的尺寸范围，颗粒变换为凹形立方体形状。我们表明，研究了γ'沉淀物的形态变化，以及其粗化行为的其他方面，与晶格错配，ε的大小不相关。相反，这些倾向更好地相关，但绝不是完美的，具有长度比例参数，L_O =Σ/ε〜2μ杆，其中σ是γ'/矩阵界面自由能，μ杆是弹性恒定。

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