Hanlon Alaina B, Matson Douglas M, Hyers Robert W
Department of Mechanical and Industrial Engineering, Engineering Lab, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Ann N Y Acad Sci. 2006 Sep;1077:33-48. doi: 10.1196/annals.1362.049.
A new hypothesis has been developed to explain the effect of internal fluid flow on the lifetime of a metastable phase in solidifying Fe-Cr-Ni alloys. The hypothesis shows excellent agreement with available experimental results, but microgravity experiments are required for complete validation. Certain Fe-Cr-Ni stainless steel alloys solidify from an undercooled melt by a two-step process in which the metastable ferrite phase forms first followed by the stable austenite phase. Recent experiments using containerless processing techniques have shown that the lifetime of the metastable phase is strongly influenced by flow within the molten sample. Simulations using a commercial computational fluid dynamics (CFD) package, FIDAP, were performed to determine the time required for collision of dendrites and compared to experimental delay time. If the convective velocities are strong enough to bend the primary arms, then the secondary arms of adjacent dendrites can touch. The points of collision form low-angle boundaries and result in high-energy sites that can serve as nuclei for the transformation to the stable phase. It has been determined that the convective velocities in electrostatic levitation (ESL) are not strong enough to cause collision. However, in ground-based electromagnetic levitation (EML), the convective velocities are strong enough to cause the dendrites to deflect so that the secondary arms of adjacent dendrites collide. There is quantitative agreement between the numerically determined time to collision and the experimentally observed delay time in EML. The strong internal velocity due to convection within the EML samples is the reason for the observed difference in delay times between ESL and EML. Microgravity testing is essential because the significant change in nucleation behavior occurs between the ranges accessible by ground-based ESL and EML. Testing in microgravity using EML will permit a large range of internal convective velocities including those that are inaccessible in 1 g.
已提出一种新假说，用于解释内部流体流动对凝固态Fe-Cr-Ni合金中亚稳相寿命的影响。该假说与现有实验结果吻合良好，但需要进行微重力实验才能完全验证。某些Fe-Cr-Ni不锈钢合金从过冷熔体中通过两步过程凝固，其中先形成亚稳铁素体相，随后形成稳定的奥氏体相。最近使用无容器加工技术进行的实验表明，亚稳相的寿命受到熔融样品内流动的强烈影响。使用商业计算流体动力学（CFD）软件包FIDAP进行模拟，以确定枝晶碰撞所需的时间，并与实验延迟时间进行比较。如果对流速度足够强，能使一次枝晶臂弯曲，那么相邻枝晶的二次枝晶臂就可以接触。碰撞点形成低角度边界，并产生高能位点，这些位点可作为向稳定相转变的核心。已确定静电悬浮（ESL）中的对流速度不足以引起碰撞。然而，在地面电磁悬浮（EML）中，对流速度足够强，能使枝晶发生偏转，从而使相邻枝晶的二次枝晶臂发生碰撞。在数值确定的碰撞时间与EML中实验观察到的延迟时间之间存在定量一致性。EML样品内由于对流而产生的强烈内部速度是ESL和EML之间观察到的延迟时间差异的原因。微重力测试至关重要，因为在地面ESL和EML可达到的范围之间，成核行为发生了显著变化。使用EML在微重力条件下进行测试将允许获得大范围的内部对流速度，包括在1g重力条件下无法获得的速度。
