液氮冷却条件下激光快速熔凝Ni-28 wt%Sn合金组织演变

研究了在液氮冷却条件下激光快速熔凝Ni-28 wt%Sn亚共晶合金的组织演化过程. 结果显示, 熔池从上至下可以分为三个区域: 表层为平行激光扫描方向的α-Ni转向枝晶区; 中部为近乎垂直于熔池底部外延生长的α-Ni柱状晶区; 底部为少量的残留α-Ni初生相和大量的枝晶间(α-Ni+Ni₃Sn) 共晶组织. 激光熔凝区组织受原始基材组织的影响很大, 熔池中的α-Ni枝晶生长方向受到了热流方向和枝晶择优取向的双重影响. 与基材中存在的层片状、棒状和少量离异(α-Ni+Ni₃Sn)共晶的混合组织相比, 熔池内的共晶组织皆为细小的规则(α-Ni+Ni₃Sn)层片状共晶, 皆垂直于熔池底部外延生长, 并且从熔池顶部至底部, 共晶层片间距逐渐增大. 分别应用描述快速枝晶生长的Kurz-Giovanola-Trivedi 模型和描述快速共晶生长的Trivedi-Magnin-Kurz模型对熔池表层凝固界面前沿的过冷度进行估算, 发现熔池表层α-Ni 枝晶和(α-Ni+Ni₃Sn)层片共晶的生长过冷度在50.4-112.5 K 之间, 远大于相应深过冷凝固(α-Ni+Ni₃Sn) 反常共晶生长的临界过冷度20 K, 这说明文献报道的临界过冷度并不是反常共晶出现的充分条件.     

The effect of two-dimensional shear flow on the stability of a crystal interface in a supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of twodimensional shear flow on the stability of a crystal interface in the supercooled melt of a pure substance.Similar to the three-dimensional shear flow as described in our previous paper,the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude.However,compared with the case of the Laplace equation for a steady-state thermal diffusion field,due to the existence of time partial derivatives of the temperature fields in the diffusion equation the absolute value of the gradients of the temperature fields increases,therefore destabilizing the interface.The circular interface is more unstable than in the case of Laplace equation without time partial derivatives.The critical stability radius of the crystal interface increases with shearing rate increasing.The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

The effect of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt of pure substance. Similar to the three-dimensional shear flow as described in our previous paper, the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude. However, compared with the case of Laplace equation for steady state thermal diffusion field, due to the existence of time partial derivatives of the temperature fields in diffusion equation the absolute value of the gradients of the temperature fields increases, therefore destabilizing the interface. The circular interface is more unstable than in the case of Laplace equation without time partial derivatives. The critical stability radius of the crystal interface increases with shearing rate increasing. The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

Microstructure characteristics of laser forming repaired Ti60 alloy

The microstructure characteristics of laser forming repaired (LFR) Ti60 (Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.3Nb) as-deposited and annealed samples are analyzed. The microstructure of as-deposited repaired zone (RZ) consists of epitaxial columnar prior β grains, in which fine woven α laths and β-phase between α laths exist. The heat-affected zone (HAZ) experiences a continuous microstructural transition from duplex microstructure of the base metal zone (BMZ) to the microstructure of RZ. The presence of silicide precipitates is observed in both RZ and BMZ in an annealed sample by transmission electron microscopy. They are identified as (Ti, Zr) 6 Si 3 distributed mainly at the α/β interface with the size of 100 300 nm. The fine αprecipitates are detected in BMZ by electron diffraction; there was no α detected in RZ.     

Settling velocity of equiaxed dendrites in a tube

The settling velocity of equiaxed dendrites can cause macrosegregation and influence the structure of the equiaxed zone during the casting solidification process. So an understanding of the settling characteristics is needed to predict the structure and segregation in castings. The settling velocity of NH4Cl equiaxed dendrites of non-spherical geometry was studied experimentally in an NH4Cl-70wt.%H2O solution. A calculation formula was proposed to calculate the settling velocity of sediment equiaxed dendrites in a tube filled with saturated solution at a moderate Reynolds number region. The retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity were taken into account in the development of the calculation formula, and the correction function B of the drag coefficient with consideration of the retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity of the equiaxed dendrite was calibrated according to the experimental results. A comparison showed that the formula has a good accordance with the experimental results.     

Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO₂ laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210~mm/s. An implicit expression of the critical solidification velocity for the cellular--planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M--S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS was successfully achieved in the present experiments.     

脱氧胆酸钇络合物的EXAFS和FTIR表征

用ＥＸＡＦＳ，ＦＴＩＲ以及ＩＣＰ等实验方法，对脱氧胆酸钇的结构进行了初步探讨。实验结果证实，钇与脱氧胆酸配位，生成稀土络合物，其中钇周围有八个氧原子，配位数为八，Ｙ－Ｏ平均距离为０．２３４ｎｍ。脱氧胆酸中羧基氧原子，羟基氧原子均与钇发生配位作用。     

凝固界面形态演化的实验研究

本文采用典型的透明模型合金丁二腈-1.1wt%乙醇进行定向凝固实验,对从平界面失去稳定性到建立起新的稳定的界面形态之间时间相关的形态演化过程进行了详细的直接观察研究。发现:1)平界面失稳初始扰动波长在Mullins-Sekerka(缩写为MS)稳定性理论所预言的波长范围内,但与理论预言的振幅发展最快的波长不一致;2)平界面失稳初期,扰动振幅发展速率确实是振幅的线性函数,但在具体数值上实测值比理论值小得多;3)胞晶和枝晶形态演化过程具有不同的特征,导致这种差别的主要原因是稳态一次间距与初始扰动波长之间相对尺 关键词：