Bridgman Crystal Research Articles

Structure and mechanical properties of unidirectionally solidified low alloy steels

AbstractTwo low alloy steels have been unidirectionally solidified in a liquid metal cooling Bridgman crystal grower. The dendrite morphology and dendrite arm spacing have been determined as a function of distance along the bars for solidification at various rates and under different temperature gradients. Microsegregation in the as solidified material was studied by electron probe microanalysis. The tensile properties of heat treated unidirectionally solidified material were determined and their values of elongation to failure found to be considerably greater than for the conventionally cast material. Similarly, the impact energy values of heat treated unidirectionally solidified material are higher than those of the conventionally cast alloy. The tensile and impact properties are discussed in terms of the strain incompatibility present during deformation both at the dendritic grain boundary and at the individual dendrite. Incompatibility of strain leads to a propensity for secondary cracking at these bo...

Structure and mechanical properties of unidirectionally solidified low alloy steels

Two low alloy steels have been unidirectionally solidified in a liquid metal cooling Bridgman crystal grower. The dendrite morphology and dendrite arm spacing have been determined as a function of distance along the bars for solidification at various rates and under different temperature gradients. Microsegregation in the as solidified material was studied by electron probe microanalysis. The tensile properties of heat treated unidirectionally solidified material were determined and their values of elongation to failure found to be considerably greater than for the conventionally cast material. Similarly, the impact energy values of heat treated unidirectionally solidified material are higher than those of the conventionally cast alloy. The tensile and impact properties are discussed in terms of the strain incompatibility present during deformation both at the dendritic grain boundary and at the individual dendrite. Incompatibility of strain leads to a propensity for secondary cracking at these boundaries, the amount of which depends upon the detailed morphology of the dendrite which is determined by the solidification parameters.MST/880

Finite element analysis of the control of interface shape in Bridgman crystal growth

Numerical experiments were performed on a vertical Bridgman crystal growth system. The thermal field was studied in the absence of fluid motion by solving the steady state energy equation using a finite element method. Key parameters were isolated and their influence on the interface curvature was assessed. These included insulation zone thickness, system temperatures, Biot numbers, solid-liquid thermal conductivity ratio, and ampoule location in the furnace. It was concluded that interface curvature could be controlled by the crystal grower to achieve any desired shape.

Numerical Simulation Of A 2D Crystal Growth Problem: Latent Heat Effects And Solid-liquid Interface Morphology

The influence of latent heat and natural convection in the melt and the shape of the melt-crystal interface are analyzed for a vertical Bridgman crystal growth system by direct numerical simulation. The temperature and the velocity field in the melt and in the crystal are computed using an homogenization technique. Initially the ampoule contains poly-crystal material, these container is translated inside a furnace which includes two heating zones characterized by two temperatures which are respectively greater and lower than the melted temperature of the material. The two heating zones in the furnace are separated by an adiabatic zone when the solid-liquid phase change occur. The control of the solidification conditions (phase change front velocity, gravity ...) permits to produce with these technique very high quality single crystals. The results presented in this paper show the effects of various gravitational conditions upon the flow in the melted part of the material inside the ampoule. We have also analyzed the effect of the latent heat upon the shape of the melt-crystal interface and the convective cells in the melted pool.

Electron Mobility in AgCl

Transit-time measurements of electron mobility in AgCl have been extended to very low temperatures. Results are presented for an air-grown Bridgman crystal and for a Kyropoulos crystal of high purity. A maximum mobility of about 250 ${\mathrm{cm}}^{2}$/volt-sec at 80\ifmmode^\circ\else\textdegree\fi{}K was found for the former and a maximum of 480 ${\mathrm{cm}}^{2}$/volt-sec at 55\ifmmode^\circ\else\textdegree\fi{}K for the latter. It appears that imperfections strongly influence the mobility at low temperatures. At intermediate temperatures the data are in best agreement with the theory of interaction with optical modes of vibration of the lattice from which it is found that ${\ensuremath{\mu}}_{0}=30({e}^{\frac{280}{\ensuremath{\tau}}}\ensuremath{-}1)$ ${\mathrm{cm}}^{2}$/volt-sec. This corresponds to a polaron effective mass of $\frac{{m}^{*}}{{m}_{e}}=0.28$ where ${m}_{e}$ is the free electron mass.