| This work was carried out for some observations on crystal growth under pressure in an ammonium chloride/water analogue system, with a view to explore possible similarities with the mode of solidification in metals. An ammonium chloride solution having 37% wt. composition was used, and crystallisation phenomena were observed in this solution frozen at atmosphere and under pressure, depending on the pouring temperatures. Crystallisation phenomena under pressure greatly differed from the ones at atmosphere; they were as follows; (i) Cooling rate was much faster under pressure than that at atmosphere, stimulating dendrite fragmentation. (ii) Growth rate of columnar and/or equiaxed grains was increased under pressure. (iii) A sweeping effect ahead of the advancing interfaces was much more effective under pressure. (iv) A recess of columnar or equiaxed zone near the mold wall by applying pressure stimulated the formation of new crystals, but this was not in atmosphere. (v) Equiaxed zones were formed mainly by simultaneous nucleation at the lower pouring temperature, and at the higher ones mainly by convection. These effects were more effective under pressure. |
|
