液态三元Fe-Cr-Ni合金中快速枝晶生长与溶质分布规律

采用落管方法实现了液态三元Fe-Cr-Ni合金的深过冷与快速凝固,合金液滴的冷却速率和过冷度均随液滴直径的减小而迅速增大.两种成分合金近平衡凝固组织均为粗大板条状α相.在快速凝固过程中,不同直径Fe_(81.4)Cr_(13.9)Ni_(4.7)合金液滴凝固组织均为板条状α相,其固态相变特征很明显,随着过冷度增大,初生δ相由具有发达主干的粗大枝晶转变为等轴晶.Fe_(81.4)Cr_(4.7)Ni_(13.9)合金液滴凝固组织由α相晶粒组成,随着过冷度增大,初生γ相由具有发达主干的粗大枝晶转变为等轴晶,其枝晶主干长度和二次分枝间距均显著下降,晶粒内溶质的相对偏析度也明显减小,溶质Ni的相对偏析度始终大于溶质Cr.理论计算表明,与γ相相比,δ相枝晶生长速度更大.在实验获得的过冷度范围内,两种Fe-Cr-Ni合金枝晶生长过程均由热扩散控制.

Rapid dendrite growth mechanism and solute distribution in liquid ternary Fe-Cr-Ni alloys

Stainless steels with excellent hardness and corrosion resistance performance have been widely used in industrial production. Ternary Fe-Cr-Ni alloys, as a model alloy of nickel chromium stainless steels, are of great importance in the fields of material science. Under non-equilibrium solidification condition, alloys may have new microstructure and improved performance. In this paper, two liquid ternary Fe-Cr-Ni alloys are deeply undercooled and rapidly solidified in a 3-m drop tube to investigate the microstructure evolution and solute distribution of alloy droplets with different sizes. In the drop tube experiments, the Fe-Cr-Ni alloy samples with a mass of 1.5 g are placed in a φ16 m mm×150 mm quartz tube with a 0.5-mm-diameter orifice at its bottom and heated by induction heating device in a high vacuum chamber. Then the samples are melted and overheated to 200 K above their liquidus temperatures for several seconds. The alloy melt is ejected out of the small orifice and dispersed into numerous droplets after adding high pressure helium gas flow. The alloy droplets with diameters ranging from 68 μm to 1124 μm are achieved. After experiments, the alloy droplets with different sizes are mounted respectively. Then they are polished and etched. The drop tube technique provides an efficient way to study the rapid solidification mechanism of alloys. Besides the experiments, the dendrite growth velocities of primary phase in two Fe-Cr-Ni alloys are calculated theoretically using the modified LKT/BCT model. As droplet size decreases, both cooling rate and undercooling increase exponentially and the morphologies of two alloys become well refined. Under the near-equilibrium solidification condition with a cooling rate of 10 K/min, both alloys consist of coarse lath-like α phase. After rapid solidification of Fe_81.4Cr_13.9Ni_4.7 alloy droplets during free fall, the microstructure presents a lath-like α phase, resulting from the solid-solid phase transition. As undercooling increases, the primary δ phase is converted from the coarse dendrite with long trunk into equiaxed grain. For Fe_81.4Cr_4.7Ni_13.9 alloy, the microstructure is composed of α phase grains. The transition of primary γ phase from coarse dendrite with long trunk to refined equiaxed grain occurs as the undercooling increases. Meanwhile, both dendrite trunk length and secondary dendrite arm spacing decrease drastically, suggesting that the rapid solidification is the main reason for grain refinement. Moreover, the relative segregation degree of solute Cr and Ni inside α phase grain also decreases obviously with the increase of undercooling, and the microsegregation of Ni is more remarkable than that of Cr. This suggests that the high cooling rate and undercooling cause the solute to be distributed evenly. Compared with that of γ phase, the dendrite growth velocity of δ phase is large and its dendrite tip radius is small. The two phase transform from solute diffusion controlled growth into thermal diffusion controlled growth as undercooling increases to 8 K. When undercooling is larger than 8 K and within the experimental undercooling range, the dendrite growth of both Fe-Cr-Ni alloys is controlled by thermal diffusion.