L1_0相和L1_2相结构原位转变的微观相场法模拟

MICROSCOPIC PHASE-FIELD METHOD SIMULATION FOR THE IN SITU TRANSFORMATION OF Ll_0 PHASE AND Ll_2 PHASE STRUCTURE

以Ni_(80)Al_(13)Cr_7合金为研究对象,采用描述单个格点位置原子占位的时间和空间演化为特征的微观相场法,研究了Al和Cr原子在(100)和(200)面上的原子占位变化与时效早期Ni_3(Al,Cr)结构演化之间的关系.研究表明,在时效最初阶段, Al和Cr原子在(100)和(200)面上的成分序参数并不发生变化,然而在两面上的长程序参数相等并随时间逐渐增大,以等成分有序化的形式发生第一次原位转变,形成低有序度的L1_0结构预析出相.当有序度增大到一定值时,Al和Cr原子的成分序参数和长程序参数在(100)面上迅速增大,而在(200)面上则迅速降低,相结构发生第二次原位转变,L1_0结构逐渐向L1_2结构转化.

As a Ni3Al precipitation strengthening alloy, Ni-Cr-Al alloy is one of typical structural materials applied in high temperatures. Much work has been done concerning its structure transformation from fcc to L12 during the process of ordering and phase separation. However, the study so far we know on the phase transformation in very early precipitation stage, especially on the phase transformation of unstable pre precipitation phase, was not yet sufficient. Microscopic phase-filed method is used for describing the temporal and spatial evolution of atomic site in lattice. This method is employed to study the pre-precipitation during the structure transformation from fcc to L12 in NisoAl13Cr7 alloy in this paper. The relationship between the occupation probability changes in A1 and Cr atoms on （100） and （200） planes respectively and structure evolution of Ni3（Al, Cr） will be inveatigated. The simulation results demonstrate that at in very early aging stage, the composition order parameters of Al and Cr atoms keep unchange on both （100） and （200） planes. However their long range order parameters are equal in these two planes and gradually increase with aging time until the first in situ transformation by congruent ordering at that time the Llo pre-precipitation phase with low long range order is formed. By prolonging aging time, their long range order parameters continue increase once they attain to the respective certain values, the composition order parameters and the long range order parameters of Al and Cr atoms on （100） plane become increasing rapidly but those on （200） plane decreasing quickly, The second in situ transformation occurs and the Llo is gradually transformed into L12.