Investigation On The Phase Equilibria And Crystal Structure Of Key Ternary Systems In The Zn-Mn-Ni-Sn-Si System

Multi-component phase diagrams have found increasingly wide applications in industry, such as the design of new materials and the formulation of the processes for the thermal processing and heat treatment. CALPHAD (CALculation of PHAse Diagram) method is the most convenient way to obtain the multi-component phase diagrams. The thermodynamic models of the multi-component alloy systems obtained by the CALPHAD could be used in the phase field investigation to simulate the non-equilibrium transformation of the alloys. The results from phase field simulation throw light on understanding the microstructure evolution. The CALPHAD method coupled with the first principles calculation and the phase field method are the base for attaining the prediction of materials microstructures and the design of their properties. The accuracy of the thermodynamically calculated multi-component phase diagrams with the CALPHAD method strongly depends on the accurate thermodynamic description of the related low order systems including binary and ternary systems. However, the latter highly rely on the high-quality experimental phase diagrams of binary or ternary systems.Mn, Ni, Si, Sn and Zn are the important alloying elements in Al alloys. Moreover, Mn, Ni, Si and Sn are also the key elements related to the galvanizing. Due to the excellent performances, the aluminum alloys are widely used in the aerospace and car industry. The galvanizing coatings are employed to protect the steel from corrosion. Knowledge of the interaction of the alloying elements would help the scientific design of the alloys and the improvement of the galvanizing process. In this work, the phase diagrams of the Mn-Ni-Sn, Mn-Ni-Zn, Ni-Sn-Zn, Mn-Sn-Zn and Mn-Ni-Si ternary systems, and the crystal structures of the related compounds were investigated. Some important results are summarized as follow:(1) The phase equilibria of the Ni-Sn-Zn ternary system at 500℃were established. A new ternary compound 8 (Ni4Sn4Zn2) was discovered in the present work. This compound has simple cubic structure with cell parameter of a= 0.88350 (7) nm. Further investigation on the crystal structure revealed that this compound is isostructural to CuNi5Sn9Zn and belongs to space group P-43m. In the present work, the powder diffraction data of this new compound was firstly presented too.(2) The isothermal section of the Mn-Ni-Sn ternary system at 500℃was firstly constructed. A quite different conclusion was addressed comparing to previous works on the Mn-Ni binary system. The Mn-Ni system was found to contain only Fccγ-MnNi and aMn at 500℃when equilibrium is reached. Moreover, a new ternary compound with the composition of about Mn22Ni36Sn42 was discovered. This compound adopt hexagnal structure with cell parameter of a=0.42047(8) nm and c= 0.52767(6) nm. In the composition of Mn42Ni3oSn28 for the MnNi2Sn Heusler alloy, stress-induced martensitic tranformation was identified at the first time. The martensitic phase was found to adopt orthorhombic structure. The homogeneity area of the Heusler alloy MnNi2Sn was determined in the present work.(3) The isothermal sections of Mn-Ni-Zn system at 400,500 and 600℃were determined for the first time. Two new ternary compounds were discovered in the present work. One was the T phase with a formula close to (MnNi)Zn6.5. The T phase was determined to have space group of F-43m with cell parameter of a=1.81450(7) nm and to be isostructural to (FeNi)Zn6.5. Rietveld refinement results was obtained to be Rp=8.68, Rwp=12.28 and S=1.59. Another phase wasτ1 with close composition of Mn7Ni9Zn8 and with tetragonal structure and cell parameter a= 0.2733 nm and c= 0.3428 nm. These two phases were observed at 400 and 500℃and are absent at 600℃.(4) The phase equilibria of the Mn-Sn-Zn system were investigated and established for the first time. A new ternary compound with bcc structure and cell parameter of a=0.92508(5) nm was found. The homogeneity range of this new ternary compound was determined too.(5) The isothermal section of the Mn-Ni-Si system at 900℃was estabished. A new ternary compound was discovered with composition at about Mn45Ni4oSi15。This new ternary compound was observed to undergo martensitic transformation during quenching. The crystal structure of the martensitic phase was determined to be hexagonal structure with cell parameters of a= 0.47783(8) nm and of c=0.76847(9) nm. The crystal structure of Mn15Ni4oSi45(N) was first identified to adopt hexagonal structure with cell parameters of a=1.70738(8) nm and of c=0.7841(9) nm. Moreover, the structure type and the space group of this compound were determined to be Co4NbSi3 and with No.191. The previously identified S and I phases were confirmed to be the same compound in the Mn-Ni-Si system. This compound was found to be isostrutural to V41Ni36Si23 with monoclinic structure. The cell parameters of this compound was calculated to be a=1.359 nm, b=2.358nm/β=100.1°c=0.901 nm.