The Microstructures, Mechanical Properties and Deformation Mechanisms of Fe-Mn-Al-C Alloys with Spinodal Decomposition

• Main Authors: Lin, Chih-Lung, 林志龍
• Other Authors: Liu, Tzeng-Feng
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/41219549638735109657

博士 === 國立交通大學 === 材料科學與工程學系所 === 102 === The relationship between microstructures and tensile properties of an Fe-30wt.%Mn-8.5wt.%Al-2.0wt.%C alloy (Alloy A), and the deformation mechanisms in an Fe-30wt.%Mn-9.5wt.%Al-2.0wt.%C alloy (Alloy B) after tensile test have been examined by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, LECO 2000 image analyzer and Instron tensile testing machine, respectively. On the basis of the experimental examinations, the results can be summarized as follows： [1] The as-quenched microstructure of both the alloys A and B is the austenite phase containing a high density of extremely fine κ'-carbides were formed during quenching by spinodal decomposition. The unique κ'-carbides formation mechanism is quite different that observed in the FeMnAlC (C≦1.3wt.%) alloys, in which the fine κ'-carbides could only be observed in the aged alloys. Owing to the presence of the high density of the extremely fine κ'-carbides within the austenite matrix, the ultimate tensile strength (UTS), yield strength (YS) and elongation (El.) of both A and B alloys can reach 1105 MPa, 883 MPa and 54.5%, and 1150 MPa, 950 MPa, 53%, respectively. Evidently, the as-quenched mechanical properties of both A and B alloys are superior to those of the as-quenched FeMnAlC (C≦1.3wt.%) alloys examined by previous workers. [2] Since the '-carbides already exist in the as-quenched alloys, both the aging time and temperature for obtaining the optimal combination of strength and ductility can be significantly reduced. For instance, when the alloy A was aged at 550℃ for 3 hours and the alloy B was aged at 450℃ for 9 hours, the obtained YS can be reach 1262 MPa, and 1406 MPa and the El maintains to be 32.1 and 32.5%, respectively. Under the same elongation, our results showed over 30% enhancement in strength as compared with the optimally aged C≦1.3wt.% FeMnAlC alloys. More importantly, the obtained strength-ductility combination has evidently exceeded by a sizeable margin over the targeted specifications put forth by the US DOE in 2012 and by American Steel Association for the third generation advanced high-strength steels (3GAHSS) to be on market in 20172025. [3] When the alloy B was aged at 450℃ for 9 hours, the nano-size κ'-carbides grew along the <100> directions, and the volume fraction of the grown κ'-carbides increased dramatically to about 68%. Therefore, the austenite matrix was well-divided to isolated γ-phase nano-channels. Consequently, after tensile test fractured, no long slip lines could be observed and a very high density of dislocations homogeneously distributed within the γ-phase nano-channels. Moreover, these dislocation were all oriented normal to the γ/κ'-carbides interfaced. It means that the plastic deformation was dominated by “bursting dislocation nucleation” within the isolated austenite nano-channels. Furthermore, transmission electron microscopy observation revealed that the dislocations near the γ/κ' interface were formed in pair-wise manner, which is a unique feature of Taylor lattice. It is worth while to note here that (a) we discovered, for the first time, a novel deformation mechanism in polycrystalline metals, which is governed by “bursting dislocation nucleation”, and (b) we are the first to directly observe the dislocation Taylor lattice in atomic resolution images.