Warm Deformation And Hardening Mechanism Of M₇C₃ Carbide In High Chromium Cast Iron

Metal carbide is a typical hard phase in steel and iron materials. Elastic-plastic deformation and fracture can occur in carbide during the service of workpieces, which has a great effect on the macro-properties of materials. So it is worth finding out the behaviors and mechanisms of plastic deformation that occurred in carbide, and then investigating the influence of deformation on the mechanical properties, which are all very helpful to the development of new strengthening method of materials. Also this has an important academic value and a good prospect in engineering applications.In this thesis, M7C3 carbide in high chromium cast iron was chosen as the study project. Firstly, specimens were compressed at 450 °C, and then the micro-Vickers hardness of(Fe,Cr)7C3 carbide before and after warm deformation was tested. Meanwhile, deformation behaviors of(Fe,Cr)7C3 carbide were observed systematically and its hardening mechanism was analyzed comprehensively by applying the methods of XRD、SEM、TEM、HRTEM and first-principles calculations.The results show that the micro-Vickers hardness values of(Fe,Cr)7C3 carbide after being compressed to 15% and 20% at 450 °C were 1766.20 HV and 1793.50 HV, which increased by 4.2% and 5.8% respectively compared to the original hardness value of 1695.36 HV before deformation. This indicates that work hardening occurs in(Fe,Cr)7C3 carbide due to warm deformation.TEM observations show that dislocations and twins generated in(Fe,Cr)7C3 carbide due to warm deformation. With the strain increasing, the dislocations began to slip, forming slip bands. Besides, the burgers vector of the edge dislocation was 1/3 ]0112[ and the slip plane was( 0001) according to the investgations of HRTEM.Through the observations of XRD and TEM Kikuchi diffraction pattern, lattice distortion also can be found in(Fe,Cr)7C3 carbide after deformation, which reflected as lattice contraction and the related strain of lattice constant a was about 2.0%. In addition, shear and torsion deformation happened and sub-grain boundaries emerged in(Fe,Cr)7C3 carbide when the strain became larger. The values of different torsion angles were measured to be 5.4 ° and 9.4 ° by Kikuchi diffraction in TEM and Moiré patterns in HRTEM respectively.In a word, the hardening mechanism of(Fe,Cr)7C3 carbide was concluded by comprehensive analyses of XRD, TEM, HRTEM and GSFE calculations, as follows: lattice distortion, shear and torsion deformation, the formation of dislocations and sub-grain boundaries, slip and cross-slip of dislocations under the action of shear stress.