Study of Fracture in Nb-Al Alloys and Pure Metals by Computer Molecular Dynamic Simulation
- PDF / 400,970 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 22 Downloads / 176 Views
STUDY OF FRACTURE IN Nb-Al ALLOYS AND PURE METALS BY COMPUTER MOLECULAR DYNAMIC SIMULATION DONGHYUN KIM, P. C. CLAPP AND J. A. RIFKIN Center for Materials Simulation, Institute of Materials Science, University of Connecticut
Storrs, CT 06268
ABSTRACT In molecular dynamic studies of 15,000 atom arrays of A15 Nb 3AI, BCC Nb and FCC Al containing crack under external stress in Mode I loading, it has been verified that fracture behavior can be predicted (starting in the vicinity of the crack) in terms of competition between dislocation nucleation and Griffith crack propagation. BCC Nb and FCC Al appears to be ductile and A15 Nb 3AI appears to be brittle, in agreement with theoretical predictions. The elastic solution used for predictions was proposed by Rice[l] for the anisotropic material. The interatomic interactions used in the simulation were Embedded Atom Method (EAM) potentials developed by Voter and Chen[2] for the Al-Al and by Rifkin[3] for the NbNb and Nb-Al.
INTRODUCTION 1) Background and motivation Computer Molecular Dynamics (CMD) simulation is an atomistic simulation that is based on interatomic potentials and volume forces developed semi-empirically via Embedded Atom Method and tested by comparing simulation predictions to other data not used in constructing the potentials. The EAM is a simple procedure for computing the electronic contribution to cohesion in transition metals. The EAM is based on the Hohenberg-Kohn theorem[4] which states that the energy contribution of an atom in an array of interacting atoms is a function of the local electron density, due to all other atoms. Unlike the band structure approach, the EAM does not require a periodic array of atoms. Consequently it can be used for disordered alloys, surfaces, cracks, dislocation cores, grain boundaries, stacking faults, and liquid-solid phase interfaces. By observing microscopic mechanisms like crack propagation, and dislocation emission during CMD simulation, we can get important input for predicting fracture. Nb 3Al was selected for study, because it is one of the candidate materials for high temperature applications in aerospace jet engines, but is limited by unacceptable levels of fracture toughness at ambient temperature. Thus we were motivated to examine the atomistic mechanisms of fracture in Nb 3AI at various temperatures; an examination which is uniquely possible with the CMD simulation. 2) Main focus of investigation Rice[l] recently proposed a new model which can calculate the level of applied stress intensity factors(K1i) required for the dislocation nucleation at the crack tip of anisotropic material based on the Peierls concept[5]. Kic is shown by them to be proportional to (y.)", where y.,, the unstable stacking energy, is a new solid state parameter identified by the analysis. It is the maximum energy needed in the block-like sliding along a slip plane. Based on the above information, the main concern of this study is to predict the brittle Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
508
vers
Data Loading...
