Self-Induced Internal Corrosion Stress Transgranular Cracking in Gradient-Structural Ploycrystalline Materials at High Temperature

Self-induced internal corrosion stress transgranular cracking is investigated theoretically and experimentally linking grain boundary wetting (GBW) and grain boundary diffusion (GBD) to improve the ability to reveal the micro mechanism of crack in compositional gradient-structural intermetallic materials. Theoretical analysis shows that the grain boundary wetting and diffusion induce the diffusion-coupled dynamic internal stresses, and their interaction leads to crack nucleation. The experimental results show a stress concentration zone have been established at the grain boundary interface where the cracks preferentially nucleate and then extend through the inside of the grain to both sides, forming a typical transgranular fracture.

High-Temperature Mechanical Spectrometer for Internal Friction Measurements

A new high temperature mechanical spectrometer, based on an inverted torsion pendulum, has been constructed for the measurement of the internal friction and the dynamic shear elastic modulus in two different working modes: (a) as a function of temperature (300 – 1800 K) at imposed frequency, during heating or cooling; and (b) as a function of frequency (10-3 – 10 Hz) in isothermal conditions. The whole installation is computer controlled by a dedicated software specifically developed. We describe the different parts of this new installation, as well as its performances in both temperature and frequency through an original example study on a high temperature structural intermetallic of Fe-Al.

STRUCTURAL PHASE STABILITY OF Ti3Al UNDER HIGH PRESSURE

Recently, the structural stability of the high temperature structural intermetallic Ti 3 Al has been investigated experimentally and a transition from the DO 19 to the DO 24 structure has been reported. We have performed band structure calculations as a function of reduced volume by tight binding linear muffin-tin orbital (TBLMTO) method to look into these transitions to understand their mechanisms in terms of its electronic structure. The mechanism of the DO 19 to the DO 24 structure is understood to be due to shifting of the Fermi energy into a valley in the density of states curve, thereby driving the system to a more stable structure.