|Budget Amount *help
¥6,700,000 (Direct Cost : ¥6,700,000)
Fiscal Year 1991 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1990 : ¥5,600,000 (Direct Cost : ¥5,600,000)
It has been commonly accepted that a microstructure consisting of gamma/ alpha_2/gamma lamellae improves the ductility of gamma titanium aluminides. In this program, we have established that there are two formation mechanisms of gamma/alpha_22/gamma lamellar microstructures, which we proposed to call as primary lamellar microstructure : P-LMS and discontinuously corsened lamellar microstructure : DC-LMS.
1)Primary gamma/alpha_2/gamma lamellar microstructure : P-LMS
Formation of the primary gamma/alpha_2/gamma lamellar microstructure was investigated using a Ti-40at%Al binary alloy. Upon quenching a disordered hcp alpha phase, stable at higher temperatures, to gamma+ alpha_2 two phase region, ordering from alpha to alpha_2 took place first, followed by precipitation of gamma plates on the basal plane of the alpha_2 matrix as a habit plane. Rapid lengthening of the gamma plates along the only one existing habit plane resulted in a seemingly perfect planar layer alignment of gamma and alpha_2 alternate lamellae. Variant distribution in the gamma plates was experimentally determined. Relative orientation relationships were discussed among independently grown gamma plates.
2)Discontinuously corsened gamma/alpha_2/gamma lamellar microstructure : DC-LMS.
Discontinuous coarsening(DC)of the P-LMS was studied in a temperature range between 1073 and 1473 K, using four Ti-Al binary alloys containing between 40 and 47 at%Al. DC started from the original alpha grain boundary. The migration rate of the advancing front of DC decreased with reaction time. The finer intelamellar spacing of P-LMS enhanced, while the larger volume fraction of alpha_2 phase beyond 50 pct in P-LMS inpeded, DC. DC divided a primary lamellar domain, whose size is the same as the original alpha grain size, into smaller colonies, within which not only the interlamellar spacing of the lamellae was larger, but also their alignment was less planar.