• PDF / 1,790,872 Bytes
• 15 Pages / 606.24 x 786 pts Page_size
• 86 Downloads / 223 Views

DOWNLOAD

REPORT

---

11/9/03

3:55 PM

Page 2113

Microstructural Effects on the Tensile Properties and Deformation Behavior of a Ti-48Al Gamma Titanium Aluminide G. BABU VISWANATHAN, MICHAEL J. MILLS, and VIJAY K. VASUDEVAN The effects of microstructure on the tensile properties and deformation behavior of a binary Ti-48Al gamma titanium aluminide were studied. Tensile-mechanical properties of samples with microstructures ranging from near
to duplex to fine grained, near- and fully-lamellar were determined at a range of temperatures, and the deformation structures in these characterized by transmission electron microscopy (TEM). Microstructure was observed to exert a strong influence on the tensile properties, with the grain size and lamellar volume fraction playing connected, but complex, roles. Acoustic emission response monitored during the tensile test revealed spikes whose amplitude and frequency increased with an increase in the volume fraction of lamellar grains in the microstructure. Analysis of failed samples suggested that microcracking was the main factor responsible for the spikes, with twinning providing a minor contribution in the near-lamellar materials. The most important factor that controls ductility of these alloys is grain size. The ductility, yield stress, and work-hardening rate of the binary Ti-48Al alloy exhibit maximum values between 0.50 and 0.60 volume fraction of the lamellar constituent. The high work-hardening rate, which is associated with the low mobility of dislocations, is the likely cause of low ductility of these alloys. In the near-
and duplex structures, slip by motion of 1/2110] unit dislocations and twinning are the prevalent deformation modes at room temperature (RT), whereas twinning is more common in the near- and fully-lamellar structures. The occurrence of twinning is largely dictated by the Schmid factor. The 1/2110] unit dislocations are prevalent even for grain orientations for which the Schmid factor is higher for 101] superdislocations, though the latter are observed in favorably oriented grains. The activity of both of these systems is responsible for the higher ductility at ambient temperatures compared with Al-rich single-phase
alloys. A higher twin density is observed in lamellar grains, but their propagation depends on the orientation and geometry of the individual
lamellae. The increase in ductility at high temperatures correlates with increased activity of 1/2110] dislocations (including their climb motion) and twin thickening. The role of microstructural variables on strength, ductility, and fracture are discussed.

I. INTRODUCTION

ALLOYS based on the intermetallic compound TiAl are candidate materials for high-temperature aerospace applications because of their unique combination of properties. The most promising alloys, which are based on the Ti-48Al composition (in at. pct) with ternary or quaternary additions, are characterized by the two-phase Ti3Al  TiAl (2 
) lamellar microstructure;[1] duplex structures show higher tensile ductilities, for example, up to