Distinguishing Ambient-Temperature Creep Region in Deformation Mechanism Map of Annealed CP-Ti
 
Hisamune Tanakaa,* , Tomoyasu Yamadaa , Eiichi Satoa, Itaru Jimbob
aInstitute of Space and Astronautical Science, Japan Aerospace Exploration Agency,
3-1-1Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan.
b Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-1292, Japan.
 
Titanium alloys are attractive materials for structural applications in aerospace systems because of their high strength to weight ratio, high corrosion resistance and ability of superplastic blow forming. The fuel tank of the electric propulsion system of HAYABUSA, the JAXA satellite for asteroid sample return mission launched on May 2003, is made of Ti-6Al-4V alloy. During the proof test of the tank, creep phenomenon was observed at an ambient temperature and under stresses below the yield stress.

In the present study, we selected CP-Ti as a representative material for ambient-temperature creep study of H.C.P. metals and alloys. The purpose of this study is to distinguish ambient-temperature creep region in the Ashby-type deformation mechanism map of CP-Ti (-Ti).

The tested material was a CP-Ti plate of JIS TP270C. Creep tests were performed at 273-873 K, and the stress exponent and activation energy were determined. At high temperatures above 773 K the stress exponent is evaluated as 4.5, at intermediate temperatures between 573 and 673 K it is evaluated as 6.5, and at low temperatures below 423 K it is evaluated as 5.0. The apparent activation energy is evaluated as about 220 kJ/mol at high temperatures above 573 K and 10 kJ/mol at low temperatures below 423 K. It must be noted that the apparent activation energy is extremely low for ambient-temperature creep. The creep parameters reported by Ashby and Frost and those of ambient-temperature creep obtained by the present study are summarized in Table.

Based on these creep parameters as shown Table, we draw the deformation mechanism map including ambient-temperature creep region (region of (f)) in Fig. In this figure, constant strain rate contours are in every two orders. The extrapolated experimental data to =10-8 s-1 and =10-6 s-1 are also plotted. The contours line of 10-8 s-1 and 10-6 s-1 are in agreement with the experimental data points. Therefore, we proposed that ambient-temperature creep region for annealed -Ti was distinguished in the Ashby-type deformation mechanism map.

[Published in Scripta Materialia, Vol.54, (2006) 121-124]

 
Table Creep parameters for construction of deformation mechanism map including ambient-temperature creep region


Fig. Deformation mechanism map including ambient-temperature creep region. The experimental data points of 10-8 s-1 () and 10-6 s-1 () are corresponding the contour lines well.