Hydrogen-induced internal-stress plasticity in titanium
- PDF / 1,282,213 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 34 Downloads / 205 Views
hase diagram[14] showing chemical cycles used in the present study of chemically induced internal-stress plasticity at 805 or 860 ⬚C ( pH2 ⫽ 0 } 4.1 kPa) as well as typical thermal cycles used in the literature for thermally induced internal-stress plasticity in hydrogen-free Ti (T ⫽ 860 } 900 ⬚C).
Fig. 2—Isothermal creep rate at 805 ⬚C and 860 ⬚C for hydrogen-free ␣Ti and fully hydrogenated -Ti.
in Figure 2 tend toward values of n ⬇ 1 at low stresses and n ⬇ 4.3 at high stresses, in good agreement with the reported stress exponents of pure titanium in the diffusional and dislocation creep regimes.[9] The transition between these mechanisms is in the range of 1.5 to 3.5 MPa, in rough agreement with the calculated value of about 1 MPa for a 20 m grain VOLUME 32A, MARCH 2001—841
Fig. 3—Strain history at constant stress (2.25 MPa) and temperature (805 ⬚C) recorded for titanium subjected to nine chemical cycles (␣/ cycling, pH2 ⫽ 0 } 4.1 kPa, and v ⫽ 3 h⫺1) and calculated from Fig. 2 for uncycled, hydrogenated -Ti (pH2 ⫽ 4.1 kPa) and uncycled, hydrogen-free ␣-Ti (pH2 ⫽ 0 kPa). The two hydrogen charging and discharging segments are shown in the fourth chemical cycle.
size.[9] A doubling in creep rate from 805 ⬚C to 860 ⬚C is expected from the bulk diffusion activation energy[9] but is not clearly visible in Figure 2, possibly because of minor differences in grain size or dislocation densities between specimens. However, as expected from the low creep resistance of hydrogen-free bcc -Ti above 882 ⬚C,[9] hydrogenated bcc -Ti crept 3 to 5 times faster than hydrogen-free hcp ␣-Ti tested at the same temperature. Figure 3 shows the creep curve of a specimen subjected to nine consecutive hydrogen cycles (pH2 ⫽ 0 } 4.1 kPa) at constant applied stress (2.25 MPa) and temperature (805 ⬚C). The cyclic oscillations in the creep curve correspond to reversible lattice strains from hydrogen dissolution and titanium transformation. A reproducible plastic strain increment of 1.1 pct was recorded after each cycle, corresponding to an average strain rate of 9.3⭈10⫺6 s⫺1 for the experimental cycle frequency of 3.0 h⫺1. Also illustrated in Figure 3 are the creep curves for hydrogen-free ␣-Ti and hydrogenated -Ti at 805 ⬚C and 2.25 MPa constructed from creep data in Figure 2. It appears from Figure 3 that hydrogen cycling leads to an average deformation rate that is 10 times higher than creep in the weak, hydrogenated -Ti phase, even though the cycled specimen was in the slower creeping ␣Ti phase during part of the cycles. The most likely explanation for this result is that internal-stress plasticity is activated during chemical cycling, as observed in titanium subjected to temperature cycling.[3,10] Two other possible explanations for the enhancement shown in Figure 3 cannot be retained upon further analysis. First, temperature fluctuations due to hydrogen dissolution and phase transformation could increase the average strain rate; reproducible temperature changes of ⫾3 K were indeed recorded after each change
842—VOLUME 32A, MAR
Data Loading...
