Straightlyaligned Dislocation Arrays in h.c.p. Metals after Ambient Temperature Creep

Titanium alloys are beneficial materials for aerospace systems because of their high strength-to-weight ratio, high corrosion resistance, and capability of superplastic blow forming. For example, a Ti-6Al-4V alloy fuel tank was developed and used for the scientific satellite "HAYABUSA" by ISAS/JAXA in 2003. During the tank proof test, creep behavior was observed at ambient temperature and under stresses below the 0.2% proof stress (_0.2). Although small creep strain in a fuel tank might be permissible, creep in fasteners cannot be permitted because it would cause stress relaxation, which might engender a very dangerous state.

Ambient temperature creep was observed in Ti in 1949. However, a few studies examined it in Ti-5Al-2.5Sn and Ti-6Al-4V in 1960s and in Ti-0.4Mn in 1990s sporadically. In the past few years, the Mills group conducted studying this phenomenon using Ti-6Al and Ti-6Al-2Sn-4Zr-2Mo. The ambient temperature creep mechanism is inferred to be related to Andrade creep⁶⁾ and deformation twins. Recently, straight aligned dislocation arrays that are formed by solute Al atoms have been suggested. However, those are presuppositions based only on individual experiments that are unique for tested materials. Therefore, we need a macroscopic discussion whether Ti-Al solid solution system is necessary for the creep.

The purpose of this study is to investigate the ambient temperature creep mechanism. Therefore, we performed creep tests for several materials, including CP-Ti, Zr, Mg, Ti-6Al-4V, Zircaloy, AZ31, and some cubic metals and alloys, at ambient temperature. Figure 1 shows a double logarithmic plot of the steady-state creep rate and the modulus-compensated applied stress. In this figure, three groups based on the crystallographic structure and alloying of the materials is visible. One group has low stress and high steady-state creep rate, which includes pure h.c.p. metals. Solid solution h.c.p. alloys form a different group that has high stress and high steady-state creep rate. And, cubic metals and alloys lie on low stress and low steady-state creep rate region. In this paper, representative h.c.p. metals were selected as specimens: CP-Ti, Mg, and Zn, which show the remarkable creep behavior. Since the h.c.p. metals have their own c/a ratios and the lowest Peiels potential slip systems, we investigate the c/a dependency of the creep behavior. Creep tests were performed at several temperatures, 203-873 K, to study the temperature dependency of the deformation mechanism. Finally, transmission electron microscope (TEM) observation was performed for the three crept h.c.p. metals and their dislocations were characterized.

Ambient temperature creep was observed in all three h.c.p. metals having different c/a ratios. Figure 1 shows Arrhenius plots of CP-Ti, Mg, and Zn. It shows two regions. One is the low temperature dislocation creep region, which has larger apparent activation energies of 50-150 kJ/mol. Another is the ambient temperature creep region that shows very low apparent activation energies of 10-18 kJ/mol. Even for Zn, room temperature, which is 0.43 of the melting temperature, belongs to the ambient temperature creep region.

TEM observations were performed after creep time of about 45000 s. At that period, the primary creep is ending and the secondary creep is starting. Associated TEM bright-field images are shown in Fig. 2 showing a similar dislocation structure among the three metals: straightly-aligned dislocation arrays without any dislocation cutting. The Burgers’ vectors of the three samples were identified as being the same direction, <1210>. The dislocation line directions were determined as <0001> for CP-Ti and Mg and <1210> for Zn. Therefore, the activated dislocations and slip systems were edge dislocations and prismatic slip for CP-Ti, edge dislocations and prismatic slip for Mg and screw dislocations and basal slip for Zn, as summarized in Table 1. In the case of Mg, the mixed dislocation and basal slip was activated in a few grains because of their rolling texture.

Ambient temperature creep was observed in all three h.c.p. metals having different c/a ratios. In addition, straightly-aligned dislocation arrays worked in grains. And these dislocation arrays were on slip systems having the lowest and the second lowest Peiels potential without any dislocation cuttings. Because of it, only one slip system was activated in a grain. For Mg, the slip system having the second lowest Peiels potential, prismatic one, was dominated because of their strong rolling texture. Therefore, one slip system was observed in a crystal grain, indicating that dislocation cuttings do not occur; then deformation proceeded.

If creep deformation proceeds, a accommodation mechanism has to work. For high temperature creep, dislocation core or volume diffusion activates. But, the apparent activation energies for ambient temperature creep were very low, 10-18 kJ/mol, which are much smaller than these diffusion's (fig. 3). In addition, ambient temperature creep depends on grain diameter. The grains were getting bigger, the steady-state creep rate and creep strain decrease. For example, Zn with average grain diameter of 1440 m ruptured with intergranular fracture after about 8000 s. If one slip system activated during deformation, h.c.p. structure don't implement the Von Mises law. Thus, dislocations can't go through to a next grain. Therefore, results showed dislocation pileup and stress concentration around grain boundary. Thus, dislocations were accommodated by grain boundary.

Apparent activation energies were very low, ca. 10 kJ/mol. It is not enough to activate body or dislocation core diffusion. Ambient temperature creep showed weak grain diameter independence. The steady-state creep rate and creep strain decrease. Therefore, we consider that grain boundaries work as accommodation process.