Tetsuya Matsunaga

Ti alloys are used as compressor blades and disks in jet engines due to their high specific strength and good oxidation resistance at operation temperature. However, Ti alloys cannot be used above 600 degrees C because creep properties and oxidation resistance deteriorate. To overcome the above problems, the effect of alloying element on oxidation resistance was investigated and it was found that Sn deteriorated oxidation resistance and Nb improved oxidation resistance. Then, we have attempted to design new Ti alloys without Sn, but including Nb because Nb improved oxidation resistance. To expect solid-solution hardening, Zr was also added to the alloys. In this study, the creep behavior of Ti-10Al-2Nb-2Zr and Ti-10Al-2Nb-2Zr-0.5Si alloys was investigated. The creep test was performed at temperature range between 550 and 650 degrees C and stress range between 137 and 240 MPa. The stress exponent and the activation energy for creep were analyzed using an Arrhenius equation. The stress exponent was 5.9 and 3.4, and the activation energy was 290 and 272 kJ/mol for Ti-10A1-2Nb-2Zr and Ti-10A1-2Nb-2Zr-0.5Si, respectively. This indicates the creep deformation mechanism is dislocation (high-temperature power law) creep governed by lattice diffusion.

This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 μm. Although the alloy shows negligible SRS at strain rates of &gt 10-5 s-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10-8 s-1. The activation volume (V) was evaluated as approximately 100b3 at high strain rates and as about 15b3 at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.

Ultrafine-grained interstitial-free steel fabricated by the accumulative roll-bonding method was subjected to tensile tests and analyses of AFM, TEM and XRD to identify the effects of interaction between dislocations and grain boundaries (GB) on the deformation mechanism. The AFM analyses indicated that the main deformation mechanism of this material changed from dislocation motion to grain boundary sliding (GBS) with decreasing strain rate. TEM observations and XRD analysis revealed showed that dislocations piled up at GB and the dislocation density decreased with increasing strain. Those suggest the dislocations are absorbed into GB during deformation, activating slip-induced GBS.

Creep tests were conducted at low temperatures for ultrafine-grained aluminum (UFG Al) fabricated by accumulative roll bonding. The samples showed remarkable creep behavior with a stress exponent of about three, a grain-size exponent of almost zero, and a low apparent activation energy of 20 kJ/mol. This creep behavior is similar to that of low-temperature creep of coarsegrained (CG) Al, though UFG Al shows creep under stresses below its 0.2% proof stress while CG Al show it under stresses above that. © (2013) Trans Tech Publications, Switzerland.

Hexagonal close-packed metals and alloys show significant creep behavior with extremely low activation energies at and below ambient temperature even below their 0.2% proof stresses. It is caused by straightly-aligned dislocation arrays in a single slip system without any dislocation cuttings. These dislocation arrays should, then, pile up at grain boundary (GB) because of violation of von Mises' condition in H.C.P. structure. The piled-up dislocations have to be accommodated by GB sliding. Electron back scatter diffraction (EBSD) analyses and atomic force microscope (AFM) observations were performed to reveal the mechanism of GB sliding below ambient temperature in H.C.P. metals as an accommodation mechanism of ambient temperature creep. EBSD analyses revealed that crystal lattice rotated near GB, which indicates the pile up of lattice dislocations at GB. AFM observation showed a step caused by GB sliding. GB sliding below ambient temperature in H.C.P. metals are considered to compensate the incompatibility between neighboring grains by dislocation slip, which is called slip induced GB sliding.

The deformation behaviour of high-purity aluminium at low temperatures was investigated in order to re-examine Ashby-type deformation mechanism map. All specimens with different purities showed significant creep below room temperature. Under the same stress and temperature, the steady-state creep rate increased with increasing purity of the material. They showed stress exponents around 5.0 and apparent activation energies around 20 kJ/mol at temperatures below about 400 K, and 4.0 and 70-80 kJ/mol at temperatures above that temperature. The grain size had no effect in the low temperature region. From the microstructural observation, secondary slip system was observed. These features imply that pure aluminium deforms in the different mode from the ambient temperature creep of h.c.p. metals which has similar activation energy.

Only hexagonal close-packed (h.c.p.) materials show creep behaviour significantly at ambient temperature or less even below their 0.2% proof stresses with their stress exponents of 3.0 and their apparent activation energies of 20 kJ/mol. Transmission electron microscopy revealed dislocation arrays as a planar slip without any tangled dislocations inside each grain. Atomic force microscopy and electron backscatter diffraction pattern analyses brought about the occurrence of grain boundary sliding. The grain-size exponent was evaluated as 1.0, which means grain boundaries work as the barrier of the dislocation motion. Ambient-temperature creep of h.c.p. materials is schematically illustrated as that lattice dislocations move inside each grain without any obstacles and then pile up at grain boundaries. To continue the creep deformation, these dislocations are absorbed by grain boundaries to accommodate the internal stress and lead to grain boundary sliding.

Hexagonal close-packed metals and alloys show significant creep behavior at ambient temperature, even below their 0.2% proof stresses. That creep behavior arises from straightly aligned dislocation arrays in a single slip system without any dislocation cuttings. These dislocation arrays pile up at grain boundary (GB) because of violation of von Mises' condition. Therefore, GB sliding must accommodate the piled-up dislocations. In this study, electron back scatter diffraction (EBSD) analyses and atomic force microscope (AFM) observations revealed an accommodation mechanism in ambient temperature creep region. Lattice rotation occurred near GB during creep, as revealed by EBSD analyses, indicating the pile up of lattice dislocations there. GB sliding during creep was revealed by AFM observations.

Metals with a hexagonal closed packed (HCP) structure show creep behavior at ambient temperature. Features of this phenomenon are: (1) it appears in all and only HCP structure metals and alloys; (2) dislocations are contributing; and (3) it shows very low apparent activation energy (ca. 10 kJ/mol). Transmission electron microscope (TEM) observation was conducted on crept specimens of commercially pure Ti (CP-Ti), pure Mg, and Zn. Results showed no dislocation tangle. The dislocation arrays were aligned straightly inside the grain. The dislocation array consisted of one dislocation type. One slip system was activated in the ambient temperature creep condition. Therefore, it was considered that work hardening does not occur, and that creep deformation continued.