K Edagawa

High temperature specific heat of Al-Pd-Mn and Al-Cu-Co quasicrystals

Abstract For the purpose of investigating the behavior of thermally excited phasons, specific heat has been measured by differencial scanning calorimetry (DSC) for icosahedral Al–Pd–Mn (i-Al–Pd–Mn) in the temperature range from 550 to 1080xa0K and for decagonal Al–Cu–Co (d-Al–Cu–Co) in the range from 890 to 1070xa0K. The measured specific heat per atom at constant pressure cp has been converted to that at constant volume cv by use of the data of bulk modulus previously reported and the data of thermal expansion coefficient which we measured by X-ray diffractometry. cv of i-Al–Pd–Mn is about three kB up to about 700xa0K, as expected from the Dulong–Petit’s law. However, it starts deviating upward from three kB at about 700xa0K, increases dramatically and reaches five kB at 1080xa0K. The Similar behavior has been observed also for d-Al–Cu–Co. The origin of such a dramatic increase in cv is discussed in light of possible thermal excitation of phasons.

Plastic deformation of icosahedral Al-Pd-Mn single quasicrystals to large strains II. Deformation mechanism

Abstract In part I, we have shown experimentally that the large amount of work softening is due to decreasing activation enthalpy for plastic deformation with increasing plastic strain. Based on the Peierls mechanism of the dislocation glide, the plastic deformation process in a quasiperiodic lattice is modelled. In the model, we assume high and low activation enthalpies, corresponding to large and small Peierls potentials, of the kink pair formation in the quasiperiodic Peierls potential. In an unstrained high-quality quasicrystal, the dislocation glide is essentially controlled by the high activation enthalpy whereas, in a highly strained state where a high density of phason defects is introduced, the dislocation glide is controlled by the low activation enthalpy; parts with a large Peierls potential can be overcome by the side motion of kinks produced at parts with a small Peierls potential.

Dislocations in quasicrystals

Quasicrystals have additional elastic degrees of freedom not found in conventional crystals, which are termed phason degrees of freedom. Dislocations in quasicrystals are special in that they are inevitably accompanied by the phason strain in addition to the conventional elastic strain. Characteristic features of such quasicrystalline dislocations and plastic properties of quasicrystals are reviewed. Deformation mechanisms in quasicrystals by dislocation motion are discussed.

HRTEM observation of phason flips in Al–Cu–Co decagonal quasicrystal

Abstract In-situ high-temperature, high-resolution transmission electron microscopy (HRTEM) has been performed on an Al–Cu–Co decagonal quasicrystal, to investigate thermal fluctuation of phasons. A tiling pattern with the edge length of 2 nm was constructed by connecting white spots in the HRTEM image. Local tile-rearrangements, which can be interpreted as phason flips, have been observed at temperatures above 1073 K. The frequency of the phason flip is higher at thin areas near the edge of the sample than at thick areas. The origin of such a sample-thickness dependence of the frequency of the phason flip is discussed using a simple model calculation.

Thermal expansion of icosahedral Al-Pd-Mn and decagonal Al-Cu-Co quasicrystals

Thermal expansion measurements have been performed by X-ray diffractometry on icosahedral Al-Pd-Mn (i-(Al-Pd-Mn)) in the temperature range between 10 and 700K and on decagonal Al-Cu-Co (d-(Al-Cu-Co)) in the range between 100 and 750K. The linear thermal expansion coefficient alpha(T) of i-(Al-Pd-Mn) is about half of that for the pure aluminium phase at room temperature and does not show a negative thermal expansion at low temperatures. The degree of anisotropy in the thermal expansion of d-(Al-Cu-Co) is small; the ratio of the average alpha between the tenfold periodic direction and a quasiperiodic direction perpendicular to it is 0:93 +/- 0:05. The Gruneisen parameter gamma(T) has been evaluated from the measured alpha(T). gamma for d-(Al-CuCo) is almost isotropic, similar to alpha. The gamma values for the two phases lie in the range between 1.5 and 1.8, which are comparable with those for conventional metallic crystals. They are almost constant over the temperature range studied.

Plastic deformation of icosahedral Al-Pd-Mn single quasicrystals to large strains I. Experiments

Abstract Icosahedral Al-Pd-Mn single quasicrystals, grown by the Czochralski method, were deformed in compression along a twofold direction to large strains. The true stress versus true strain curves showed three stages: the initial work-hardening stage, a large work-softening stage followed by a weak work-hardening stage. The minimum flow stress was about half the maximum flow stress. The stress relaxation experiments showed temperature-dependent activation volume versus applied stress relations. Assuming that the flow stress is composed of the effective stress, the temperature-dependent dragging stress and the internal stress, the effective stress dependence of the activation enthalpy was analysed for both the maximum-flow-stress state and the minimum-flow-stress state. The activation enthalpy of the latter state was found to be about 20% smaller than the former state, and the work softening is due to a decrease in the activation enthalpy with increasing plastic strain.

High-temperature specific heat of quasicrystals and a crystal approximant

Abstract Specific heat measurements have been performed by DSC in the temperature range from 430 to 1080 K for an icosahedral quasicrystal of Al–Pd–Mn, a decagonal quasicrystal of Al–Cu–Co and a crystal approximant of Al–Pd–Fe. The measured specific heat per atom at constant pressure c p ( T ) has been converted to that at constant volume c v ( T ) by use of the data of bulk modulus previously reported and the data of the thermal expansion coefficient which we measured by dilatometry. c v ( T ) of all three samples stay at around 3 k B up to about 700 K, as expected from Dulong–Petits law. For the two quasicrystals, however, c v ( T ) increases dramatically and reaches 4.7 k B at 1080 K. In contrast, the deviation in c v ( T ) of the crystal approximant is small; it is about 3.6 k B at 1080 K. The origin of the large deviations in c v ( T ) of the quasicyrstals from Dulong–Petits value is discussed in light of thermal excitation of phasons.

Plasticity of Al-Ni-Co decagonal single quasicrystals

Abstract Compression experiments have been performed for Al-Ni-Co decagonal single quasicrystals grown by a floating zone method with different orientations of the compression axes: the tenfold direction ( c ∥ ), a twofold direction perpendicular to it ( c ⊥ ) and a direction inclined from the tenfold direction by about 45° ( c 45° ). The yield stresses obtained range from 700 to 1050xa0MPa at 973xa0K, which are much larger than those previously reported. The anisotropy in the yield stress is less than a factor of two. For the c 45° compressed samples, a slip deformation has been observed on a twofold prismatic slip plane and in the tenfold slip direction. Long straight screw dislocations have been observed by transmission electron microscopy (TEM) in the c 45° compressed samples, indicating the existence of deep Peierls-potential valleys for the dislocation glide. The c ∥ and c ⊥ compressed samples have shown slip deformations along more than two types of pyramidal slip planes.

Deformation mechanism of quasicrystals

Abstract Characteristics of the high-temperature compressive deformation behavior of icosahedral and decagonal quasicrystals are reviewed. In icosahedral quasicrystals, the true stress versus true strain relation shows a pronounced yield drop, followed by gradual softening towards a minimum at a large true strain of about 0.5 and then changes to hardening at higher strain; the yield and flow stresses are quite sensitive to temperature and strain rate. In decagonal quasicrystals, the yield stress depends weekly on orientation in spite of its anisotropic structure. In both types of quasicrystals, the yield stress is higher in higher quality samples. The deformation mechanism of the quasicrystals is discussed in terms of dislocations glide in the quasicrystalline lattice governed by the Peierls mechanism. The Peierls mechanism in quasicrystals differs from that in crystals in that the dislocation glide is accompanied by a phason atrain relaxation and the Peierls potential is not periodic but quasiperiodic.

Periodic approximants of quasicrystals in the Al–Rh–Si ternary system

Abstract A 2/1 cubic type periodic approximant of a Mackey icosahedron type (MI-type) quasicrystal was found to form in conventionally solidified and annealed samples of Al 65 Rh 27 Si 8 . An almost single 2/1 cubic approximant phase is formed in the compositional range of 26–28 at.% Rh and 7–9 at.% Si. In the compositions with lower Si concentration, the 1/0 cubic approximant phase is formed. No phase transformation from the 2/1 cubic approximant was observed by long time annealing (48 h) of Al 65 Rh 27 Si 8 at 1273 K, suggesting that the 2/1 cubic approximant phase is a thermodynamically stable phase at this temperature. The 1/0 cubic approximant phase is found to form also in Al–Rh–Pd and Al–Ru–Si systems in a comparative investigation with Al–Rh–Si system.