佐藤 英一

Superplastic deformation behavior was examined in high-purity 3mol% Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with fine and coarse grains, and their deformation mechanism map was drawn. The fine grain material below 0.3m showed two deformation regions with stress exponent 2 at high stresses and 3 at low stresses, while the coarse grain material above 1m showed two deformation regions with stress exponent 1 at high stresses and 3 at low stresses. These deformation regions can be predicted by the obtained deformation mechanism map.

Metal Matrix Composite containing Fe₂Tb_0.3Dy_0.7 particles in Al matrix is strengthened by the effect of magnetostriction. The composite is fabricated by powder metallurgy, and the average diameter and the volume fraction of the particle are 2μm and 5%, respectively. The magnetostriction induced by the magnetic field is relaxed at high temperature. Turning off the magnetic field at room temperature produces the residual stress. It is found that the compressive stress induced in the matrix develops the large tensile strength.

A composite consisting of Al matrix and giant magnetostriction particles was fabricated by powder metallurgy. The composite was heat treated under a magnetic field, where the particles induced giant magnetostriction, and after turning off the magnetic field at room temperature, the matrix possessed compressive residual stress. Tensile tests revealed that the above magnetization heat treatment improved the yield strength of the composite. The result proposed the new magnetostriction strengthening mechanism.

Composite creep deformation was analyzed, based on a continuum plasticity representation of the matrix, in an ideal composite at high temperatures in the case of negligible interfacial diffusion and sliding. A general formula of the steady-state creep strain rate was derived for a composite consisting of an ellipsoidal rigid reinforcement and a creeping matrix with a stress exponent of one. A closed-form solution was then derived for a composite with a cylindrical reinforcement under pure shear deformation in a two-dimensional analysis. The resultant creep deformation satisfies the requirements of impotency, volume conservation and interfacial continuity. Traces of two types of edge dislocations were analytically drawn; they show that dislocations climb over the reinforcement, retaining no dislocations either in the matrix or at the interface. Also, two types of dislocations simultaneously climb up and down at any portion in the matrix through dislocation core shuffling without long-distance diffusion. Finally, it was concluded that plastically-accommodated creep is characterized by two types of dislocations that simultaneously climb over a reinforcement, generating a heterogeneous creep strain increment without long-distance diffusion.

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is common superplasticity ceramics and its deformation behavior at high temperatures is influenced strongly by trace impurities such as silica and alumina. Low purity materials more than 0.1% show only one region of stress exponent 2, while high purity materials show two deformation behavior region with stress exponent 2 at high stresses and 3 at low stresses. In the present study, another deformation region with stress exponent 1 at lower stresses is observed in high purity 3Y-TZP, by means of constant cross-head speed compression tests and constant load compression creep tests.