Ryoichi Monzen

The bcc-to-9R martensitic transformation of Cu precipitates and the relaxation process of elastic strains in an Fe-Cu alloy

Abstract High-resolution electron microscopy experiments have been performed to explore the bcc—9R transformation and the subsequent elastic relaxation of Cu precipitates in an Fe—Cu alloy aged at 550°C. It was found that both electron irradiation (at an electron energy of 400 kV) and thermal annealing caused rotation of the close-packed (009)9R planes in twinned 9R Cu precipitates. For 400 kV electron irradiation, such rotations were observed in precipitates smaller than about 12nm in diameter. For specimens cooled from the ageing temperature of 550°C to a given temperature up to −60°C, and then annealed at 400°C, the rotation of (009)9R planes was found to occur only in precipitates above a size which depended on the temperature to which the specimen had been cooled. This critical size ranged from about 9 nm for specimens cooled to 400°C, to 4 nm for specimens cooled to −60°C. It is argued that these critical sizes are indicative of the sizes at which coherent bcc precipitates transform martensitically at different temperatures. At the ageing temperature of 550°C, the transformation to 9R takes place when precipitates reach a size of about 12nm. The number of twin segments in the transformed 9R precipitates is determined by the transformation, depending on the precipitate size. The annealing-induced plane rotations are shown to be connected with the diffusional relaxation of elastic strains, which are created upon the martensitic transformation. From the precipitate size and annealing time dependence of the rotations, it is concluded that the elastic strains relax by atomic diffusion along the interfaces between the Fe matrix and Cu precipitates. The activation energy for the interfacial diffusion is evaluated to be 1.7eV.

Structural changes of 9R copper precipitates in an aged Fe-Cu alloy

High-resolution transmission electron microscopy and conventional transmission electron microscopy have been used to investigate in detail the transformation processes of twinned 9R copper precipitates to final stable structures via a 3R structure in a thermally aged Fe-Co model alloy. In 9R precipitates of size greater than about 13nm, the motion of the twin boundaries and the elimination of the regular stacking faults on every third (009)9R close packed plane were observed to occur nearly simultaneously. Direct evidence was found that 3R is a non-cubic structure obtained when the regular stacking faults on the (009)9R basal planes are removed. In larger precipitates (of size up to about 26 nm), lattice plane rotations and plane spacing changes in 3R variants took place towards stable fcc and fct structures, suggesting the occurrence of lattice relaxation involving the diffusion of atoms. The fct structure had larger lattice constants a=b=0.369 nm and c=0.366 nm than bulk fcc copper (a=0.361 nm). Precipitates of size 26-40 nm consisted of nearly twin-related variants with both fcc and fct segments, and aligned with the iron matrix according to the Kurdjumov-Sachs orientation relationship. Larger precipitates were observed to be fcc and fct single crystals.

NANOMETER GRAIN BOUNDARY SLIDING IN CU : 011 SYMMETRIC TILT BOUNDARIES, MISORIENTATION DEPENDENCE AND ANISOTROPY

Abstract Sliding on [011] symmetric tilt boundaries in Cu has been studied between 323 and 584 K, using an electron microscope technique and bicrystals of an aged Cu-1.05%Fe-0.45%Co alloy. The b.c.c. FeCo particles formed by aging on boundaries suppress the sliding so that it amounts to less than 1 nm. The sliding has been detected by observing a change in the direction of Moire fringes formed by the matrix {111} and the particle {110}. Finely dispersed f.c.c. FeCo particles in grains abutting boundaries completely prevent crystal dislocations playing any role in boundary sliding. The sliding increases with increasing temperature and saturates to a value determined by the size and distribution of the boundary particles and the applied shear stress. A boundary with a higher energy can slide at a lower temperature, as low as 350 K. A curve showing the ease of sliding against the misorientation angle is similar, with cusps, to the energy vs misorientation angle curve. The sliding parallel to the tilting axis, [011], occurs at lower temperatures than that in the direction normal to the tilting axis.

Boundary sliding and elastic distortion in Cu bicrystals with boundary precipitates

Abstract The direct detection of elastic distortion caused by boundary sliding which is blocked by precipitates on a boundary has been attempted in Cu. B.c.c. precipitates have been introduced to boundaries by aging bicrystals of Cu-1.4 Fe-0.6 Co. Moire fringes are formed by the matrix {111} f and precipitate {110} b planes in electron microscopy. The elastic distortion due to blocked sliding causes the rotation of the Moire fringes. The rotation is detected in foils annealed after loading which has introduced the sliding and elastic distortion in a bulk state. The Moire fringes rotate in the same direction for all the precipitates examined by annealing of foils. The direction and magnitude of rotation are in accordance with those predicted by elasticity analysis. The rotation on annealing does not occur in a sample which is loaded and subsequently unloaded at the same temperature; the fact indicating that a boundary slides back upon unloading owing to an internal stress.

Coarsening kinetics of Cu particles in an Fe-1.5 % Cu alloy

The Ostwald ripening of spherical and rod-shaped Cu precipitate particles in an Fe-1.5 mass% Cu alloy aged between 873 and 1023 K has been examined by measuring both the particle size by electron microscopy and the Cu concentration in the α-Fe matrix by electrical resistivity. The average radius of spherical and rod-shaped particles increases with ageing time t as t 1 / 3 , in agreement with the growth by diffusion-controlled coarsening. The kinetics of the depletion of supersaturation with t obey the predicted t - 1 / 3 time law. The Cu/a-Fe interfacial energy and the diffusivity of Cu in α-Fe have been independently derived from the data on coarsening. The interfacial energies are measured as 0.57 and 1.1 Jm - 2 for the spherical and rod-shaped particles respectively. The pre-exponential factor and the activation energy for bulk diffusion are found to be 4.54 × 10 - 5 m 2 s - 1 and 241 kJmol - 1 .

Diffusional relaxation around martensitically transformed Fe-Co particles in a Cu matrix

Abstract Small precipitate f.c.c. Fe-Co particles in a Cu matrix transform martensitically into b.c.c. particles by simple cooling a Cu-1.4 mass% Fe-0.6 mass% Co alloy to the liquid nitrogen temperature. Transmission electron microscopy has revealed that the small transformed particles are free from dislocations around them. However, the transformation strains create elastic fields in and around the particles. Successive annealing results in the occurrence of the diffusional relaxation of the elastic fields as envisaged by the changes in the Moire fringes in the Fe-Co particles. The mechanism and kinetics of the diffusional relaxation are analyzed both experimentally and theoretically. It is found that diffusion at the interfaces between the Cu matrix and Fe-Co particles causes the relaxation and that the activation energy of the interfacial diffusion is 1.6eV.

Discontinuous precipitation in {100} planes in a Cu–5.7 wt% Ag alloy single crystal

The formation and growth of disc-shaped aggregates consisting of rod-shaped Ag precipitates and Ag-depleted Cu matrix in the {100} planes in single crystals of a Cu–5.7 wt% Ag alloy have been investigated by transmission electron microscopy. The formation of the aggregates begins on dislocations and their growth progresses through a discontinuous process without existing dislocations. The aggregates grow at a constant velocity in an isothermal condition, maintaining a constant inter-rod spacing. A kinetic analysis of the aggregate growth using the model of Zener has yielded values of the diffusivity of Ag in Cu. The estimated activation energy for diffusion is 185 kJ mol−1.

Mechanical Properties of Cu-4.0wt%Ni-0.95wt%Si Alloys with and without P and Cr Addition

The effects of P and Cr addition and two-step aging on the microstructure and mechanical properties of a Cu-4wt%Ni-0.95wt%Si alloy have been examined. The addition of 0.02wt%P improves both strength and elongation because it suppresses discontinuous precipitation reaction. The Cr addition to the Cu-Ni-Si-P alloy decreases greatly the grain size, resulting in an increase in elongation. Two-step aging, pre-aging at 300 or 350oC and subsequent second-step aging at 450oC, causes an increase in strength without reducing elongation. The increase in strength is attributable to the decrease in inter-precipitate spacing by the two-step aging. The two-step aged Cu-4wt%Ni- 0.95wt%Si-0.02wt%P-0.02wt%Cr alloy attains a tensile strength of 830MPa, an elongation of 13% and an electrical conductivity of 35%IACS.

Effect of applied stress on precipitation of Guinier–Preston zones in a Cu–0.9 wt.% Be single crystal

A stress-oriented formation of plate-shaped Guinier–Preston (GP) zones in single crystals of a Cu–0.9 wt.% Be alloy aged at 200°C has been found. A compressive stress applied in the [001] direction assists the formation of the GP zones perpendicular to the [001] axis, while a tensile stress results in the preferential formation of the GP zones parallel to the stress axis. The applied stress strongly affects the nucleation of the GP zones. Free ageing at 275°C after compressive-stress ageing at 200°C produces plate-shaped γ′′ precipitates perpendicular to the stress axis, indicating a continuous change in the structure of the GP zones into that of the γ′′ phase. From length-change measurements during ageing, the misfit strains of the GP zone or γ′′ precipitate in directions parallel and perpendicular to the plate plane are estimated, respectively, as −0.01 and −0.10 for the GP zone and −0.01 and −0.11 for the γ′′ precipitate.

Determination of activation energy for nanometre-scale grain-boundary sliding in copper

Abstract The activation energy Q for grain-boundary-dependent sliding in copper has been measured below 573 K from experiments using an electron microscope on Cu-Fe-Co alloy bicrystals with various [011] symmetric tilt boundaries. The technique utilizes the sliding-induced rotation of moire fringes in b.c.c. Fe-Co particles on grain boundaries and is capable of detecting sliding of less than 1 nm. It is concluded that grain boundaries slide in a viscous manner with inherent viscosity \ eta = \ etaoT exp(QIRT), where \ etao is a constant related to the boundary structure. The misorientation angle dependences of the values of Q and \ etao display cusps and are similar to that of the boundary energy in copper. The larger the degree of disorder in the atomic arrangement at a boundary, the lower is the boundary viscosity.