Kouichi Maruyama

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Abstract Fine-grained alloys of Mg-3Al-1Zn-0.2Mn in wt.% (AZ31B) were obtained by an equal-channel angular extrusion technique and subsequent annealing at elevated temperatures. Tensile tests were performed at room temperature at a strain rate of 1x10-3 s-1. The alloys exhibited an apparent steady-state deformation region and a large tensile elongation of 47%. The deformed microstructure at an elongation of 2% indicated substantial cross-slip to non-basal planes induced by plastic compatibility stress associated with grain boundaries. The non-basal segment of dislocations was found to consist of 40% of the total dislocation density at a yield anisotropy factor of only 1.1 instead of an expected value of 100 obtained from single-crystal experiments. The deformed microstructure at an elongation of 16% indicated recovered regions within twins as well as untwinned matrices. These results indicate that dynamic recovery can occur in Mg alloys at room temperature.

Strengthening effect of Zn in heat resistant Mg–Y–Zn solid solution alloys

Abstract The addition of zinc is effective in improving creep strength of Mg–Y solid solution alloys at temperatures 550–650 K because zinc suppresses the non-basal slip that operates predominantly at such temperatures. This suppression is caused by the formation of planar defects on the (0xa00xa00xa01) plane and confinement of dislocations in the basal plane due to a decrease in stacking fault energy.

Effect of W on recovery of lath structure during creep of high chromium martensitic steels

Effect of W on creep strength of martensitic steels was investigated paying special attention to microstructural degradation during creep. Though tempered martensitic lath structure is stable at the elevated temperatures without stress, its recovery takes place substantially during creep. After the recovery, lath width and dislocation density in lath interior reached the stationary values determined by creep stress. There was no difference in the stationary values between the two steels with (TAF650 steel) and without (Mod.9Cr–1Mo steel) W. However, recovery processes of the lath structure are significantly different between the two steels. The growth of lath width and the annihilation of dislocations in lath interior are slower in W containing TAF650 steel than those in Mod.9Cr–1Mo steel. Accumulation of creep strain is suppressed in TAF650 steel because of the slow recovery of its lath structure. The retardation of the recovery of lath structure results in the lower creep rate and the higher creep rupture strength of the W containing steel. The slow recovery of lath structure in the W containing steel is ascribed to pinning effect of M23C6 and Laves phase (Fe2W) precipitated on lath boundaries.

Void formation by thermal stress concentration at twin interfaces in Cu thin films

A void formation mechanism was investigated in an electroplated copper thin film on Ta/SiO2/Si. Microstructural observation after thermal cycling indicated that void formation occurred at intersecting points or terminating corners of annealing twins. The calculated stress distribution was compared with experimental results of the void formation tendency. An excellent correlation was found between void formation sites and stress concentration sites. Electron diffraction analysis revealed that most twin interfaces in Cu thin films are incoherent {322} planes. The stress concentration drives diffusion along incoherent twin interfaces of {322} and leads to void formation at twin interfaces and corners.

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys.

Stress-induced phase transformation during superplastic deformation in two-phase Ti–Al–Fe alloy

Abstract Ti–5.5Al–1Fe alloys consisting of the h.c.p.-α phase and the b.c.c.-β phase were investigated for microstructural changes during superplastic deformation at temperatures from 1050 to 1200xa0K. Observed changes occurred in two steps: (1) agglomeration of the β phase to grain boundaries perpendicular to the tensile axis and (2) subsequent increase of the β volume fraction. The β volume fraction after failure was found to increase with increasing deformation temperature. The first step was considered to be induced by the gradient of traction force acting upon grain boundaries. The second step was considered to be induced by stress concentration at grain boundaries of the α phase where the β phase was depleted by agglomeration to the perpendicular boundaries. The phase equilibrium under stressed condition was calculated by increasing the Gibbs energy of the α phase by 500xa0J/mol relative to that of the β phase. An excellent quantitative agreement was found between calculated results and experimental results of the β volume fraction and the Fe composition in each phase. The present work indicates that the phase transformation accompanied by diffusion can be induced by application of stress of the order of 100xa0MPa. This new type of stress-induced phase transformation can decrease the β transus temperature by more than 100xa0K.

Effects of crystallographic texture on stress-migration resistance in copper thin films

The crystallographic texture of heat-treated Cu thin films and its effects on stress-migration resistance were studied as a function of film thickness within a range of 50–900 nm. All as-deposited films had (111) texture. After heat treatment at 723 K, texture transition from (111) to (100) was observed in films of thickness greater than 300 nm. The (111) texture films after heat treatment showed severe stress migration; in contrast, the (100) texture films showed no noticeable stress migration. The observed stress-migration resistance in the (100) texture films can be attributed to the absence of twins and to lower thermal stress as compared with the (111) texture films.

Effects of lamellar spacing on mechanical properties of fully lamellar Ti–39.4mol%Al alloy

Abstract Effects of lamellar spacing on mechanical properties of a fully lamellar TiAl alloy were investigated at room temperature and at 950K by compression tests. Lamellar spacing was varied from 20 to 590 nm, keeping the same grain size of 90 μm. The Hall–Petch relation holds between lamellar spacing and yield stress of the alloy. However, the yield stress saturates at a value of about 1 GPa below a critical lamellar spacing of about 100 nm. The Hall–Petch slope, critical lamellar spacing and saturation stress can be explained consistently by the pile-up model of dislocations at lamellar interfaces. Contrary to the beneficial effect on yield stress, the refinement of lamellar spacing deteriorates ductility of the alloy probably due to the suppression of the hard deformation modes.

Microstructural influences on stress migration in electroplated Cu metallization

Stress migration in advanced Cu interconnects leads to device failure and to poor production throughput. In this work, microstructural effects on stress-migration resistance were investigated in two types of electroplated Cu metallization having a 〈111〉 texture and a random texture. Transmission electron microscopy showed incoherent twins in the 〈111〉 textured films whereas coherent twins in the random textured films. The incoherent twins were found to accompany stress-induced voids because of a weak bonding at twin interfaces. Unlike conventional Al interconnects, a strong 〈111〉 texture should be avoided to minimize stress-migration failure in Cu interconnects.

Effects of discontinuour coarsening of lamellae on creep strength of fully lamellar TiAl alloys

Abstract High temperature creep of a binary Ti-42mol%Al alloy with fully lamellar structure was studied to examine effects of lamellar spacing on creep strength. Strain hardening is more significant in a finer lamellar material, resulting in higher creep strength at high stresses. Discontinuous coarsening of lamellae takes place during creep, and is more substantial in the finer lamellar material at low stresses. Because of the microstructural degradation, the strengthening by fine lamellae diminishes at low stresses. Some specimens were annealed at high temperatures to finish the discontinuous coarsening prior to creep testing. In these specimens, the strengthening by fine lamellae becomes effective even at low stresses.