Kei Ameyama

Improved mechanical properties in dissimilar Ti-AISI 304L joints

The effect of low temperature post weld heat treatment on the tensile strength and bend test properties of dissimilar friction welds between titanium and AISI 304L stainless steel joints is investigated. Post weld heat treatment at temperatures less than 873 K has no effect on joint tensile strength properties, but markedly improves bend test properties. The highest bend angle is produced using a post-weld heat treatment at 773 K for 1 h (the Larson-Miller parameter corresponding to this treatment is 15.5×103 K h−1). Low temperature heat treatment improves bend ductility, because stress relaxation occurs with minimal increase in the transition region width at the bondline region. Dissimilar joint bend testing properties decrease markedly when the width of the transition region exceeds 1–2 μm. An explanation for the detrimental effect of thick transition regions at the joint interface region on the mechanical properties of dissimilar joints is proposed. It is suggested that the development of significant triaxial stress due to the constraint imposed by large, needle-shaped intermetallic particles promotes premature joint failure in joints containing thick transition regions.

Influence of silicon in aluminium on the mechanical properties of titanium/aluminium friction joints

The influence of post-weld heat-treatment and of residual silicon in aluminium on the mechanical properties of dissimilar friction joints between titanium and aluminium was investigated. Although joint tensile strength and bend test properties were drastically reduced following post-weld heat treatment, the responses of Ti/h.p. Al and Ti/c.p. Al joints were quite different. The tensile strength and bend test properties of Ti/h.p. Al joints were markedly decreased by heat-treatments involving shorter holding times at lower temperatures.Joint failure in post-weld heat-treated joints was associated with Al3Ti formation at the bondline region. The growth rate of the Al3Ti intermetallic layer at the joint interface was much faster in post weld heat-treated Ti/h.p. joints. More than 20 at%Si segregated in the region between the titanium substrate and the Al3Ti intermetallic phase in heat-treated Ti/c.p. Al joints. It is suggested that silicon segregation retards Al3Ti formation by acting as a barrier to titanium and aluminium diffusion at the joint interface.

The Development of High Performance Ti-6Al-4V Alloy via a Unique Microstructural Design with Bimodal Grain Size Distribution

The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique microstructure with bimodal grain size distribution, termed as “harmonic structure.” The Ti-6Al-4V compacts with harmonic structure design were successfully prepared via a powder metallurgy approach consisting of controlled mechanical milling and spark plasma sintering of the pre-alloyed Ti-6Al-4V powders. The microstructural evolution at each stage of processing has been investigated to establish a correlation between the processing conditions and the microstructural evolution. The Ti-6Al-4V compacts with heterogeneous harmonic structure exhibited better mechanical properties as compared to their homogeneous fine/coarse-grained counterparts. An attempt has also been made to explain the deformation mechanism of the harmonic-structured Ti-6Al-4V specimens with the help of the experimental evidences. The superior mechanical properties of the harmonic structure Ti-6Al-4V were found to be related to the peculiar topological distribution of strong fine-grained and ductile coarse-grained regions, which promotes uniform distribution of strain during plastic deformation and results in improved mechanical properties by avoiding the localized plastic deformation in the early stages of deformation.

Mechanical properties of pure titanium and Ti-6Al-4V alloys with a new tailored nano/meso hybrid microstructure

Abstract Pure titanium and Ti-6Al-4V alloy powders are treated by a mechanical milling process, which is one of the severe plastic deformation processes. The mechanical milling enables to produce a nano-grain microstructure very easily and has been applied to many powder materials. The bimodal microstructure in those mechanically milled powders is composed of a nano-grain structure with a grain size of about 50 nm in the surface-near region and of a work-hardened microstructure in the core region of the powder. We applied the hot roll sintering process to pure titanium and to Ti-6Al-4V mechanically milled powders. These compacts have a hybrid microstructure that consists of nano-grain structure with grain sizes of 200 to 500 nm and a micron size meso-grain structure. The hybrid microstructure materials demonstrate enhanced mechanical properties compared to conventional materials.

Fabrication of Nano Grain Tungsten Compact by Mechanical Milling Process and Its High Temperature Properties

Mechanical milling (MM), which is one of SPD process, is applied to W powder and W-Remixed powder. MM processed W and W-Re powders easily form nano grain structure even though they have high melting temperature. The nano grain formation mechanism in these powders is as follows: multi axial deformation of the powders by milling, at the temperature of 333K at most, produces pan-cake grain structure at first. Extremely dense dislocations result in grain sub-division, and finally nano grain structure with high angle boundary forms. Nano grains with approximately 10 nm in diameter are obtained. The MM powders are sintered using Spark Plasma Sintering process. Sintering and high temperature deformation behaviors of the MM powders are also investigated. The MM treatment enables W powder to be able to sinter at 1273 K while the powder without MM could never sintered at the same temperature. Re addition prevents grain growth during sintering and thus increases hardness of the compacts. A large deformation of W-10mass%Re sintered compact, whose grain size is approximately 450nm, is observed at elevated temperatures.

Superplastic behaviour of Ti-48at.%Al two-phase titanium aluminide compacts made from mechanically alloyed powder

Abstract An HIP compact of MA-processed powder having a nominal composition of Ti-48at.% Al was produced. The compact consisted of a large amount of TiAl(λ) and a small amount of Ti 3 Al ( α 2 ), in a completely ultra-fine equiaxed grain structure. This two-phase compact showed typical superplastic deformation behaviour. A maximum elongation of 550% was obtained. A strain exponent, n = 2, and grain size exponent, p = 2, were determined from the results of a strain-rate-change test and a creep test at constant initial stress using samples having various grain sizes, respectively. The activation energy for creep, Q c at constant stress was calculated to be 350 kJ/mole. It is concluded that the superplastic deformation mechanism of the material under study is grain boundary sliding controlled by lattice diffusion in the TiAl phase.

Preferred orientation relationship of intra- and inter-granular precipitates in titanium alloys

Abstract In a previous study by one of the authors, it had been reported that intra-granular α (HCP) precipitates have a Burgers orientation relationship (O.R.) with the β (BCC) matrix in Ti–22V–4Al and Ti–15V–3Cr–3Sn–3Al alloys. On the other hand, grain boundary α precipitates were shown to have a tendency for parallelism of the {110} β //{1 1 01} α planes, i.e. they display a Potter O.R. with one of the conjugate β grains. A three-dimensional near-coincidence site lattice (3-D-NCS) model, coupled with transmission electron microscope (TEM) observations, has been developed to explain these observations. The shape of an embryo predicted by the 3-D-NCS analysis is compared for the Burgers and Potter O.R., assuming the morphology is only determined by the anisotropy of the structural part of the interfacial energy. An intra-granular precipitate with Burgers O.R. has a lower free energy of formation for the critical nucleus than one with the Potter O.R., in agreement with the observations. However, α precipitates with the Potter O.R. are preferred at grain boundaries, again in agreement with the experimental observations.

Dislocation loop and cavity formation under He-ion irradiation in a Ti-rich TiAl intermetallic compound

Abstract Damage structure in a Ti-rich TiAl intermetallic compound has been investigated by transmission electron microscopy after He-ion irradiations up to 3 × 10 21 ions/m2 at 623 and 773 K. The irradiations resulted in loop-shaped or dot clusters in the γ-TiAl and α 2 - Ti 3 Al grains. The cluster density in TiAl was over one order of magnitude lower than that in Ti3Al. The clusters in TiAl irradiated to 3 × 10 21 ions/m2 at 773 K were identified as interstitial-type faulted loops lying on {111} planes with the Burgers vector direction of 〈111〉. Cavities were created along the grain boundaries as well as in the matrix in TiAl at 773 K. The EELS analysis indicated that the cavity formation was associated with injected He atoms.

Effect of bimodal harmonic structure design on the deformation behaviour and mechanical properties of Co-Cr-Mo alloy

In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmonic structure design, were successfully prepared via a powder metallurgy route consisting of controlled mechanical milling of pre-alloyed powders followed by spark plasma sintering. The harmonic structured Co-Cr-Mo alloy with bimodal grain size distribution exhibited relatively higher strength together with higher ductility as compared to the coarse-grained specimens. The harmonic Co-Cr-Mo alloy exhibited a very complex deformation behavior wherein it was found that the higher strength and the high retained ductility are derived from fine-grained shell and coarse-grained core regions, respectively. Finally, it was observed that the peculiar spatial/topological arrangement of stronger fine-grained and ductile coarse-grained regions in the harmonic structure promotes uniformity of strain distribution, leading to improved mechanical properties by suppressing the localized plastic deformation during straining.

Formation and annealing behavior of defect clusters in electron or He-ion irradiated Ti-rich Ti–Al alloys

In order to clarify the effect of He atoms on the formation and annealing behavior of defect clusters in Ti–Al alloys, a Ti–47 at.% Al intermetallic compound has been irradiated with electrons and He-ions. Helium-ion irradiation enhances the nucleation of defect clusters, especially of interstitial loops, at temperatures from 623 to 773 K in both γ-TiAl and α2-Ti3Al grains of the sample. However, there is little difference between the annealing temperature ranges of defect clusters in TiAl grains formed by He-ion or electron irradiation at 623 K. The dot-shaped clusters and interstitial loops grow scarcely during annealing, but are annihilated by annealing up to 923 K. Cavities are formed after irradiation with He-ions below 10 dpa at 773 K, but no cavities are formed by electron irradiation up to 30 dpa. The cavities in γ-TiAl and α2-Ti3Al grains survive after annealing even at 1053 K for 1.8 ks, keeping their density and diameter to be nearly the same as those in the as-irradiated grains.