Tadahiko Furuta

Thermomechanical properties of P/M β titanium metal matrix composite

Abstract A high performance β titanium metal matrix composite (TMC) reinforced with in situ TiB particles has been developed via a low-cost blended elemental (BE) powder metallurgy process. The TMC shows anomalous mechanical properties as a titanium based material in tensile and fatigue strength, Youngs modulus and wear resistance. Besides the high performance, the TMC has the potential to be fabricated at a lower cost far below that of the practical titanium alloys made by the conventional ingot metallurgy process. The as-sintered density of the TMC reached to 99% of theoretical with a unique activation sintering mechanism and has an excellent hot-workability superior to that of the conventional unreinforced Ti–6Al–4V alloy.

Phase-stability dependence of plastic deformation behavior in Ti-Nb-Ta-Zr-O alloys

The authors investigated the effects of alloy content on mechanical properties to make clear a correlation between plastic deformation behavior and β-phase stability in Ti-Nb-Ta-Zr-O alloys. It was realized that there was specific compositional area in which the alloy exhibited little work hardening and minimum Young’s modulus value. The specific area was expressed by the bond order (Bo), based on theDV-Xα method, of 2.87 and the averaged electron/atom ratio (e/a) of 4.24, which corresponded to those of a multifunctional β titanium alloy, “Gum Metal.” These electronic conditions also minimized ideal strength required for plastic deformation without any dislocation activity. The deformation behavior of alloys in the specific compositional area revealed that the unique behavior could be characterized by a “giant fault.” It was also confirmed that such a compositional area corresponded to the phase boundary between the α″ martensite and β phases at room temperature.

Lattice softening for producing ultrahigh strength of iron base nanocrystalline alloy

One can increase the strength of metallic materials by pinning dislocations with nanoscale obstacles, as the dislocations facilitate plastic deformation. However, simultaneous achievement of the ultrahigh strength and the ductility is extremely difficult in conventional metallic materials. Here we show that the ultrahigh strength iron base alloy with enhanced ductility, whose strength is approaching ideal strength and being twice as much as the upper limit of conventional alloys, can be realized by introducing the paradox concept of lattice softening. Designing atomic arrangement with specific electronic structure creates the lattice softening, and a nanograined structure is then produced by subsequent processing with severe plastic deformation.

High Strength and High Uniform Ductility in a Severely Deformed Iron Alloy by Lattice Softening and Multimodal-structure Formation

Despite high strength of nanostructured alloys, they usually exhibit poor uniform ductility. For many applications, it is an important issue to design new nanostructured alloys which have both high strength and high uniform ductility. In this study, an Fe–Ni–Al–C alloy with ultrahigh tensile strength of 1.9–2.2 GPa and high uniform ductility of 16–19% was developed by concurrent employment of several strategies: (i) appropriate choice of chemical compositions for lattice softening, (ii) severe plastic deformation using the high-pressure torsion method for grain refinement, and (iii) control of strain level for multimodal-structure formation composed of equiaxed nanograins, lamellar coarse grains and fine precipitates.

Severe plastic deformation in Gum Metal with composition at the structural stability limit

Abstract The effects of severe plastic deformation on the mechanical properties and deformation behavior of Gum Metal with a nominal composition of Ti-36Nb-2Ta-3Zr-0.3O (mass.%) were investigated using high-pressure torsion (HPT). Applying the HPT process, the alloy shows ultra-low Youngs modulus, high strength, extended elastic limit, high ductility, and very low hardening during HPT. The tensile strength of Gum Metal increases with increasing equivalent strain, while work hardening is much smaller than reported values for other common metallic materials. The deformation structure is composed of ultrafine grains produced by transgranular shear through application of HPT. These results suggest that the strengthening mechanism of Gum Metal is attributable to a unique plastic deformation which operates in the region where the local stress reaches values close to that of ideal strength.

Thermomechanical Studies of Yielding and Strain Localization Phenomena of Gum Metal under Tension

This paper presents results of investigation of multifunctional β-Ti alloy Gum Metal subjected to tension at various strain rates. Digital image correlation was used to determine strain distributions and stress-strain curves, while infrared camera allowed for us to obtain the related temperature characteristics of the specimen during deformation. The mechanical curves completed by the temperature changes were applied to analyze the subsequent stages of the alloy loading. Elastic limit, recoverable strain, and development of the strain localization were studied. It was found that the maximal drop in temperature, which corresponds to the yield limit of solid materials, was referred to a significantly lower strain value in the case of Gum Metal in contrast to its large recoverable strain. The temperature increase proves a dissipative character of the process and is related to presence of ω and α″ phases induced during the alloy fabrication and their exothermic phase transformations activated under loading. During plastic deformation, both the strain and temperature distributions demonstrate that strain localization for higher strain rates starts nucleating just after the yield limit leading to specimen necking and rupture. Macroscopically, it is exhibited as softening of the stress-strain curve in contrast to the strain hardening observed at lower strain rates.

Infrared thermography applied for experimental investigation of thermomechanical couplings in Gum Metal

Abstract Results of initial investigation of thermomechanical couplings in innovative β-Ti alloy called Gum Metal subjected to tension are presented. The experimental set-up, consisting of testing machine and infrared camera, enabled to obtain stress–strain curves with high accuracy and correlate them to estimated temperature changes of the specimen during the deformation process. Both ultra-low elastic modulus and high strength of Gum Metal were confirmed. The infrared measurements determined average and maximal temperature changes accompanying the alloy deformation process, allowed to estimate thermoelastic effect, which is related to the alloy yield point. The temperature distributions on the specimen surface served to analyse strain localization effects leading to the necking and rupture.

Shear Strength Measurement of Gum Metal during High-Pressure Torsion

In the present study, in situ measurements of applied torque and compressive load were conducted during high-pressure torsion (HPT) on Ti-23%Nb-0.7%Ta-2.0%Zr-1.2%O (in at %) , Gum Metal, by using four active strain-gage method. The shear stress was then calculated from the measured torque. The in situ measurements revealed that the maximum shear stress reaches ~2 GPa during HPT. This value is comparable to the ideal shear strength of Gum Metal, which was reported as ~1.8 GPa from experiments using single crystals. The deformation mechanism strongly depends on body-centered cubic (bcc) phase stability at an early stage of HPT straining, where the shear stress is well below the ideal shear strength. On the other hand, the deformation mechanism may be insensitive to the bcc phase stability at a later stage of HPT straining, where plastic deformation occurs at a strength close to the ideal shear strength.

Deformation in Ti-Nb-Ta-Zr-O Alloy at Near Ideal Strength

Recent experimental results on phase stability and deformation behavior in a multifunctional Ti-36Nb-2Ta-3Zr-0.3O alloy, Gum Metal, are summarized and its deformation mechanisms are discussed. The crystal structure of the alloy is essentially unstable to tensile loading in <110> direction, but the microstructure of the cold worked state stabilizes the crystal structure. Work hardening in Gum Metal was far smaller than the other materials even when huge amount of strain is accumulated by severe plastic deformation. By comparing actual applied stress during plastic deformation with ideal shear strength, the alloy is likely to deform at near ideal strength with stress concentrations along with the highly inhomogeneous deformation behavior.

Mechanical Properties of Al‐Zn‐Mg‐Cu Alloys Processed with High‐Pressure Torsion

It has been reported that tensile strength of the 7075 alloy, a commercial age-hardenable Al-Zn-Mg-Cu based alloy, is significantly improved by high-pressure torsion (HPT). The present research has been performed to study the effects of alloy compositions and process conditions on mechanical properties in Al-Zn-Mg-Cu alloys processed with HPT. Several Al-Zn-Mg(-Cu) alloys were melted and cast, and disc specimens of 10 mm diameter and 1 mm thick were machined from the homogenized ingots. The disc specimens were solution treated and subjected to HPT with a compression stress of 2 GPa at a rotation speed of 1 rpm. The torque at the steady state increased with increasing amount of alloying elements. The strength after HPT also increased with increasing amount of alloying elements. The tensile strength of Al-10%Zn-2%Mg-2%Cu alloy, in mass %, was increased to about 900 MPa by 10 turns of HPT processing.