Naoyuki Nagasako

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.

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.

Strengthening of Alloys with Elastic Anisotropy by Severe Plastic Deformation

Effects of the elastic anisotropy on deformation behavior are examined in a Ti-23%Nb-0.7%Ta-2%Zr-1.2%O (in at %) alloy, Gum Metal, and in an Fe-19%Ni-34%Co-8%Ti alloy with body centered cubic (bcc) crystal structure, and microstructural development in the iron based alloy during severe plastic deformation (SPD) process is discussed. Strong elastic anisotropy with reduced shear modulus, C11- C12, results in low ideal shear strength, which implies dislocation mediated plasticity easily occurs at lower stress. On the other hand, high pressure torsion (HPT), a typical SPD method, realizes very high shear stress during processing, which seems to reach the ideal shear strength in these alloys. Significant refinement of the grain size to 20 - 50 nm in the Fe-Ni-Co-Ti alloy is discussed in relation to the unique deformation mechanism which might be activated at ideal shear strength.

Deformation mechanism and grain refinement behavior in Gum Metal

2005年に多機能チタン合金「ゴムメタル 1)」に関する解説 記事 2)を本誌に執筆させていただいた。その際,本合金が 弾性異常を伴う特殊な結晶構造安定性を有していること,ま た理想強度レベルまで転位運動が抑制され,非転位型機構に より塑性変形が進行する可能性について紹介した。非転位型 変形機構に関しては,国内外から多くのご関心をいただき, 現在に至る7年の間に多くの知見が得られた。そこで本稿の 前半では,ゴムメタルの変形機構に関する最近の研究進展状 況を紹介する。いまだ変形機構の詳細については不明な点も 残っているが,本合金が理想強度まで強化されていること や,その際の変形挙動が既存の金属材料とは異なることが客 観的な実験事実により明らかになってきた。 本合金が理想強度レベルで変形することから,加工中の組 織変化も従来金属材料とは異なる特徴を有する。そこで後半 では本特集号の主旨にも合せて,ゴムメタルの冷間加工中の 結晶粒微細化挙動に関する筆者らの研究内容をまとめた。な お,ゴムメタルはTi–Nb系をベースとした合金であるが,非 常に限られた組成範囲の特殊な合金であることにご注意いた だきたい。本合金の周辺組成を有する合金の変形機構や結晶 粒微細化挙動については,形状記憶・超弾性合金や生体用低 ヤング率合金の開発の観点から,多くの系統的な研究がなさ れている 3)。本稿ではそれらについては言及しないが,ゴム メタルの変形機構に関連するものについては一部紹介させて いただく。