NEWS LETTERNEWS LETTERJune 2007 No.2

http://zaiko6.zaiko.kyushu-u.ac.jp/spd/index_e.html

Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects Program Leader Zenji Horita (Kyushu Universty)

Establishing facilities for production of high density lattice defects through giant straining process (presented by A01(a) group)

Cryo-ball milling facility for ultimate straining at Ritsumeikan University

Ball milling under liquid nitrogen temperature (Cryo-ball milling) is feasible with the facility shown in Fig.3. A maximum strain of ~10 is introduced into a material by the ball milling process and such a high strain is stored in the material. High-density lattice defects are then expected to be created and they facilitate to produce various phenomena.

HPT facility with capacity of 20kN at Toyohashi University of Technology

A bigger version of atmosphere-controlled 20kN-HPT facility is shown in Fig.2. This facility can produce disk-shape samples (maximum 40 mm in diameter and 1 mm in thickness) with high-density lattice defects through torsion straining under high compression pressure. Various processing conditions such as strain rate, ambient temperature, atmosphere and so on can be controlled precisely with this facility. Furthermore, changes in specimen temperature, load and torque can be measured.

ECAP facility having back pressure at Kyushu UniversityFigure 1 is a view of a modified facility: not only ECAP tools last longer but also it is feasible with harder materials including application of back-pressure. It is now possible to produce ECAP samples up to dimensions of 15 mm in diameter and 100 mm in length.

An important role of A01(a) group is to provide samples to other research members. Facilities for GSP processes are then newly installed in this project. In Kyushu University, ECAP facility was upgraded so that ECAP is performed to create high-density lattice defects in a controlled manner. Toyohashi University of Technology has installed a 20kN-HPT facility so that it is now possible to produce bigger samples with various factors controlled such as temperature, rotation speed, atmosphere, etc. Cryo-ball milling process became feasible in Ritsumeikan University. Ball-milling is performed in a liquid nitrogen temperature so that ultimate straining process is realized with this facility.

Fig. 2 (a) View of precise-controlled 20kN-HPT facility and (b) anvils with atmosphere-controlled system.

Fig.1 View of modified ECAP facility with back pressure.

Fig.3 Outlook of cryo-ball milling facility

June 2007 No.2

Metallic materials are usually hardened when they are plastically deformed. However, this is correct for high purity Al if imposed strain is small. When the strain exceeds a certain limit, hardening does not occur but rather softening follows. The result was presented at ReX&GG (Recrystallization and Grain Growth) in Jeju Island, Korea. As shown in the figure, this strain limit is about 2 when ECAP is used. The softening is more intense as the purity level is higher. Dislocation theory says hardening occurs by the creation of dislocations. However, the dislocations disappear after further straining according to the microstructural observations. The softening does not happen in Cu and Au even when the purity is high. This suggests that the stacking fault energy (SFE) has a significant effect on the grain refining behavior. Dislocations are easy to move and therefore to annihilate because of high SFE in Al. Due to this inherent nature of Al, the grain size of high purity Al is hardly lowered below one micron. How and where dislocations disappear can be a key subject of the examination.The proceeding of the ReX&GG appears in Mater. Sci. Forum, Vol.558-559, pp.1273-1278 (2007).

Quantitative correlation between plastic strain and fine-grained microstructures in low carbon steel bars fabricated by multi-pass warm caliber rolling was made clear using 3D finite element and EBSD analyses. The maximum plastic equivalent strain was 5.9. The relationships between the plastic strain and the microstructural features such as misorientation angle, grain size and texture were examined. Elementary process of dislocation emission from a Frank-Read type dislocation source in a crystal grain was studied in detail by a three-dimensional dislocation dynamics (DD) simulations. A new theory of continuum mechanics-based crystal plasticity was developed on the basis of the DD simulation results. The new theory successfully reproduced the grain size-dependent yield phenomenon of metal polycrystals (Fig.). A higher-order extended crystal plasticity model was also developed. The theory incorporates influences of the geometrically necessary dislocation-induced internal stress fields and extra-boundary conditions for crystallographic slip. With this theory, we expect to predict the mechanical responses of fine-grained polycrystalline materials more rigorously. Phase field simulations for the microstructural evolution of fine-grained materials has been performed in order to investigate the forming mechanism of the bimodal structure in grain size distribution. It is found that development of the bimodal structure is attributed to anisotropy in grain boundary mobility.

Within the sub-group A02(c), collaborative studies on peculiar mechanics

of giant-strained metals have been energetically started between Tokyo Inst.

Tech., Ibaraki Univ. Kyushu Univ. and Osaka Univ. As one of the recent

results, interesting mechanical properties of ultrafine grained pure-Ti have

been found. Figure shows stress-strain curves of the commercial purity Ti

ARB processed and annealed. Both strength and ductility continuously

changed with increasing annealing temperature, so that a good strength-

ductility balance was achieved in this material. The results were presented

at the 11th World Conf. on Titanium held in Kyoto: “Ultrafine-Grained CP-

Ti fabricated by Severe Plastic Deformation and Annealing”, D.Terada,

S.Inoue, H.Kitahara and N.Tsuji.

Fig.

High purity Al is softened with straining by process of severe plastic deformation

UFG-Ti Managing both Strength and Ductility

A01(a)

Macro- to meso-scopic mechanical response and microstructure evolutionA01

(b)

A02(c)

Zenji Horita (Kyushu University), Minoru Umemoto (Toyohashi University of Technology), Kei Ameyama (Ritsumeikan University), Yoshikazu Todaka (Toyohashi University of Technology)

Masaharu Kato, Susumu Onaka (Tokyo Institute of Technology), Kenji Higashida (Kyushu University), Yo Tomota (Ibaraki University), Nobuhiro Tsuji (Osaka University)

Tetsuya Ohashi (Kitami Institute of Technology), Mitsutoshi Kuroda (Yamagata University), Yoshiyuki Saito (Waseda University), Tadanobu Inoue (National Institute for Materials Science)

June 2007 No.2

In ultra-grained materials, it is difficult to maintain dislocation sources and form dislocation cells or sub-grains in the grains; therefore, it can be presumed that the grain boundary becomes an important dislocation source and sink. The right figure shows the interaction between dislocations and the grain boundary-dislocation pile-up, dislocation absorption, and dislocation transmission-expressed by the multi-scale atomic simulations*. The influence of the grain boundary structures on the above phenomenon is investigated, and the critical forces on the dislocation in small-angle tilt grain boundaries for it to eject from the boundaries are evaluated.*T. Shimokawa, et.al., Phys. Rev. B, Vol. 75, pp.144108(1-11) (2007)

A severe bulk strain, ~1, intentionally introduced into an Al-Ag specimen by the Equal-Channel Angular Pressing (ECAP) process, caused almost spherical Guinier-Preston (GP) zones and {111} planar phases to be sheared by dislocation motion. Distortion of GP zones in the Al-Ag system was first observed by Nicholson and Nutting, but the 3-dimensional morphology of the deformed GP zones was not studied. The presence of fine scale distorted GP zones parallel to the {111} slip planes using Z-contrast electron tomography (3D-ET). The GP zones lie within localized shear bands, which result from the introduction of the severe strain. The local shear strain measured at the nanoscale within the shear band was determined to be ~1.8 – 0.2.[See also; Inoke et al, Acta Mater, 54, 2957-2963, (2006).]

In the group A03(f), first-principles calculations of defects, dislocations and boundaries are performed in order to understand the basic structural and mechanical properties of high-density defect materials. Large-scale calculations based on the density-functional theory are performed by using recent efficient algorithms. Figure shows an example of the power of the ab initio scheme to reproduce the stable lattice constants of fcc metals within errors less than 2% compared with experiments (Wang, Tanaka and Kohyama). Optimization of the lattice constant of fcc Cu by the

PAW scheme using QMAS code developed in AIST (Ishibashi et al.).

Z-contrast 3D-ET and ECAP

Test calculations of bulk metals by the first-principles method

A02(d)

A03(e)

A03(f)

Microdynamics Analysis on Advanced Materials Containing Ultra-High Density Lattice Defects

Masahiko Morinaga (Nagoya University), Yoshiaki. Akiniwa (Nagoya University), Tetsu. Ichitsubo (Kyoto University), Hideharu Nakasima (Kyushu University), Kenji Kaneko (Kyushu University), Tadakatsu Ohkubo (NIMS)

Masanori Kohyama (National Institute of Advanced Industrial Science and Technology), Shingo Tanaka (National Institute of Advanced Industrial Science and Technology), Masato Yoshiya (Osaka University), Tokuteru Uesugi (Osaka Prefecture University)

A. Nakatani (Osaka Univ.), T. Shimokawa (Kanazawa Univ.), K. Saito (Kansai Univ.), T. Matsushima (Tsukuba Univ.)

Contact AddressZenji Horita, ProfessorDepartment of Materials Science andEngineering, Faculty of Engineering, Kyushu University,Fukuoka, 819-0395, Japan.TEL: +81-92-802-2958FAX: +81-92-802-2992e-mail:horita@zaiko.kyushu-u.ac.jp

June 2007 No.2

Latest information

Bulk Nanostructured Materials (BNM): from fundamentals to innovations August 14-18, 2007, Ufa, Russia

http://www.ipam.ugatu.ac.ru/bnm-2007

6th Pacific Rim International Conference for Materials (PRICM-6), a part of session 6 dedicated for SPD

November 6-9, 2008, Jeju Island, Korea http://www.pricm-6.org

2008 TMS Annual Meeting, including a session on Ultrafine Grained Materials, Fifth International Symposium (UFG V)

March 9-13, 2008, New Orleans, LA, USA http://www.nanoSPD.org

The 4th International Conference on Nanomaterials by Severe Plastic Deformation (NanoSPD4)

August 18-22, 2008, Goslar, Germanyhttp://www.nanospd4.org

New Contributed Reserch Members (Joined in April 2007)

Development of Ultrafine Grained Structures during Severe Plastic Deformation and the Thermal Stability

Multiscale Crystal Plasticity Simulation for Ultrafine-Graining at Giant Strain Based on Dislocation Patterning

Characterization of deformation behavior in the vicinity of grain boundaries for ultra-fine grained materials

Additional strengthening by high density dislocation alignment in severe plastic deformed alloys

Wear behavior of Advanced Materials Containing Ultra-High Density Lattice Defects

Crack Propagaion Behavior in Ultrafine Grained Materials

Effect of hydrogen on mechanical properties of ultrafine-grained materials produced by severe plastic deformation processes.

Molecular dynamics simulation to understand phenomena around grain boundaries of polycrystalline metals

Atomistic simulation study of the Influences of Giant Straining History on the Formation of Fine Grained Structure and Mechanical Properties

Positron microprobe analysis of open-volume type defects induced by severe plastic deformation

Quantitative analysis of defects in severely-deformed materials by mechanical-loss spectroscopy

Positron annihilation study on severe plastically deformed materials

Grain boundary structure and its stability in advanced materials with giant straining process

A01(a)

A01(b)

A01(b)

A02(c)

A02(c)

A02(c)

A02(c)

A02(d)

A02(d)

A03(e)

A03(e)

A03(e)

A03(e)

Taku Sakai

Kazuyuki Shizawa

Takahito Ohmura

Mayumi Suzuki

Yoshimi Watanabe

Hiromoto Kitahara

Takumi Haruna

Toshihiro Kameda

Ryosuke Matsumoto

Masanori Fujinami

Hiroshi Numakura

Hideki Araki

Seiichiro Ii

UEC Tokyo (University of Electro-Communications)

Keio University

National Institute for Materials Science

Graduate School of Environmental Studies, Tohoku University

Nagoya Institute of Technology

Kumamoto University, Graduate School of Science and Technology

Kansai University

University of Tsukuba, Graduate School of Systems and Information Engineering

Kyoto University, Graduate School of Engineering

Chiba University, Faculty of Engineering

Osaka Prefecture University, Graduate School of Engineering

Osaka University, Graduate School of Engineering

Sojo University, Faculty of Engineering

Todaka awarded the Joint JIM/TMS Young Leaders International Scholar Program

Todaka received the trophy as the Young Leader International Scholar at the TMS ceremony in 2007.