別紙４（課程博士）
Date of Submission:

平成	  27	  年	  	  1 月	  	 Department

Student ID Number

機械工学専攻学籍番号Applicant’s name

第 083202 号

氏名

戸高 義一
Supervisors

指導教員

足立 望

16 日

伊崎 昌伸

Abstract論 文 内 容 の 要 旨 	  ( 博 士 )Title of Thesis
博士学位論文名

Tailoring mechanical properties of bulk metallic glass through controlling microstructure byplastic deformation(塑性変形による組織制御に基づくバルク金属ガラスの力学的高機能化)(Approx. 800 words)(要旨	  1,200 字程度)
Bulk metallic glasses (BMGs) have excellent mechanical properties as structural materials, such as highstrength, fatigue limits, and corrosion resistance. However, BMGs fracture with zero ductility, especially under
tensile stress. Such catastrophic failure is caused by a formation of single shear band across a sample, and thestrain localization owing to the significant softening within the shear band. It is widely believed that the plastic
deformation of BMGs is governed by the activation of shear transformation zone (STZ) model, which considers
stress relaxation through the local atomic rearrangement at the clusters consisted of a few to hundreds of atoms.
The preferential STZ activation sites are expected to have larger free volume (FV) fraction because such site canrearrange the structure without affecting surrounding. The cause of poor plasticity in conventional BMG is small
number of STZs activated during loading. In order to improve plasticity of BMGs, increasing the number of STZ
activation during loading and controlling the easiness of STZ activation are quite important.
As the STZ activation seems to occur at the site with higher FV, FV is thought to be the key factor tocontrol the mechanical properties of BMGs. The FV fraction is easy to reduce from the as-produced state throughstructural relaxation by sub-Tg annealing. The only way to introduce FV is plastic deformation. However, there isno valid and feasible deformation process to introduce the FV fraction in a BMG, since cracks are easy to form
during deformation process using conventional process such as cold rolling.
Alternatively, the β-relaxation, which is the single atomic diffusion-like structural relaxation process inBMGs is closely related to the STZ activation. The energy barrier for the STZ activation, WSTZ, and the activationenergy for β-relaxation, E , which. That is WSTZ ≈ E . The relation between E and glass transition temperature, Tg,
βββhave been also reported: E = 26RTg. Since the Tg is the material’s constant, these two relations imply that theβease with which STZs can be activated is strictly limited by the composition.
In this study, it is shown that the number of STZ activation sites and the WSTZ can be controlled using
high-pressure torsion (HPT) process, which can deform brittle material under high pseudo-hydrostatic pressure,and the mechanical properties of BMGs are consequently improved. The physical origin of the mechanicalproperty evolution induced by HPT process has been also investigated in this study.
FV increment after HPT process was confirmed by both density measurement and thermal analysis. Thedensity decrease caused by the HPT process indicates that the FV fraction higher than that achieved byconventional deformation process can be obtained using HPT process. The tensile strength was not decreased
even after HPT process, indicating that the cracks were not formed in the sample during HPT process.

The relaxation enthalpy, ΔΗ, owing to the structural relaxation of FV became larger with increasing applied
strain in HPT process in accordance with the density measurement. The parameters derived from the Arrhenius
temperature dependence of relaxation time in enthalpy relaxation were in good agreement with the characteristicsof β-relaxation in general BMGs reported previously. However, the activation energy distribution of β-relaxation
significantly became broader after HPT process, indicating that the site with lower activation energy for
β-relaxation was generated by HPT process. Considering the E is equivalent to WSTZ, the decrease in yield stress
βcan be well explained. Furthermore, since the E has broad distribution comparing with general BMGs, STZs canβbe activated continuously throughout wide stress range, and strain hardening therefore occurred in theHPT-processed sample.
Nanoscale microstructure introduced by the HPT process was investigated by the unique technique
combining small-angle X-ray and neutron scattering technique, to evaluate the structural origin of pronounced
β-relaxation appeared after the HPT process. It is found that Cu should be segregated near the FV to explain themeasured scattering intensity. The segregation of the smallest Cu atoms in the FV seems effectively works toreduce the E and to improve the mechanical property.
βAfter the HPT-process, the tensile plastic elongation of 0.34 % was obtained in Zr50Cu40Al10 BMG. The
yield stress showed significant decrease from 1800 MPa to 1477 MPa after HPT process, while tensile strength
increased over 100 MPa after HPT process. During plastic deformation, HPT-processed sample exhibited strain
hardening, which cannot be observed in conventional BMGs deform via shear band formation. The shear bandsobserved in samples after tensile test could not explain the plastic elongation achieved in the tensile test. Whenthe stress above yield stress is applied to HPT-processed sample, the stress was relaxed with the same timedependence as β-relaxation without forming shear band, whereas nondeformed sample showed no stressrelaxation. This result indicates that the plastic deformation in HPT processed sample is governed by thepronounced β-relaxation.
The one of the biggest advantage of crystalline materials is easily controllable mechanical property
after the production by heat treatment and/or deformation processes. In the field of BMGs, in contrast, it
has long been recognized that mechanical properties are decided by the composition. However, this study
revealed that such compositional limitation could be overcome using the plastic deformation process, andthe key factor governing the plastic deformation in BMGs is β-relaxation process. These findings open up
new avenues for designing plastic bulk metallic glasses focusing on the β-relaxation.