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77 巻 (2013) 9 号 p. 391-397
Mg-Al-Ca 系ダイカスト合金における高温クリープ変形中の転位組織解析
寺田芳弘, 村田純教, 里達雄

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and explain the creep properties of the Mg-Al-Ca AX52 die-cast alloy at 473 K. TEM observations indicate that dislocations are introduced within the primary α-Mg grain interior in the die-casting process, which consist of both basal and non-basal segments. The non-basal segments of the dislocations, having smoother curvature in the as die-cast state, partially exhibit steps parallel to the basal plane during high temperature exposure. The basal segments of the dislocations bow out and glide on the basal planes under stress, and the jogs follow the basal segments with the help of climb during creep. The easy glide of the basal segments of the dislocations controls the creep rates immediately after the stress application of the creep tests, while the creep mechanism for the alloy has been identified as the dislocation climb. By comparing the dislocation movements for the Mg-Al-Ca AX52 die-cast alloy with those for the Mg-Al AM50 die-cast alloy, it is inferred that the eutectic intermetallic phase covering the primary α-Mg grains decreases the climb velocity of the jogs during creep.

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2014年論文賞
76 巻 (2012) 7 号 p. 429-436
中炭素鋼の温間変形抵抗と加工材強度
越智達朗, 宮西慶, 戸田正弘, 加田修

The purpose of this study is to clarify the optimum approach to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working of medium carbon steel. Increase in warm deformation resistance during warm working by addition of Cr is small. On the other hand, Cr has the effect of suppressing the annihilation of dislocations in the air cooling process after warm working. Thus, by adding Cr, the tensile strength after warm working is effectively improved, without increasing warm deformation resistance during warm working. Increasing of Si is effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working, too. Si has an effect of the solid solution hardening. This effect by increasing Si is due to the large temperature dependence of solid solution hardening effect. The control of microstructural factors, that are pearlite fraction and cementite spacing, is not effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working.

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2013年論文賞
76 巻 (2012) 7 号 p. 449-455
Ti-4Fe-7Al 合金の焼戻しに伴う特異現象
竹元嘉利, 越智昌宏, 瀬沼武秀, 高田潤, 清水一郎, 松木一弘

Unprecedented phenomena were discovered by tempering the Ti-4Fe-7Al alloy quenched from the β (bcc) field. The alloy became very hard when it was tempered at 450℃ for several minutes, and severely rugged surface was generated. The inverse shape recovery phenomenon was also discovered when a quenched specimen that had been bent at room temperature was heated. The tempered microstructure showed almost β grains and some of the usual martensitic acicular structure areas. However, electron back scattering pattern (EBSP) measurements showed that the β-like grain was not the bcc structure but was the hcp or orthorhombic structure. X-ray diffraction (XRD) measurements clarified that an orthorhombic α″ structure (a=0.299₅ nm, b=0.491₃ nm, c=0.465₉ nm) was formed from the β phase by tempering. Moreover, this α″ structure was confirmed to be a type of martensitic transformation because no concentration distribution was detected in scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis of the microstructure. It was suggested that the essential M_s point of the alloy should be higher than room temperature; however, the martensite transformation could not operate by fast quenching.
The newly-discovered α″ martensite is formed without atomic diffusion by heating. When the β grain transforms into the single α″-variant, a very huge lattice strain is generated, resulting in the severely rugged surface or the inverse shape recovery phenomenon.

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2013年論文賞
76 巻 (2012) 10 号 p. 597-606
押込み試験とモデリングによるベキ乗則材料の単軸クリープに関する構成式の予測
高木秀有, 道明, 藤原雅美

Indentation creep tests and finite element simulations were performed on a model material to show that a constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate maintain constant values of p_s and ε_in(s), respectively, the contours of the equivalent stress and the equivalent plastic strain rate in the region beneath the conical indenter expand according to the increase in the indenter displacement while maintaining the geometrical self-similarity. These findings indicate that a pseudo-steady state deformation takes place around the indenter tip. The representative point exhibiting the creep behavior within the limited region, which actually determines the indenter velocity, is defined as the location where the equivalent stress σ_r is equal to p_s/3. The equivalent plastic strain rate ε_r at this point is found to be ε_in(s)/3.6 in the case that the creep stress exponent is 3. The stress exponent and the activation energy for creep extracted from the results of Al-5.3 mol%Mg solid-solution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for σ_r and ε_r agree well with the values for the applied stress and the corresponding creep rate in tensile creep tests at the same temperature. The above results show not only that the creep characteristics of advanced materials, which are often available in minute quantities or as small-volume specimens, can be obtained from carefully designed indentation creep tests, but also that the constitutive equation for tensile creep can be predicted with sufficient precision through indentation creep test results.

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2013年論文賞

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