Yasuhiro Morizono

Transmission electron microscopy of twins in martensite in Ti-Pd shape memory alloy

Abstract Twins in the B19 martensite in the TiPd shape memory alloy have been investigated by conventional transmission electron microscopy (CTEM) and electron diffraction. There were three twinning modes, i.e. 111 Type I, 〈121〉 Type II and 101 compound twins, in the martensite. The 111 Type I and 〈121〉 Type II twinnings which were conjugate to each other coexisted in a same martensite variant. The 111 Type I twins were dominantly observed and the 〈121〉 Type II twins were less frequently observed. The former twinning was considered to be a lattice invariant shear. The Type II twin plate appeared in two types of forms. The first one was directly connected to the Type I plate. In other words, the twin plate was inclined at crystallographically defined angle. The second one branched off from the Type I plate. Since there was no martensite variant consisting wholly of the 〈121〉 Type II twins throughout the present observations, the 〈121〉 Type II twins were considered to be a deformation twin due to the elastic interaction during the transformation. The 101 compound twinning was also considered to be a deformation twin which was introduced as result of elastic interactions during the transformation since the twin had an isolated fashion in the martensite variant consisting of 111 Type I twins.

Effect of alumina addition on hydroxyapatite biocomposites fabricated by underwater-shock compaction

Abstract Hydroxyapatite [HAp; Ca 10 (PO 4 ) 6 (OH) 2 ] biocomposites are ceramic materials that exhibit excellent bioactivity, but their inherent low fracture strength and toughness render them unusable for practical applications. Alumina (Al 2 O 3 ) is added to improve in mechanical properties without diminution of biocompatibility. Powder mixtures of HAp and Al 2 O 3 in various volume ratios (90:10, 80:20, and 70:30) were compacted by underwater shock waves generated by explosives. High explosives of 6900 m s −1 detonation velocity served as the energy generators. The compacts were sintered at 1473, 1573, and 1673 K in order to investigate the influence of sintering temperature. All compacts were found to be free of defects and to have undergone continuous compositional changes to α tricalcium phosphate and tetracalcium phosphate. K 1c showed maximum value with HAp–30vol.% Al 2 O 3 that have been sintered at 1673 K for 7.2 ks, and the value of 3.0 MPa m −1/2 is 2.5 times that of HAp monolithic material. The results of optical microscopy, scanning electron microscopy, X-ray diffraction, and other analyses are reported.

Bonding characteristics and diffusion barrier effect of the TiC phase formed at the bonding interface in an explosively welded titanium/high- carbon steel clad

Microstructural aspects and bonding characteristics of the explosively welded titanium/high-carbon steel clad of the as-welded and postannealed states were investigated. Amorphous and βTi phases were observed at the interface in the as-welded clad. These were considered to be the trace of melting and subsequently rapid solidification of thin layers along the contact surface of both the parent materials. The melting layer was considered to be responsible for the substantial bonding. The TiC layer was formed at the bonding interface by postannealing, which served as a barrier for diffusion of species across the interface and suppressed the formation of Fe-Ti intermetallic compounds. As a result, high bonding strength was preserved even after prolonged annealing at elevated temperatures.

Microstructure of Bonding Interface in Explosively-Welded Clads and Bonding Mechanism

Microstructural aspects and bonding characteristics of the explosive welded Ti/Ti, Ti/steel and Ti/Ni clads were investigated mainly by TEM microscopy. It was found that the bonding interface was composed of layers of metastable phases such as amorphous phases, icosahedral quasicrystal and intermetallic compounds. These metastable phases are formed as a trace of melting followed by subsequent rapid solidification of thin layers of about 200 nm in thickness along the contact surface of both the component materials. It is concluded that the very thin melting layer is responsible for the bonding of explosively welded Ti base clads and especially amorphous phase formed in the bonding interface keeps high bonding strength. Introduction The industrial cladding processes are hot rolling, diffusion bonding, and explosive welding. Many combination of metals and metallic alloys successfully can be bonded by explosive welding, which normally requests an inclined impact between two metallic objects creating metal jet. It has been considered that the bonding is achieved by a kind of cold pressure welding without thermally activated process in the explosive welding. On the other hand, another bonding mechanism in explosive welding was proposed that the bonding is performed by metallurgical bonding through the thin liquid phase formed by impact of Materials Science Forum Online: 2004-09-15 ISSN: 1662-9752, Vols. 465-466, pp 465-474 doi:10.4028/www.scientific.net/MSF.465-466.465

Changes in ferroelectric domain structure with electric fatigue in Li 0.06(Na0.5K0.5)0.94NbO3 ceramics

Piezoelectric materials should have stable performance during electric cycling in service. Because piezoelectricity-related properties depend on the ferroelectric domain structure, it is essential to investigate the fatigue characteristics of domain structures under electric loading. Piezoresponse force microscopy (PFM) was used to observe changes in ferroelectric domain structure with cyclic electric loading in MnO2-doped Li0.06(Na0.5K0.5)0.94NbO3 (LNKN) ceramics. The PFM observations revealed that polarization loss occurred owing to electric fatigue, and that domain width and length fraction of 180° domain walls increased with increasing number of cycles. The decreases in the piezoelectric constant d33 and the electromechanical coupling coefficient kp observed owing to electric fatigue are compared with fatigue of the domain structure.

Effect of heat treatment on formation of columnar ferrite structure in explosively welded titanium/hypoeutectoid steel joints

Explosive welding of titanium to hypoeutectoid steel (0.09 mass%C) was carried out, and interfacial aspects of as-welded and heat treated states have been investigated with a focus on microstructures of the steel. In as-welded joint, plastic flow occurred by high velocity collision was observed in the vicinity of the interface. The steel in the joints showed equiaxed structure consisting of ferrite and pearlite even after prolonged heat treatment up to 1173 K. Columnar grains were generated in the steel near the interface by the heat treatment at 1223 K and above. Although the region of the columnar ferrite structure increased with increasing heating temperature and holding time, texture with specific crystal orientation was not confirmed. It was found that such a microstructural change in the steel was closely related to constituent phases formed at the bonding interface. The formation mechanism of the columnar structure was also discussed.

Diffusion barrier effect of carbide layer on bonding characteristics of Ti/steel clad

Bonding characteristics and interfacial microstructures in explosively welded Ti/stainless steel clad of the as-welded and annealed states were investigated. In case of Ti/SUS430 ferritic stainless steel combination, the average shear strength of an as-welded clad was 555 MPa, and metastable phases such as amorphous and fine crystalline phases were observed at the interface. These were considered to be the trace of melting and subsequently rapid solidification at the contact surface of both the parent materials. By annealing below 1173 K, the strength gradually decreased with increasing holding time. The average shear strength of the clad annealed at 1073 K for 360 ks was 242 MPa, while that of the clad annealed at 1273 K abruptly decreased down to 107 MPa with increasing holding time up to 360 ks. The reaction layer formed at the interface consisted only of TiC in the former. On the other hand, the coexistence of TiC, TiFe, TiFe 2 and χ was observed at the interface in the latter. The TiC in the former was considered to serve as a barrier for diffusion of Ti, Fe and Cr across the interface and to suppress the formation of intermetallic compounds. As a result, the growth of reaction layer was inhibited and high bonding strength was preserved even after prolonged annealing. The results of the Combination of Ti and SUS304 austenitic stainless steel were also discussed.

Shock Consolidation of Sm-Fe-N Magnetic Powders and Magnetic Properties

Sm2Fe17Nx compound is a prospective candidate as a material for high performance permanent magnets because of its good intrinsic magnetic properties with a Curie temperature of 747K, a room-temperature anisotropy field of 14T and a room-temperature saturation magnetization of 1.5T. However, Sm2Fe17Nx compound decomposes to -Fe and Sm nitride above 873K and conventional powder metallurgy processing techniques fail to meet the processing requirements. Shock consolidation is a viable alternative to process this compound. Fully dense Sm2Fe17Nx bulk materials were fabricated by cylindrical explosive consolidation technique using water as a pressure transmitting medium. Explosive consolidation is performed under cold state and fully dense materials can be obtained without any degradation of the characteristics of the powder states. Sound compacts were obtained without any cracks or teas, and the value of (BH)max of Sm2Fe17Nx compact is 23.8 MGOe.

Bonding and Separation Behaviors between Ti-Sn Alloys and High Carbon Steel

Ti-Sn binary alloys (Ti-5 to 20 mol% Sn) were diffusion-bonded to high carbon steel between 1073 and 1273 K for 3.6 ks in a vacuum to investigate the influence of the alloy composition on the interfacial microstructures. Ti-5 and 10 mol% Sn alloys were attached firmly to the steel at a bonding temperature of 1273 K. A continuous TiC layer was formed along the interface, while voids were observed between the TiC layer and the steel. Although the joints with Ti-15 and 20 mol% Sn alloys were also prepared at 1273 K, these joints separated near the interface after the bonding treatment. The TiC layer was formed in the separated surface of Ti-Sn alloy, and Fe in the steel diffused into the Ti-Sn alloy. This indicates that the Ti-15 and 20 mol% Sn alloys established contact with the steel at elevated temperatures until just before the separation. The specimens bonded at 1173 K also denoted the same tendency. However, the Ti-15 mol% Sn/steel joint bonded at 1073 K showed a shear strength of more than 50 MPa. The mechanism and the application of the interface separation are discussed on the basis of the microstructural observations.

FABRICATION OF TRICALCIUM PHOSPHATE COMPACTS BY UNDERWATER-SHOCK CONSOLIDATION

The present study is to fabricate dense tricalcium phosphate (TCP) compacts by our newly developed underwater-shock consolidation technique and to investigate the characteristics of the compacts. By adding Al2O3 powder to β-TCP powder, the biocomposites were fabricated to improve the fracture toughness. Sound compacts of α- and β-TCP powders and β-TCP/Al2O3 biocomposite powder were fabricated without any cracks and tears. The relative densities of α- and β-TCP compacts were about 85% in as-compacted state and more 94% after annealing at 1373K for 7.2ks. Compressive strengths of α- and β-TCP compacts are 160 and 140MPa after annealing at 1373K for 7.2ks, respectively. Compressive strength and fracture toughness of β-TCP/Al2O3 biocomposites after annealed at 1373K for 7.2ks were 188MPa and 2.54MPa·m1/2, which are comparable to values of human bone, respectively.