Shuji Hanada

Transmission electron microscopic observations of mechanical twinning in metastable beta titanium alloys

Microstructure and crystallography of a stress-induced plate in metastableβ titanium alloys, Ti-V, Ti-Mo, Ti-Nb, Ti-Fe, were investigated by a combination of two surface trace analysis and transmission electron microscopy. Stress-inducedω phase transformation or {332} 〈113〉 twinning appeared in circumstances whereβ phase was very unstable. It was found that there were two types of {332} 〈113〉 twinning, depending on whether one variant ofω phase was preferentially induced in a twin or not. Stress-inducedω phase not relating to {332} 〈113〉 twinning was not observed in matrix. This suggests that stress-inducedω phase transformation is accompanied with {332} 〈113〉 twinning. Preferential formation of oneω variant was influenced not only by alloy system and its composition, but also by cooling rate from solution treatment temperature. The relation between athermalω structure and stress-inducedω phase transformation or plastic deformation mode is also discussed.

Correlation of tensile properties, deformation modes, and phase stability in commercial β-phase titanium alloys

The effect of plastic deformation mode on tensile properties of quenched commercial β-phase titanium alloys has been investigated at approximately constant grain size and oxygen content. In addition, stability of β-phase has been estimated from ω-reflections or diffuse streaking in electron diffraction patterns in a manner similar to the previous works on binary β-phase titanium alloys. Dominant mode of plastic deformation is {332}<113> twinning in the alloys with large instability of β-phase, such as Ti-11.5Mo-6Zr-4. 5Sn and Ti-15Mo-5Zr, and is crystallographic slip in the alloys with small instability of β-phase, such as Ti-15Mo-5Zr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-15V-3Cr-3Al-3Sn, Ti-8Mo-8V-2Fe-3Al, and Ti-13V-11C-3Al. Twinning leads to low yield strength and large elongation, while slip results in high yield strength and small elongation in agreement with binary and termary β-phase titanium alloys.

Microstructure and room temperature fracture toughness of Nbss/Nb5Si3 in situ composites

The microstructure and room temperature fracture toughness of binary Nbss/Nb5Si3 and ternary Nbss/Nb5Si3 in situ composites alloyed with Mo are investigated at hypo- and hypereutectic compositions, where Nbss denotes the niobium solid solution. The binary and ternary alloys consist of coarse primary Nbss particles and fine eutectic at a hypoeutectic composition, while they are composed of fine eutectic at near-eutectic compositions. The room temperature fracture toughness of binary arc-melted alloys is 12 MPa m1/2 at the hypoeutectic composition and decreases rapidly to about 4.5 MPa m1/2 at near-eutectic compositions. In the arc-melted alloys, 5%Mo addition increases the fracture toughness up to 9–15 MPa m1/2 depending on Si content. The fracture toughness of the arc-melted alloys with eutectic microstructure is higher than that of the directionally solidified (DS) alloys with fine microstructure mostly aligned perpendicular to the direction of crack propagation. No significant influence of Mo addition on the toughness is observed for the DS alloys. Scanning electron micrographic observations confirm that the fracture toughness is increased by large scale bridging of thick primary Nbss particles in the hypoeutectic composition for the binary alloys, and by complicated bridging of Nbss with maze-like structure at near-eutectic compositions for the ternary alloys. The low fracture toughness of the DS alloys is discussed on the basis of unfavorable interface decohesion.

Application of the selected area channeling pattern method to the study of intergranular fracture in Ni3Al

Abstract The selected area channeling pattern method was applied to the characterization of grain boundaries on the basis of the coincidence lattice site model and to the detection of plastic strain accompanying intergranular fracture in cast or recrystallized Ll 2 type Ni 3 Al and Ni 3 (Al, Ti) polycrystals. The sum of the numbers of low angle and twin (Σ3) boundaries observed was less than 20% of the number of all boundaries studied. An evident resistivity of low angle and Σ3 boundaries against intergranular cracking was found. The boundaries of grains having the coincidence orientation relation (Σ5-Σ29) and random boundaries were preferentially broken under tensile stress. The amount of plastic strain accompanying intergranular fracture of Ni 3 Al was found to depend upon processing conditions (as-cast or as-recrystallized state), deformation temperature, and substitution of titanium for aluminum. It is pointed out that the amount of plastic strain is consistently larger in materials having higher resistivity against intergranular cracking.

Grain boundary fracture of L12 type intermetallic compound Ni3Ai

AES analysis of intergranular fracture surfaces of Ni3Al showed that grain boundaries are free from any detectable amount of impurity segregation. From this finding it was suggested that grain boundary brittleness in Ni3Al is not due to the segregation of harmful elements. X-ray diffraction and SEM observation of the fracture surfaces detected a plastically strained layer of which thickness is compa-rable to grain size.

Deformation characteristics in Β phase Ti-Nb alloys

Deformation modes in Β-phase Ti-Nb alloys containing from 36 to 52 wt pct Nb have been investigated as a function of composition, crystallographic orientation, or deformation temperature using both two surface trace analysis and transmission electron microscopy. 332 (113) twinning occurs in the low niobium content alloys independent of orientation and temperature. With increasing niobium content, 332<113> twinning or (111) slip occurs dependent on orientation and temperature. The operative twinning systems are explained by considering both the polarization of twinning shear and the Schmid factor. The 332 (113) twinning is suppressed in the high niobium content alloys and also suppressed by aging or oxygen addition in the low niobium content alloy. These characteristics have previously been observed in Β phase Ti-V and Ti-Mo alloys. The occurrence of 332 (113) twinning in Β-phase Ti alloys is related to the instability of Β phase.

The bone tissue compatibility of a new Ti-Nb-Sn alloy with a low Young's modulus.

A Ti-Nb-Sn alloy was developed as a new β-type titanium alloy which had a low Youngs modulus and high strength. The Youngs modulus of the Ti-Nb-Sn alloy was reduced to about 45 GPa by cold rolling, much closer to human cortical bone (10-30 GPa) than that of Ti-6Al-4V alloy (110 GPa) and other β-type titanium alloys developed for biomedical applications. The tensile strength of the Ti-Nb-Sn alloy was increased to a level greater than that of Ti-6Al-4V alloy by heat treatment after severe cold rolling. In this study the cytotoxicity of Ti-25Nb-11Sn alloy was evaluated in direct contact cell culture tests using metal disks and the bone tissue compatibility - examined using metal rods inserted into the medullary canal of rabbit femurs. The remarkable findings were that: (1) there were no significant differences in the relative growth ratio and relative absorbance ratio between cells grown with the Ti-Nb-Sn alloy, Ti-6Al-4V alloy and CP-Ti in direct contact cell culture tests; (2) there were no significant differences in the load at failure between the Ti-Nb-Sn alloy and Ti-6Al-4V alloy in pull-out metal rods tests; (3) there were no significant differences in new bone formation around metal rods between the Ti-Nb-Sn alloy and Ti-6Al-4V alloy in histological evaluations. The new Ti-Nb-Sn alloy with an elasticity closer to that of human bone is thus considered to be bioinert while also having a high degree of bone compatibility similar to that of Ti-6Al-4V alloy.

Oxidation behavior of Mo5SiB2-based alloy at elevated temperatures

Abstract A Mo 5 SiB 2 -based alloy having composition of Mo–12.3 mol% Si–24.9 mol% B was produced by arc-melting in an Ar atmosphere, and its oxidation behavior was investigated at temperature between 973 and 1673 K. At and above 1273 K, transient and steady state oxidation stages were clearly observed. The occurrence of the transient and steady state oxidation is interpreted in terms of rapid volatilization of MoO 3 and B 2 O 3 under ambient O 2 pressure at the initial stage and the passive oxidation after completely sealing the substrate by silicate glass. Development of two layers onto the substrate, i.e. SiO 2 glass scale and Mo solid solution interlayer including SiO 2 dispersions, strongly supports the interpretation. Dissolution of B into the SiO 2 scale was not confirmed because of low B concentration that was under a detectable limit of EPMA and TEM-EDS. It is suggested that the SiO 2 glass scale formed on the Mo 5 SiB 2 -based alloy is more protective than as expected.

Martensite transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions

Abstract Effects of ternary additions, Co and Cr, on martensite transformation temperatures and mechanical properties of offstoichiometric NiTi alloys were investigated. Martensite transformation start temperature (M s ) and austenite transformation start temperature (A s ) were determined by means of differential thermal analysis (DTA) in the temperature range between 77 and 423 K. Mechanical properties of yield stress and work-hardening coefficient were investigated using B2-stabilized NiTi alloys, whose alloy compositions were based on Ni-49 mol% Ti. It has been shown that (1) M s and A s considerably decrease with decreasing Ti concentration on the Ti poor side of stoichiometry (the concentration of Ti is less than 50 mol%); (2) M s and A s change slightly when Ti concentration is more than 52 mol%, where the alloys are in the two phase region; and (3) M s and A s decrease with increasing Cr and Co content with a constant Ti concentration. In the present analysis for evaluating the effect of ternary additions on M s , we use M s change by adding 1 mol% of ternary elements. M s changes by Cr addition are −65 K mol% −1 on the Ti rich side and −46 K mol% −1 on the Ti poor side. M s changes by Co addition are −15 K mol% −1 on the Ti rich side and −30 K mol% −1 on the Ti poor side. Both Cr and Co atoms are suggested to locate Ni sites preferably if only M s changes are considered. Characteristic stress-strain curves have indicated that the stress induced martensitic transformation (SIMT) occurred at 77 K. Above room temperature, work hardening coefficient at a permanent strain of 1% was found to be between 2 and 11 GPa. It should be emphasized that yield stress and work hardening coefficient increase with increasing test temperature between room temperature and about 650 K in most alloys. This strength anomaly is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion, probably in a similar manner to B2-type CoTi.

Microstructure and high temperature strength at 1773 K of Nbss/Nb5Si3 composites alloyed with molybdenum

Abstract Nbss/Nb5Si3 composites alloyed with and without Mo, where Nbss denotes Nb solid solution, are prepared by various processes such as arc-melting (AM), isothermal forging (IF), directional solidification (DS) and powder metallurgy (PM), followed by heat treatment at 1973 and 2123 K. The microstructures and mechanical properties at 1773 K of the composites are investigated under various processing and heat treatment conditions. In binary alloys the microstructures of AM composites are characterized by a dispersed structure of Nbss in Nb5Si3 matrix, while those of PM composites are composed of an aggregate structure. In ternary alloys microstructures consist of a maze-like structure for AM composites, an equiaxed structure for IF composites and lamella structure aligned to the growth direction for DS composites. The yield strength at 1773 K of AM binary alloys is higher than that of PM binary alloys. The increase in yield strength at 1773 K of the ternary composites by Mo addition is attributable to solid solution hardening in Nbss. The highest yield strength at 1773 K is obtained with an aligned lamella microstructure produced by DS, which may be associated with fine lamella spacing. It is suggested that the yield strength at 1773 K of α-Nb5Si3 is superior to that of β-Nb5Si3 alloyed with Mo.