Takayuki Takasugi

Factors affecting the intergranular hydrogen embrittlement of Co3Ti

Abstract The influence of intergranular hydrogen embrittlement on the mechanical behavior and fracture behavior of Co3Ti and 0.1 wt% B-doped Co3Ti compounds were studied. The significant environmental effect was investigated at ambient temperatures. The elongation and tensile strength showed lower values in the sequence of vacuum, air and hydrogen conditions though the yield strength was insensitive to the environment. The remarkable strain rate dependence was also investigated. As the strain rate decreased, the elongation and tensile strength decreased in a sigmoidal form and again the yield strength remained constant. The decreases of the elongation and tensile strength were investigated with decreasing test temperature below 400°C at the lower strain rate. These embrittlements were severer in the nearstoichiometric alloys of Co-23 at.% Ti than in the off-stoichiometric alloys of Co-21 at.% Ti. The addition of 0.1 wt% B did not actually affect the mechanical behavior of undoped Co3Ti compounds. Fracture behavior was fairly consistent with the mechanical behavior; there was a change in fracture mode from the transgranular fracture mode, through a mixed mode, to the intergranular fracture mode as the strain rate decreases, by the alternation from vacuum to hydrogen conditions and as the Co atoms become excess. The reduction of ductility and tensile strength occurred in the present Co3Ti compound was suggested to be due to hydrogen-assisted intergranular embrittlement. The dynamic and atomistic mechanism by which the cohesive strength of grain boundary was affected by the hydrogen was proposed as the probable explanation.

Electronic and structural studies of grain boundary strength and fracture in Ll2 ordered alloys—II. On the effect of third elements in Ni3Al alloy

Abstract It was proposed in the Paper I that the grain boundary of the polycrystalline Ni 3 Al compound is intrinsically weak due to the heterogeneous bonding environment at the grain boundary region. In this paper, the effect of the substitutional third elements on the mergranular embrittlement of the Ni 3 Al compound was systematically investigated at room temperature. Various kinds of third elements ranging from groups IIIa to V b in the periodic table were selected and alloyed in the Ll 2 structure range. First the phenomenological aspects such as the mechanical tests, metallographic and fractographic observations were described. It was shown that the third elements (Mn and Fe) which have the similar electronic chemical bonding nature with the Ni atom prohibited the grain boundary fracture when they substitute for the Al site. Several factors responsible for these phenomena were considered. As a consequence, the value of the valency difference between the third element and constitutive solvent atom substituted by the third element seems to control the grain boundary strength of the ternary Ni 3 Al compound. The grain boundary is strengthened when the value of the valency difference is positive. The possible fracture mechanism was proposed, based on the crystal structures connected with the electronic chemical bonding nature at the grain boundary region. The modification in that the electronic chemical bonding environment of the grain boundary region becomes more homogeneous by the alloying makes the grain boundary stronger.

Electronic and structural studies of grain boundary strength and fracture in L12 ordered alloys—I. On binary A3B alloys

Abstract The grain boundary fracture behaviors at ambient temperature were investigated on the binary Ll 2 -type A 3 B ordered alloys. The selected alloys consist of the elements of group VIII in the first row of the periodic table as the A atom and the widely different elements ranging from groups IV a to IV b as the B atom. First these behaviors were phenomenologically investigated by the mechanical tests, the metallographic and fractographic observations. The results show that the A 3 B alloys with elements of the b -subgroup as the B atom were mtergranularly brittle, while those with elements of the a -subgroup (transition elements) as the B atom were ductile. Next the possible factors affecting these phenomena were considered. As a consequence, the atomistic concept including the structure and electronic chemical bonding environment of the grain boundary region was proposed for the most probable understanding of the present phenomena. The absolute valency difference between two constituent atoms seems to be a criterion for knowing the degree of the grain boundary strength; as the value of the valency difference increases the grain boundary becomes stronger. The heterogeneous distribution of electronic charge due to A-B bondings at the boundary plane appears to control the macroscopic (average) grain boundary strength.

Strengthening and ductilization of Ni3Si by the addition of Ti elements

Abstract The mechanical properties of Ni 3 Si polycrystals alloyed with Ti and doped with boron were investigated by compressive and tensile tests. The yield stress increased with increasing Ti concentration and with decreasing Ni concentration at all testing temperatures. The peak temperature in the yield stress increased with increasing Ti concentration. The activation energy for the thermal stress term producing the positive temperature dependence decreased with increasing Ti concentration and was correlated with the phase stability of L1 2 (Ni 3 Si) relative to D0 24 (Ni 3 Ti). Ductility was obtained by the addition of Ti. Higher elongation values were observed in the alloys consisting of higher Ti concentration and higher Ni concentration. Further improvement of the tensile elongation was found by the addition of a small amount of boron. The variation of the elongation with temperature showed a peak at intermediate temperature. The elongation behavior correlated well with the variation of the fracture patterns. The ductilization of the Ni 3 Si observed in this work verified the alloying method to improve the grain boundary cohesion of L1 2 type ordered alloys which has been proposed by the present authors.

Environmental effect on mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

The environmental effect on the mechanical properties of boron-doped and undoped Ni3(Si, Ti) polycrystals was investigated by tensile testing in air from room temperature to 1073 K, and the results were compared with those obtained previously by tensile testing in vacuum. The environmental effect for the Ni3(Si, Ti) alloys was significant at ambient temperatures whereas that for the boron-doped Ni3(Si, Ti) alloys was considerable at elevated temperatures. When these samples at associated temperatures were tensile tested in air and also at low strain rate, intergranular fracture was dominant. It was suggested that the environmental embrittlements at low and high temperatures were due to hydrogen and oxygen absorbed from the air, respectively, and were caused by the weakening of the grain-boundary cohesion. It was proposed that boron competing with hydrogen, for site occupation or for its effectiveness at grain boundaries, has the effect of suppressing hydrogen embrittlement, whereas it was suggested that the low-melting phases, consisting of boron and oxygen (and/or constituent atoms), may be responsible for the ductility loss in the boron-doped Ni3(Si, Ti) alloys.

High temperature strength and ductility of polycrystalline Co3Ti

Abstract Polycrystalline Co 3 Ti (Ll 2 ordered structure) was shown deformable in tension between room temperature and 1050°C for compositions between 20 and 23 at.% Ti. The yield stress showed a positive temperature dependence up to 800°C, consistent with previous results, and the ultimate tensile stress monotonously decreased with increasing temperature. The elongation is a maximum around 400°C, and a minimum around 800°C, and increases at higher temperature. There is a correlation between ductility and fracture mode: the transgranular fracture mode results in higher ductility. The ductility increase at sufficiently higher temperatures is due to the frequent occurrences of dynamic recrystallization. Composition dependences of the mechanical properties and related fractography were interpreted in terms of deviations from stoichiometry.

Mechanical properties of Ni3Al containing C, B and Be

The mechanical properties of the flow strength, ductility and fracturing in the Ni3Al polycrystals containing C, B and Be were extensively investigated by tension and compression tests. The strengthening by the dopant was very significant at ambient temperatures as well as at temperatures showing the anomalous temperature dependence of the yield stress while was almost negligible at temperatures above the peak for every ternary alloy. The activation energy for the thermal stress term producing the anomalous positive temperature dependence was, by the addition of every dopant, enhanced in the just-stoichiometric (25 at.%Al) alloys while reduced in the off-stoichiometric (24 at.%Al) alloys. The yield stress at 77 K implying the athermal term (solid solution strengthening) increased linearly with increasing concentration of each ternary atom, the slope of which was very remarkable for the addition of C and B atoms. The evaluation in the correlation between the increment of the yield stress ΔσyΔC and the increment of lattice strain ΔϵΔC, per atom portion of the ternary addition indicated that the solid solution strengthening by C, B and Be atoms in Ni3Al alloy was not fully expained only by the size effect in the elastic interaction between solute atoms and dislocations. The ductilization was found for the addition of B and Be atoms but not for the addition of C atom, and then disappeared above 1000 K for the addition of B atom and above 800 K for the addition of Be atoms. These ductilization behavior was discussed in terms of the species and concentration of the dopant, alloys stoichiometry and test environment.

Plastic flow of Co3 Ti single crystals

Abstract The flow stress of Co 3 Ti single crystals (Ll 2 structure) by compression test was measured as a function of temperature, orientation, chemical composition and strain rate. The critical resolved shear stress (CRSS) showed a rapid increase with decreasing temperature below about 500 K, a remarkable increase with increasing temperature above about 500 K, and then a sharp decrease above the maximum temperature (about 900–1100 K). It was found that in all samples octahedral {111} slip occurs over the entire range of test temperatures. An exceptional slip, i.e. a cube {100} slip was found in sample having orientation axis near [111] and tested above the maximum temperature. The CRSS depended on neither orientation nor strain rate below the minimum temperature but depended on both orientation and strain rate above the minimum temperature. The maximum temperature was dependent of orientation, chemical composition but almost independent of strain rate. It was suggested that the plastic flow of Co 3 Ti single crystals below the minimum temperature is responsible for the dislocation movement of the superpartials dissociated on the (111) plane with the SISF. The plastic flow between the minimum and maximum temperatures is due to thermally activated cross-slip from (111) to (001) plane. It was also suggested that the decrease of the CRSS above the maximum temperature is probably due to the intrusion of diffusive process, i.e. unlocking process on cross-slipped (111) dislocation motion.

The influence of hydrogen on deformation and fracture processes in Co3Ti polycrystals and single crystals

Abstract The hydrogen embrittlement of L1 2 -type Co 3 Ti compounds was investigated by the deformation and fracture experiments using the “bulk” specimens and the “thin” specimens for TEM observation in both forms of polycrystals and single crystals. The mechanical behavior for the “bulk” specimens was strongly sensitive to the environmental gas, the strain rate and the hydrogen gas pressure, regardless of polycrystals or single crystals. In the most embrittled state, the polycrystals showed the intergranular fracture mode and the single crystals did the cleavage-like fracture mode. TEM observations for the “thin” specimens exhibited that the specimens tensile-tested in vacuum showed extensive generations and propagations of the dislocations from a propagating micro crack while the specimens tensile-tested in hydrogen gas did show the extensive introductions of stacking faults together with dislocations along the either side of the cracked surfaces. This microstructural feature was investigated in both forms of polycrystalline and single crystalline. As micro mechanism of hydrogen embrittlement for the Co 3 Ti compounds, it was suggested that hydrogen accumulated at a tip of propagating micro crack introduced the stacking faults, hindered the emissions and motions of dislocations and thereby resulted in decreasing the associated plastic works.

Geometrical consideration on grain boundary structure of L20 and L12 superlattice alloys

Abstract The grain boundaries in two superlattice alloys of AB L20 which is b.c.c. in disordered state and of A3B L12 which is f.c.c. in disordered state were geometrically considered. All the geometrical configurations based on the coincidence site lattice (CSL) theory were constructed without introduction of any accommodations of atoms. The defects of fair bondings were recognized on grain boundary region in terms of the nearest and the next nearest neighbors. The defect bondings such as A-A and B-B interatomic interactions distributed among the fair bondings of A-B interatomic interactions on grain boundary region and were characterized into several types, depending on the appearance of these faults, i.e. antiphase boundary (APB), stacking fault (SF) and complex stacking fault (APB + SF), analogy to the planar faults in the perfect lattice. The fault energies attributed to these defects were evaluated in all the geometrical configurations on this rough assumption. The fault energies had levels between 0.2 and 0.6 in grain boundary of L20 superlattice while the fault energies had levels between 0 and 1.2 in L12 superlattice when these are normalized by the energy level of APB in perfect lattice. Stable configuration with least energy was predicted in each CSL boundary. Possible implications of the results of geometrical study for understandings of the characteristics of superlattice grain boundaries were discussed.