Volume 89, Issue 9

Grain boundary (GB) diffusion of 110m Ag was measured in polycrystalline α-Zr in the temperature range 574 to 1024 K using microtome sectioning and γ-spectrometry to evaluate the diffusion profiles. An Arrhenius relation was determined for the GB diffusion parameter P = sδD gb , ( s is the segregation coefficient, δ the GB width and D gb the GB diffusion coefficient) with an activation enthalpy Q gb = 152±3 kJmol –1 and a pre-exponential factor P0=6.4−2.3+3.6.10−13 m3s−1 $ P_0 = 6.4_{ - 2.3}^{ + 3.6} .10^{ - 13{\rm{ }}} {\rm{m}}^3 {\rm{s}}^{ - 1} $ . The microstructure of the well annealed Zr material of low impurity content was examined by transmission electron microscopy. A fine subgrain network built by dislocation walls was found. In the temperature interval 733 to 786 K the penetration profiles were evaluated in terms of subgrain boundary (SB) diffusion characterised by the parameter P sb = s sb δ sb D sb as well as by the parameter P . The results of the measurements are compared with our previous results on GB diffusion of Zr and Co in α-Zr. The influence of SB diffusion on GB tracer diffusion measurements is discussed. The value of the segregation enthalpy of Ag in α-Zr grain boundaries is estimated.

The enthalpy of formation of the ordered phase (Ni x Co 1– x ) 0.50 Al 0.50 , (Ni x Co 1– x ) 0.58 Al 0.42 with (0 ≤ x ≤ 1) and Ni 0.30 Co 0.15 Al 0.55 with the B2 (CsCl) structure was measured by differential solution calorimetry at 1073 K as a function of composition in order to describe the constitutional defects. The binary Ni 0.50 Al 0.50 -phase exhibits the most negative enthalpy of formation (–71.27 kJ mol –1 ). The absolute value of the enthalpy of formation decreases by substituting nickel for cobalt with constant aluminium content. The composition dependence of the enthalpy of formation can be described by the disorder model of Wagner and Schottky. The model provides information about the composition dependence of the concentration of the existing defects (vacancy, antistructure atom) of the ordered ternary phases.

The systematics of tetragonality in L1 0 (AuCu-I) type intermetallics is analysed by means of a structural mapping procedure. It is found that the Miedema parameters (metallic electronegativity and electron density at the Wigner-Seitz cell surface) order the tetragonality systematics into two broad classes, exhibited respectively by transition metal compounds and by transition – nontransition type mixed compounds. In the former category, the axial ratio ( c/a) revealed a decrease with an increase in electronegativity difference, while the reverse trend is observed for the mixed type intermetallics. The effect of ternary addition and its preferential occupation of one of the sublattices of L1 0 structure have been found to influence strongly the tetragonality. The observed systematics has been explained in terms of a synthesis of crystal physical and chemical viewpoints on structure and bonding in intermetallics. In particular, it is hypothesised that an antiferromagnetic like coupling between interplanar and intraplanar interactions is responsible for the relative stability of L1 0 over B2 (CsCl) crystal structure. It is argued that a dominantly attractive (stabilising) interlayer interaction serves to compress the lattice along the [001] axis, which is promoted by a charge transfer taking place between the constituent elements segregating to individual sublattices. On the other hand, the presence of a net repulsive interaction in the {002} planes is what is responsible for increasing the tetragonality of the lattice and pushing the structure away from a B2 packing.

The present paper describes phase stability of nitrogen-containing defects in Ti-64 and Ti-17, and the effect of nitrogen on the chemistry of the titanium nitride, αTi and βTi phases in these alloys. Ti-64 and Ti-17 were alloyed with nitrogen in a controlled manner by vacuum arc melting of alloying elements together with δTiN powder. Nitrogen was substituted for all elements proportionately using their compositions in atom per cent. Nitrogen concentrations of 18 and 7 at.% were investigated. Microstructural examinations and electron beam microprobe analysis of δTiN, αTi, and βTi within the Ti-64 and Ti-17 alloyed with nitrogen and heat treated at both 1300 °C and 1000 °C are described. The phase chemistries of αTi and βTi modified with nitrogen are compared with chemistries of the same phases without nitrogen. The addition of nitrogen to both Ti-64 and Ti-17 reduces the solubility of A1 in αTi to a degree that the concentration of A1 in αTi is less than that in βTi. Mo and Cr (in Ti-17) and V (in Ti-64) partitioning to βTi is also increased in both Ti-17 and Ti-64 modified with nitrogen. The degree of partitioning increases with increasing nitrogen concentration.

DTA, DSC and X-ray powder diffraction have been used to reinvestigate the In–Se phase diagram. Phase equilibria have been determined and the In 2 Se 3 polymorphism has been thermodynamically studied.

A method to assess a five-component equilibrium phase diagram and to plot it on the concentration triangle is suggested. The advantages and limitations of this method are discussed. The Al–Si–Fe–Cu–Mg phase diagram, including solidification reactions, phase field distribution in the solid state and polythermal sections, is analyzed. With taking into account the change of equilibrium phase composition and phase fractions upon cooling, the aging ability of Al–Si– Fe–Cu–Mg alloys is considered.

The interface reactions in the diffusion couple Ti–SiC have been investigated at 1673 K. The diffusion couples were prepared by depositing 1 μm of Ti with MSIP on SiC substrates of 2 mm thickness and annealing for up to 108 h under inert atmosphere. Thin film XRD, EPMA dimple edge linescan profiling, TEM, as well as SEM and BSE micrographs served to analyse the phases formed by diffusion processes in the reaction zone. The intermediate ternary phase Ti 3 SiC 2 (T1), already formed after 10 min of annealing, remained the major phase of the reaction zone even after 108 h annealing at 1673 K. After 1.5 h the small portion of TiSi 2 , formed after 10 min of annealing, was decomposed again, and a phase (Ti, Si)C 1 -x , which crystallized in the TiC 1- x structure, was detectible in small amounts with a Ti/Si atomic ratio of up to 74/26. This phase only occurred in close proximity to the SiC substrate. The amount of (Ti, Si)C 1- x did not markedly increase up to 108 h of annealing. It could be shown that the formation of Si-free TiC 1- x observed in some cases at the surface of the couple did not happen in the pure diffusion couple Ti–SiC, but was always related to carbon intake from the gas phase.

Tensile samples of a modified 9Cr–lMo steel after tempering at 823 and 983 K subsequent to a normalizing treatment at 1373 K (designated as 550T and 7l0T, respectively) were deformed in tension at temperatures in the range of 77 to 373 K both by constant initial strain-rate tests at ⋵ = 1.1 · 10 –4 /s and strain-rate change tests between 1.1 · 10 –4 and 1.1 · 10 –3 /s at ε tot = 0.05, 0.07, 0.1, 0.12, and 0.14. The 710T specimens showed an increase in ductility and a marginal increase in strength with decreasing test temperature, whereas the 550T specimen exhibited an increased flow stress with very low ductility. The thermal component of flow stress (τ*), the activation volume (υ*) and activation energy (ΔH) were strongly affected by the microstructure. A decrease in τ* observed in the 710T specimens, compared with that of pure iron, at temperatures below 200 K can be attributed to solid solution softening by the dissolved nitrogen atoms. Further, the variations in υ* and Further, the variations in H with stress and temperature supports the importance of the Peierls mechanism for plastic deformation, and the movement of dislocations by double kink formation was found to be strongly affected by free nitrogen atoms.

The strength of ferrite in an armco iron with varying grain sizes and in five hypoeutectoid steels with varying volume fraction of the constituent phases have been studied using microhardness measurements. In armco iron, the strength of ferrite derived from its proportionality with microhardness, is found to decrease with increasing grain size according to the existing Hall-Petch relations. But the strength of ferrite in hypoeutectoid steel is found to vary considerably, even when the grain sizes in ferrite are almost constant. On the other hand, strength of ferrite in hypoeutectoid steel bears a strong functional relationship with volume fraction of the constituent phases. The variation of strength of ferrite in hypoeutectoid steel is explained on the basis of thermal residual stress exerted by the presence of pearlite.

A modified point stress criterion is proposed for predicting the notched strength of composite plates having a circular hole. The analytical results are compared well with the existing test results of several woven fabric composite plates.