Vijay K. Vasudevan

Precipitation reactions and strengthening behavior in 18 Wt Pct nickel maraging steels

The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni3Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni3Ti (initially) and Fe2Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis.

The influence of second phase Ti3Al on the deformation mechanisms in TiAl

Abstract Dislocations in samples of a heat-treated and quenched two phase Ti-46at.% Al alloy have been characterized, and those with Burgers vectors, b, given by b=½<1l0] and b=½<112] have been observed. The microstructure of deformed samples is characterized mainly by dislocations with b=½<1l0]. These dislocations have high Peierls stresses in TiAl of nominal purity, because of the directionality of bonds between the Ti atoms. Therefore, the present observations have been interpreted on the basis that the phase Ti3Al getters the interstitial elements from the TiAl, since the solubility of these elements in the former phase is significantly larger than in the latter. It is proposed that the removal of interstitials from TiAl in this way decreases the degree of directionality of bonding between the Ti atoms, and so reduces the anisotropy in Peierls stresses caused by these directional bonds. A relatively large number of twins have also been observed in the lamellae of TiAl, and this may be interpreted on t...

The geometry and nature of pinning points of ½ 〈110] unit dislocations in binary TiAl alloys

Abstract The b = ½ 〈110] unit dislocations in deformed TiAl alloys exhibit a unique morphology, consisting of numerous pinning points along the dislocation line aligned roughly along the screw dislocation direction, and bowed-out segments between the pinning points. The three-dimensional arrangement of these dislocations has been characterized in detail, based on post-mortem weak-beam transmission electron microscopy observations in deformed binary Ti-50 at.% Al and Ti-52 at.% Al alloys. The bowed segments glide on parallel {111} primary planes, and the pinning points are jogs with a range of heights, up to a maximum of about 40 nm. The substructure evolution is consistent with dislocation glide involving frequent double cross-slip and consequent jog formation. The dislocations experience a large glide resistance during the forward (non-conservative) motion of these jogs. Pinning of unit dislocations is an intrinsic process in these alloys and is not related to the presence of interstitial-containing prec...

Ordering reactions in an Ni25Mo8Cr alloy

Abstract The transformations from short- to long-range order in Haynes® Alloy 242, a nominal Ni25Mo8Cr (in wt%) alloy, during isothermal aging at temperatures between 550 and 750°C are reported using microhardness measurements, and optical and transmission electron microscopy. Aging below the critical transformation temperature (∼ 775°C) led to considerable hardening; this hardening was associated with the formation of a very high volume fraction of Ni 2 (Mo,Cr) domains/precipitates. The transformation from short- to long-range order was observed to proceed by a mechanism of continuous ordering at temperatures ⩽ 700°C, whereas at temperatures (750°C) close to the critical temperature a first order nucleation and growth mechanism appeared to be operative. The sequence of transformations are correlated with the associated diffraction effects and discussed in terms of theoretical formulations and experimental observations of other studies.

The high cycle fatigue and fracture behavior of aluminum alloy 7055

This paper highlights the results of a study on the high-cycle fatigue, deformation and fracture behavior of aluminum alloy 7055. Specimens of the alloy, in the T7751 temper, were cyclically deformed over a range of stress amplitudes at both ambient and elevated temperatures. While an increase in test temperature was found to have a detrimental influence on cyclic fatigue life of the transverse orientation specimens, little influence was found on the longitudinally oriented specimens. The macroscopic fracture mode was essentially identical regardless of the orientation of the test specimen with respect to the wrought rolled plate. Cyclic fatigue fracture, on a microscopic scale, revealed features reminiscent of locally ductile and brittle mechanisms. The microscopic fracture behavior was a function of test temperature. The mechanisms governing cyclic fatigue life and fracture behavior are discussed in light of the mutually interactive influences of microstructural effects, matrix deformation characteristics and test temperature.

Effect of chromium addition on the ordering behaviour of Ni–Mo alloy: experimental results vs. electronic structure calculations

Abstract The ordering behaviour of a Ni-Mo alloy in the presence of a ternary additive, viz. Cr, has been studied. The sequence of ordering transformations in binary Ni–Mo alloys has been shown earlier to be controlled by a competition between several f.c.c.-based superlattices, viz. Ni 2 Mo (Pt 2 Mo type), Ni 3 Mo (DO 22 ), Ni 4 Mo (D1 a ) and the so-called short range ordered (SRO) structure characterized by the presence of diffraction intensity maxima at the 1 1 / 2 0 f.c.c. positions in reciprocal space. The effect of ternary addition of chromium in the selection of the superlattice structure has been examined in this paper in an alloy of composition Ni–24 at.% Mo–6 at.% Cr. The presence of Cr has been experimentally found to favour the formation of Ni 2 (Mo,Cr) (Pt 2 Mo-type) phase in preference to the Ni 3 Mo (DO 22 ) and Ni 4 Mo (D1 a ) superlattices. This leads to a sequence of transformation different from that obtained in binary alloys. The effect of Cr on the ground state phase stability is determined, in this alloy, using the first-principles local-density based full-potential augmented plane wave (FP-LAPW) method. From the differences in the electronic structures, densities of states and total energies of binary Ni 2 Cr, Ni 2 Mo (Pt 2 Mo type), Ni 3 Mo (DO 22 ) and Ni 4 Mo (D1 a ) phases and the corresponding ternary superlattice structures, an attempt has been made to predict the hierarchy of relative phase stabilities. Theoretical predictions based on these electronic structure calculations have been found to be consistent with the experimental microstructural observations of the evolutionary stages of ordering in the ternary Ni–24 at.% Mo–6 at.% Cr alloy.

Nucleation kinetics of the α→γM massive transformation in a Ti-47.5 at.% Al alloy

Abstract Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements and quantitative stereological analysis of microstructures, coupled with calculations based on the classical theory of nucleation, were performed to study the nucleation kinetics of the α→γ M massive transformation in a Ti-47.5 at.% Al alloy. Using previously determined thermodynamic data on the reaction temperature, time, undercooling and driving force for the massive transformation, the free energies for critical nucleus formation and the nucleation rates were computed for different nucleus shape models, including incoherent grain face, edge and corner nuclei and faceted grain face nuclei, and compared with experimentally determined nucleation rates. The results indicate that nucleation of singly faceted grain face nuclei and incoherent grain corner and grain edge nuclei are highly likely. Good agreement was also obtained between the calculated and experimental nucleation rates at these various sites. Based on these results, possible nucleation mechanisms during the massive transformation are discussed in light of current thinking on the nature of this transformation.

Phase equilibria and solid state transformations in Nb-rich Nb–Ti–Al intermetallic alloys

Abstract The solidification pathways and phase equilibria at 1100, 900 and 700°C in seven Nb-rich alloys of the Nb–Ti–Al system were examined. The as-cast microstructures revealed that all alloys had undergone β solidification, extending the range of the β phase field further into the ternary system than previously reported. Changes in the atomic site occupancy preferences with alloy composition were examined for the B2 ordered β 0 phase through atom location by channeling-enhanced microanalysis (ALCHEMI). A β to δ massive-like transformation observed in cast material, was reproduced in continuously cooled homogenized samples. The formation of a new phase, θ, based on the I 4 1 /amd body centered tetragonal structure was observed below 1100°C. In addition, a metastable hP18 structure, produced through an ω-type transformation of the β 0 phase, was also observed.

Observations and analyses of dislocations and stacking faults in the massive γm phase in a quenched Ti–46.5 at.% Al alloy

The defect structures in the massively formed gamma ({gamma}{sub m}) grains in a Ti-46.5 at.% Al alloy, rapidly quenched from the high-temperature {alpha}-phase field, have been studied using transmission electron microscopy (TEM). The results reveal that the defect structures are composed of dislocations, stacking faults and antiphase boundaries intimately associated with dislocations or stacking faults. Comparison of experimental and computer simulated TEM images established that wide stacking faults, which are created by the dissociations of 1/2 unit dislocations, lie on {l_brace}111{r_brace} planes and are bound by b = 1/6 Shockley partial dislocations of all possible types. Based on the observations and subsequent analyses, a model for the formation of these defects -- involving the occurrence of an intermediate disordered f.c.c. phase during the {alpha} {yields} {gamma}{sub m} massive transformation -- is proposed.

Hydrogen absorption and desorption in a B2 single-phase Ti-22Al-27Nb alloy before and after deformation

Abstract The effects of deformation on the hydrogen absorption/desorption behavior of a B2 single-phase Ti–22Al–27Nb (at.%) alloy were investigated mainly by measuring the static pressure–composition ( P – C ) isotherms from room temperature to 100°C for specimens quenched from 1200°C and deformed to 5–∼90% in compression or by cold-rolling. β hydride is formed at a very low hydrogen pressure similarly to bcc hydrogen absorbing alloys. In as-quenched specimens, further absorption to γ hydride is sluggish and there is no obviously reverse γ→β hydride transformation. Both the β→γ and reverse γ→β hydride transformations are promoted by deformation. The best hydrogen absorption/desorption properties are attained in specimens deformed to 5–10%, the deformation structure of which consists of well-aligned and uniformly distributed screw dislocations. The beneficial effects of deformation on the hydrogen absorption/desorption behavior of the alloy are interpreted in terms of similarities between atomic displacements around straight screw dislocations and those taking place during the β–γ hydride transformation.