Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Phase transformation behavior in Al-Zn-Cu alloys by hammering

The 5Cu40Zn55Al and 15Cu20Zn65Al alloys were prepared in the AI-Zn-Cu system, There exist the metastable phases ε and θ in the two alloys after homogenization treatment and furnace cooling, respectively. It is shown that the particles are refined from 3 mm to less than 10 μm after hammering the two alloys but there are still metastable phases. This means that the phase constituents of the two alloys have no changes by the deformation, which is different from that by balling. The phase constituents are not changed at room temperature by hammering, which is dependent on the deformation mechanism of hammering.     

Numerical modeling of damping capacity of Zn-Al alloys with fully lamellar microstructures

The damping behaviors of Zn-Al alloys with fully lamellar microstructures were simulated with the cell method. The influences of the grain boundary condition, the strain amplitude, the number of the lamellae in the grain （N） and the content ratio of Zn and Al in Zn-Al alloys on the damping capacity were investigated. The results indicate that the grain boundary condition has great influence on the damping capacity of Zn-Al alloys, and also affects the relationship between the damping capacity and the number of lamellae （N）. The variation of damping capacity with the strain amplitude is increasing exponentially with the strain amplitude and the damping capacity increases with the increasing of content of Zn.     

Long-range order relaxation and phase transformation in γ-TiAl alloys

Residual electrical resistivity measurements were used for characterizing γ-TiAl intermetallic compounds and for investigating the kinetics of long-range order (LRO) relaxation. In annealed and slowly cooled materials, a linear relationship between residual resistivity and aluminium content was observed in the composition range from 50 to 56.1 at.% Al. The isothermal relaxation of LRO after small temperature changes was investigated in γ-TiAl compounds (with 49.8, 50.7, 52.6 and 54.1 at.% Al). The approach to equilibrium could be satisfactorily represented by a sum of two exponential relaxation processes. The slower process was assigned to LRO relaxation. From the investigation of the temperature dependence of relaxation times, an activation enthalpy of 2.97±0.15 eV was determined for the stoichiometric alloy. This value is close to the activation enthalpy for titanium self-diffusion in TiAl. For Al contents higher than 50.7 at.%, the LRO changes were found to be superimposed on a supplementary phenomenon, which produces a continuous resistivity decrease. A TEM investigation showed that this drift was associated with the precipitation of Ti₃Al₅, which is a superstructure of the L1₀ structure.     

Phase transformation and heat treatment of titanium (α + β) alloys

The variety of phase and structural transformations occurring in two-phase ( + ) titanium alloys in heat and deformation treatments yield articles with different combinations of mechanical properties. However, specific heat treatment regimes are often prescribed on the basis of empirical regularities based on practical experience rather than on data of a comprehensive analysis of phase and structural transformations. As a result, to apply the prescribed regimes of treatment to new alloys a great number of tests have to be conducted, which is laborious and material-consuming. This work is an attempt to substantiate scientifically the principles for prescribing heat treatment regimes proceeding from the regularities of the processes of decomposition of metastable phases in high-alloyed titanium alloys.     

Isothermal forging of γ-TiAl based alloys

1　INTRODUCTIONBecauseoftheirattractive properties ,γ TiAlbasedalloysareconsideredforhightemperatureap plicationsinaerospaceandautomotiveindustries .Thesepropertiesincludelowdensity ,highspecificyieldstrength ,highspecificstiffness ,goodoxidationresistance ,andgoodcreeppropertiesuptohightem peratures.Researchanddevelopmentonγ TiAlbasedalloyshaveprogressedsignificantlywithinthelastdecade .Thisresearchhasledtoabetterunder standingofthefundamentalcorrelationsamongalloycompositionandmicros…     

Irreversibility and transformation randomness of thermoelastic martensitic transformation in Ni–Ti alloys

In this work,the in situ optical observation was carried out in complete and incomplete transformation cycles of Ni-Ti alloys.In complete transformation cycles,initial martensite plates nucleate randomly in austenite.However,in a partial transformation cycle,the existing martensite plates have an influence on guiding the formation of subsequent martensite plates.And the randomness decreases with the decrease in transformation volume involved in the partial cycle.It is suggested that the randomness of transformations contributes to the introduction of defects,and the irreversibility associates with transformation randomness of martensite plates.For instance,a higher randomness in transformations could introduce more defects and more obvious irreversibility.On the other hand,defects generated in thermoelastic martensitic transformation are responsible for the hysteresis of transformations.Therefore,the randomness of transformations also contributes to the transformation hysteresis.These results could help further understanding on some martensitic transformation phenomena of shape memory alloys,such as the nonlinear and history-dependent characteristic.     

Mechanical properties of γ-TiAl cast alloys

At present, much attention is being devoted to evaluation of the properties of -TiAl alloys in various structural states characteristic for the stage of their production (casting or powder metallurgy) and for subsequent treatment (deformation or heat treatment by various regimes). Specifically, in addition to preparing thin sheets, where the high toughness of the TiAl alloys is realized most efficiently, some works are devoted to casting articles and parts with a shape close to the final one. Therefore, it is of interest to investigate the properties of actual cast alloys in two conditions, namely, after crystallization with different cooling rates (in the initial state, with all the defects in the form of chemical inhomogeneities, micropores, different-sized grains, texture) and after complete degradation of the cast structure in the process of long-term operation at temperatures up to 1050°C. The paper concerns the properties and structures of cast and annealed -TiAl alloys alloyed with Hf, V, Mn, and B.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 11 – 14, April, 1996.     

Phase transformation behavior and shape memory characteristics of Ti−Ni−Si alloys

Transformation behavior, microstructures and shape memory characteristics of Ti−(50−X)Ni−XSi (X=2, 4, 6 at.%) and (50−X)Ti−Ni−XSi (X=2, 5, 7, 10 at.%) alloys were investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistivity measurements and constant load thermal cycling tests. Ti₅Si₃, Ni₁₆Ti₆Si₇ and Ni₄Ti₄Si₇ were formed in Ti−(50−X)Ni−XSi alloys, while Ti₅Si₄, Ni₃Si, Ni₃Ti₂ and Ni₃Ti₂Si were found in (50−X)Ti−Ni−XSi alloys. The total amount of silicides increased with increasing Si content, irrespective of Si content. The B2→B19 transformation occurred in Ti−(50−X)Ni−XSi alloys, and their transformation temperatures appeared to be almost constant. Transformation elongation associated with the B2→B19 transformation decreased with increasing Si content. In contrast to Ti−(50−X)Ni−XSi alloys, a transformation accompanied with structural change did not occur in (50−X)Ti−Ni−XSi alloys.     

The transformation of narrow dislocation dipoles in selected fcc metals and in γ-TiAl

Transformations of vacancy dipoles of dissociated edge dislocations are analyzed in Cu, Ni and γ-TiAl by molecular dynamics. Dipole heights up to 20 {1 1 1} interplanar distances are investigated at temperatures ranging from 0 K to near the melting points of Cu and Ni and slightly below the upper boundary of the single phase γ-TiAl domain. Three model configurations, hollows, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Except for dipoles one interplanar distance high, hollows are either metastable or unstable and they are never formed by mutually approaching dipolar dislocations. The three configurations transform into a variety of height- and temperature-dependent layouts including cores containing ordered free volumes, zigzagged faulted dipoles and agglomerated stacking-fault tetrahedra (SFT). At the highest temperatures, small individual SFTs are formed by short-range pipe-diffusion along the dipole cores. There is no critical height below which small-height dipoles or their debris would just simply disappear.     

γ-allotriomorphs precipitation and lamellar transformation in a TiAl-based alloy

In a Ti₄₉Al₄₇Cr₂Nb₂ alloy, the microstructural characteristics of the α/α grain boundary and of neighbouring lamellar structures are analyzed by Transmission Electron Microscopy (TEM). In particular, attempts to identify the precursors and the ordering process of the γ lamellae are described.     

Solidification of Al alloys under electromagnetic field

1　INTRODUCTIONCastingisakeystepinAlalloyprocessing .From productionweknowthatalmostallqualityproblemsarerelatedtothedefectsincasting ,mostofwhichcannotbeeliminatedinfollowingformingandheattreatments ,socastingisnotonlyaformingpro cessbutalsoaprocessforstructuralcontrol.Toapplyoutfieldisanefficientandeconomicalmethodtocon trolstructures .Theelectromagneticfieldiswidelyusedinindustry ,becauseofitshighefficiency ,econ omy ,cleannessandnocontact[15] ,forexample ,electromagneticstirring ,elec…     

Kinetics of the abnormal austenite–ferrite transformation behaviour in substitutional Fe-based alloys

The kinetics of the recently discovered abnormal austenite (γ)–ferrite (α) transformation of substitutional Fe–Co and Fe–Mn alloys were measured by dilatometry and compared with the, also measured, corresponding normal transformation behaviour. A phase transformation model, involving site saturation, interface-controlled continuous growth and incorporating an impingement correction, has been employed to extract the migration velocity of the γ/α interface. It was found that the normal transformation process could be well described assuming a constant nucleus density and interface migration velocity. The thus assessed misfit-accommodation, deformation Gibbs energy is of the same order of magnitude as the chemical Gibbs energy change driving the transformation. A large austenite grain size was shown to be the precondition for the occurrence of abnormal kinetics. The abnormal transformation process involves the occurrence of additional peaks in the transformation rate for the first stage of the transformation. An autocatalytic type of nucleation was successfully incorporated in the above model to describe the occurrence of the repeated nucleation during the abnormal transformation.     

High speed ball nose end milling of γ-TiAl alloys

Following a brief review on the development and intended application of γ-TiAl for aeroengine components, the paper details statistically designed machinability experiments involving high speed ball end milling in order to benchmark 4th generation alloy Ti–45Al–8Nb–0.2C against Ti–45Al–2Mn–2Nb +0.8%vol. TiB_2, which has been the prime focus of machinability evaluation and associated publications. Testing utilised 8 mm diameter, solid WC, AlTiN coated ball nose end cutters in a factorial design and involved measurement of cutting temperatures and workpiece surface roughness in addition to tool life and cutting forces. The trials established alloy Ti–45Al–8Nb–0.2C as providing a ∼25% increase in tool life based on a maximum flank wear criteria of 300 μm. On average, alloy Ti–45Al–2Mn–2Nb +0.8%vol. TiB₂ had a surface roughness Ra of ∼0.1 μm higher than Ti–45Al–8Nb–0.2C with values <0.6 μm Ra possible. Maximum cutting temperatures for Ti–45A–8Nb–0.2C measured using infra red pyrometry were ∼250 °C at a cutting speed of 160 m/min for a new tool, rising to ∼430 °C at a cutting speed of 340 m/min with a tool that had 300 μm of flank wear. For both γ-TiAl alloys, fracture/pullout was observed on every milled surface that was assessed and varied from a few microns to a few millimeters depending on test operating levels, within general, higher cutting speeds and tool flank wear levels causing an increased level of damage.     

A three-dimensional phase-field model for computer simulation of lamellar structure formation in γTiAl intermetallic alloys

A three dimensional phase-field model of α^′₂→α₂+γ transformation is developed to simulate the formation of coherent multi-domain lamellar structures in γTiAl intermetallic alloys. The model takes into account the effect of coherency strain associated with the lattice rearrangement accompanying the phase transformation, and the anisotropy in interfacial energy. Simulation studies based on the model successfully predicted the essential features associated with the multi-domain lamellar structures observed experimentally. It is shown that the coherency strain accommodation is the dominating factor responsible for the formation of the lamellar structure. The neighboring lamellae of γ phase are found to have either a twin or a pseudo-twin relationship, with the former being dominant. It is found that strain-induced correlated nucleation plays an important role in the formation of the twined lamellae. The lamellar thickness is determined by the interplay among the elastic strain energy, interfacial energy and bulk chemical free energy. Domains within individual lamellae are isotropic and domain boundaries are smoothly curved. No special self-accommodating morphological patterns are observed on the (0001)_α2 plane, which is very different from the pattern formation predicted for the coherent hexagonal → O-phase transformations.     

Phase transformations in heat treatment of Al−Li alloys and optimization of operational properties of semifinished products from these alloys

In recent years Al−Li alloys have become very popular in the aircraft industry. These alloys are characterized by a high specific strength and rigidity which increases substantially the load ratio and fuel efficiency of the parts. As a rule, Al−Li alloys are used in an underaged state, which provides a high, ductility over the height of the semifinished products. However, the threshold stress (σ_cr) in corrosion cracking in this state does not usually exceed 25–50 N/mm², and the yield strength is not always satisfactory. The results of a study of the mechanical properties and corrosion resistance of some Al−Li alloys are described and the ways for optimizing these properties are described on the basis of the phase transformations in the alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 24–30, January, 1998.     

Phase selection in undercooled peritectic Fe–Mo alloys

The change of the primary solidification mode of undercooled peritectic Fe–Mo melts has been studied by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 380 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. A transition from the primary bcc-Mo to the peritectic σ-phase solidification mode beyond a critical undercooling of 345 K was revealed for the Fe₄₇Mo₅₃ alloy and in a similar way for other compositions between Fe₄₅Mo₅₅ and Fe₅₄Mo₄₆. The suppression of the properitectic bcc-Mo phase was also achieved for subcritical undercooling in substrate-quenched Fe₄₅Mo₅₅ samples. In Fe₆₁Mo₃₉ a transition from the primary σ- to the peritectic R-phase solidification mode beyond a critical undercooling of 150 K was inferred from recalescence processes and X-ray investigation of as-quenched undercooled samples.     

Phase transformation of δ→σ in multipass heat-affected and fusion zones of dissimilar stainless steels

The purpose of this report is to discuss the phase transformation from δ-ferrite to σ phase in various welding regions of dissimilar stainless steel welds. Experimental results indicated the grain refinement was very obvious in the second and the third pass fusion zones (304-1 and 304-2) when the welding pass increased from 1 to 3. This refining phenomenon of δ-ferrite was attributed to the recrystallization. The order of morphological development of δ-ferrite in the multipass fusion zones was as follows: massive→columnar→equiaxed. The σ phase precipitated in the third pass fusion zone (304-2) and its solidification order is L→δ+L→δ→δ+γ→δ+σ+γ with a composition of σ phase (60 wt.%Fe-35.7 wt.%Cr-4.2 wt.%Ni) of the third pass fusion zone. The precipitation mechanism of the σ phase was as a eutectoid decomposition of δ→+σγ₂.