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Journal ArticleDOI

New γ-TiAl alloys containing Ni

S.H. Whang1, J.Y Kim1, G.C Chen1, Z.X Li1 
15 Sep 1992-Scripta Metallurgica Et Materialia (Pergamon)-Vol. 27, Iss: 6, pp 699-704

AboutThis article is published in Scripta Metallurgica Et Materialia.The article was published on 1992-09-15. It has received 6 citation(s) till now.

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Journal ArticleDOI
Abstract: Titanium aluminide based alloys are difficult to consolidate by pressureless sintering. A systematic study has been made of the effect of small additions of copper on the sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb (in at% throughout). The density of the Ti–48Al–2Cr–2Nb alloy sintered at 1375 °C for 120 min increased consistently from 74% theoretical density (TD) without Cu to >98%TD with an addition of up to 2 at%Cu. The enabling effect of a small addition of Cu is due to the formation of a wetting liquid, supported by dilatometry, differential scanning calorimetry (DSC) and Thermal-Calc analyses. Rapid sintering shrinkage occurred around 1251 °C and 1370 °C corresponding to liquid formation. Transmission electron microscopy (TEM) identified the formation of a Cu- and Cr-enriched hexagonal close-packed Ti(Al, Cr, Cu, Nb) phase with a=0.520 A and c=0.803 A. Its presence increased with increasing Cu content. The compressive, tensile and flexure properties of the as-sintered Ti–48Al–2Cr–2Nb–2Cu alloy were assessed. The tensile properties are similar or superior to those of the Ti–48Al–2Cr–2Nb alloy fabricated by metal injection moulding and sintering while the compression strength is about twice that of the as-cast Ti–48Al–2Cr–2Nb.

28 citations


Journal ArticleDOI
Abstract: The microstructure evolution and tensile properties of Ti-48Al-2Cr-2Nb-0.25Ni alloy fabricated by the induction skull melting (ISM) process were investigated. Results showed that the influence of trace Ni addition on the cast microstructure was slight, while the γ phase fraction increased relatively. After tensile tests at elevated temperature, nanoscale τ3 particles precipitated within the lamellar colony in the gauge area. The τ3 phase exhibited the specific orientation relationship with the α2 and γ phases: ( 22 4 2 ) τ 3 // ( 40 4 1 ) α 2 // ( 113 ) γ and [ 1 1 00 ] τ 3 //[11 2 0] α 2 //[ 1 10]γ . At elevated temperature, the UTS and ef were enhanced evidently by Ni addition than that of Ti-48Al-2Cr-2Nb alloy. Dislocations were pinned by the nanoscale τ3 precipitates during tensile deformation at elevated temperature. The nanoscale τ3 precipitates and increased γ phase fraction induced by Ni alloying could account for the notable tensile properties in the present work. The precipitation behavior and the main strengthening mechanism were analyzed and discussed in detail.

17 citations


Journal ArticleDOI
Abstract: The effect of microstructure on the room temperature tensile properties of polycrystalline Ti-49 and 51A1 alloys were investigated in vacuum. Aging in the γ single- and γ + α two-phase fields, followed by air cooling or controlled cooling (5K/min), results in three types of microstructures: type (I), equiaxed γ grain structure, type (II), γ grain structure with α 2 γ laths formed mainly at the γ grain boundaries, and type (III), γ grain structure with α 2 γ laths formed both in the γ grain interiors and at the γ grain boundaries. The tensile properties of the γ grain structure classified into type (I) depend on the chemical composition rather than the grain size, and Ti-rich TiAl shows higher 0.2% offset stress and lower elongation than Al-rich TiAl. The 0.2% offset stress of the γ + α 2 , two phase structure classified into type (II) and (III) depends on both the chemical composition of the γ matrix and the volume fraction of the laths, whereas the elongation is strongly affected by the volume fraction of the laths and is less sensitive to the γ matrix composition. A small amount of the laths at the γ grain boundaries results in the relatively large improvement of the elongation, and a further enhancement of them is obtained by the suitable amount of the laths in the γ grain interiors.

15 citations


Journal ArticleDOI
Abstract: Titanium aluminide based pre-alloyed powder is difficult to consolidate by pressureless sintering. The effect of a small addition of nickel (Ni) on the sintering, sintered microstructure, and mechanical properties of a Ti-45Al-5Nb-0.2C-0.2B alloy, compacted from pre-alloyed powder, has been investigated. The relative density of the alloy, sintered at 1375 degrees C for 60 min, increased progressively from 74% to >99% with increasing Ni content from 0% to 1.25 at.%. The addition of Ni leads to the formation of an intermediate hexagonal-structured Al3NiTi2 phase (known as the tau(3) phase) during heating prior to reaching 1150 degrees C. The presence of the tau(3) phase enables the formation of a Ni-containing sintering liquid through three reactions between 1250 degrees C and 1350 degrees C: tau(3) + gamma -> Liquid, tau(3) + gamma -> Liquid + alpha and tau(3) + alpha -> Liquid. The sintering shrinkage of the powder blend Ti-45Al-5Nb-0.2C-0.2B-1.25Ni occurs mostly before reaching the isothermal sintering temperature (1375 degrees C). Consequently, the sintered density increases only marginally during the subsequent isothermal sintering at 1375 degrees C from 15 min to 60 min and remains unchanged afterwards. However, achieving a near full lamellar structure requires a longer isothermal hold (120 min) at the single alpha phase region. The as-sintered Ti-45Al-5Nb-0.2C-0.2B-1.25Ni alloy shows compression strength of 2200 +/- 50 MPa, yield strength of 750 +/- 20 MPa, and strain to fracture of 28 +/- 0.5%.

13 citations


Journal ArticleDOI
Abstract: In the present work, TiAl-based intermetallic matrix composite with second phase reinforcement as Ni-P-coated carbon and graphite powders were synthesized by mechanical alloying route. Graphite powder (20–30 µm) and elemental carbon powders (1–5 µm) were coated with Ni-P by the electroless coating technique, which was added to the elemental powder mixtures of Ti-48Al-1%Cr -1%Nb with 1% composition of either of the two. The powder mixtures were subjected to mechanical alloying at 300 rpm up to 250 h using toluene as a process control agent. The samples were collected after 25 h duration and characterized. The formation of TiAl (γ) and Ti3Al (α 2) phases are confirmed by x-ray diffraction. The formations of these phases were found after milling for 75 h in case of graphite addition and 100 h in case of carbon addition in the intermetallic matrix. The mechanically alloyed samples milled for different extents of time were examined by a field emission scanning electron microscope (FESEM) and energy dispersive ...

6 citations


References
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Journal ArticleDOI
Abstract: TiAl L10 type superlattice intermetallic compound single crystals were made by a floating zone method. In the L10 type structure two kinds of dislocations exist, i.e. one is the a 2 〈110〉 ordinary dislocations and the other is the a〈011〉 superdislocations. Compression tests of specimens with five orientations were performed to obtain the fundamental knowledge on the plasticity of TiAl single crystals. The yield stresses of all orientations tested show a positive temperature dependence similarly to most Ll2 type superlattice intermetallic compounds. The peak temperatures of yield stress vs temperature (σy−T) curves varied with the orientation of specimen. Twinning deformation was observed in the 〈001〉 orientation and the stress level was highest in whole orientations tested below the peak temperature of the (σy−T) curve. The mobility of the 〈011〉 dislocations was not always lower than that of the 〈110〉 dislocations. Namely, the flow stress level for slip deformation in the [110] orientation operating the a〈011〉 superdislocations was lowest in the orientations tested. The work hardening rate showed a positive temperature dependence in the specimens operating the a 2 〈110〉 dislocations as primary slip system but it decreased monotonously with increasing temperature in the specimens operating the 〈011〉 superdislocations. The plastic properties of TiAl single crystals can not be explained either by the Kear-Wilsdorf or by the Greenberg mechanisms.

332 citations


Journal ArticleDOI
Abstract: Polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition have been grown and deformed in compression at room temperature. The yield stress and deformation behaviour depend strongly on the angle between the twin boundaries and the compression axis rather than on the crystallographic orientation of the compression axis. The yield stress is high when compression is perpendicular or parallel to the twin boundaries, and is generally very low for specimens where the twin boundaries are at an intermediate angle to the compression axis. The ratio of the highest to the lowest values of yield stress is almost 8:1. This large difference in yield stress has been found to be related to the difference in the deformation mode; for compression perpendicular or parallel to the twin boundaries, shear deformation always occurs across them while, for compression at an intermediate angle, shear deformation is parallel to the boundaries. The former mode of deformation is much harder than the...

258 citations


Journal ArticleDOI
Abstract: First-principles total-energy calculations of the elastic constants and shear fault energies of TiAl are presented. We find a large elastic-shear anisotropy along the [011] direction, and high antiphase-boundary energies due to the strong cohesion between Ti and A1 layers as well as the formation of directional d bonds in the Ti layer. Shear faults of intrinsic type, extrinsic type and twin boundary type are predicted to be prevalent owing to their low energies. The anomalous temperature dependence of the flow stress is explained by the cross-slip-pinning and fault-dragging mechanisms. The intrinsic brittleness of TiAl is discussed in terms of these results.

180 citations


Journal ArticleDOI
Abstract: The room temperature deformation behavior and microstructure of Ti 48 Al 52 and Ti 52 Al 48 alloys are compared. The material was fabricated by rapid solidification melt spinning, and examined in both as-cast and consolidated forms. The Ti 52 Al 48 alloy exhibited enhanced strength and ductility in both forms in bend tests compared with the Ti 48 Al 52 alloy. The microstructure of the Ti 52 Al 48 alloy was two-phase γ–TiAl and α 2 –Ti 3 Al. The Ti 48 Al 52 alloy was single-phase γ–TiAl and had a larger grain size than the previous alloy. The microstructure of the Ti 52 Al 48 alloy after room temperature deformation consisted primarily of {111} mechanical twins and a /2〈110〉 perfect dislocations. The comparable Ti 48 Al 52 alloy microstructure contained fewer twins, and many more a 〈101〉 and a /2〈112〉 superdislocations were present in addition to a /2〈110〉 dislocations. The superdislocations had dissociated and formed sessile faulted dipoles. The possible reasons for the differences in microstructure and mechanical behavior between these two alloys are discussed.

106 citations


Journal ArticleDOI
Abstract: Binary and ternary L10 type titanium alloys were rapidly quenched into thin foils by the hammer-anvil quenching technique. The nominal alloy compositions include Ti1-xAlx and (Ti1-xAlx)100-@#@ yMy, wherex = 0.5 to 0.6; y= 1 to 4; and M is V, Cr, Mn, Fe, Co, Ni, and Ge. Lattice parameters of Ll0 phases in these alloys were studied by X-ray diffractometer using the foils and their powders. Simultaneously, long-range order parameters of these Ll0 phases in the as-quenched foils and the annealed powders were determined from the superlattice intensity measured by X-ray diffractometer. Rapid quenching suppresses long-range ordering (S) during solidification and, thereby, reduces lattice distortion (c/a) in Ll0 titanium alloys. A small amount of alloying element has a large effect on thec/a ratio. The atomic size effect and probably substitutional site preference play an important role in such ac/a reduction. From the density measurement, it is found that antisite defects are a predominant mode in Al-rich off-stoichiometric TiAl compound alloys. Determined long-range order value is significantly low in as-quenched binary TiAl due to rapid quenching, and the highest ordering occurs at as-quenched Ti45Al55 alloy. The relationship betweenc/a andS in Ll0 Ti45Al55 may be written asc/a = 1 + 0.025 ·S.

75 citations