Ductile Ti-Based Bulk Metallic Glasses with High Specific Strength

Ti-based bulk metallic glasses (BMGs) with large compressive plasticity were developed in the Ti-rich part of Vitreloy series BMGs (Ti_65–x Zr _x Cu₉Ni₈Be₁₈ alloys with x = 0, 5, 10, 15, and 20). The current materials exhibit high fracture strength reaching ~2.3 GPa and plastic strains up to ~8.3 pct after partial substitution of Zr by Ti. The plasticity of the investigated alloys strongly depends on the Zr content, which affects the elastic constants, such as Poisson’s ratio and shear modulus. This, in turn, has an impact on the shear transformation zone (STZ) volume and, hence, on the shear banding of the glasses.     

High-temperature phase equilibria in the Al-rich corner of the Al-Ti-C system

A thermodynamic analysis of the Al-rich corner in the ternary Al-Ti-C diagram, providing phase relations and regions of phase stability, is presented. An invariant four-phase equilibrium between Al, Al₄C₃, Al₃Ti, and TiC _x takes place at 0.53 at. pct Ti, 7.10⁻⁶ at. pct C, and TiC_0.883 at 966 K. The carbon content of the TiC _x phase, which extends from x=0.48 to 0.98, exerts a significant effect on phase relationships in this ternary system. In particular, it is shown that stoichiometric TiC is not stable in the presence of liquid Al. For example, at 1300 K, a two-phase equilibrium between Al _L and TiC _x exists only in the 0.91<x<0.82 range. Thus, the interaction of Al _L with stoichiometric TiC leads to the formation of the Al₄C₃ aluminum carbide phase, whereas for x<0.82, only the intermetallic compound Al₃Ti can form at this temperature. The results of this analysis were confirmed by X-ray diffraction (XRD) measurements of relevant composites.     

High copper content bulk glass formation in bimetallic Cu-Hf system

We report in this article that strip-shaped amorphous samples with thicknesses from 0.5 to 2 mm were successfully synthesized for binary Cu-Hf alloys containing 60 to 68 at. pct Cu by the traditional copper mold casting method. The best glass former Cu₆₆Hf₃₄ with casting thickness up to 2 mm has an undercooled liquid region (ΔT=T _x −T _g , where T _g is the glass transition temperature and T _x is the onset temperature of the first crystallization event) of 51 K, which is somewhat narrow compared with other neighboring alloys in the same system. The bulk glassy Cu₆₆Hf₃₄ alloy exhibits Vicker’s Hardness (H _v ) ∼779 kg/mm², Young’s modulus ∼108 GPa, fracture strength ∼2.1 GPa, and an almost constant elastic elongation ∼1.8 pct upon compression. The discovery of Cu₆₆Hf₃₄ as a bulk glass confirms the existence of rather simple bulk-glass-forming metallic systems. Moreover, the present Cu-Hf alloys may be the highest copper content bulk metallic glasses reported to date, to the best of our knowledge.     

Elastic moduli of titanium-hydrogen alloys in the temperature range 20 °C to 1100 °C

The elastic properties of a series of polycrystalline titanium-hydrogen alloys (containing up to 25 at. pct H) were measured over the temperature range 20 °C to 1100 °C. The latter limits permitted investigation of adjacent parts of the α+δ, α, and β phase fields. A laser ultrasonic technique was employed to measure the temperature and hydrogen-concentration dependencies of the elastic constants. The room-temperature elastic properties of the alloys depended only slightly on hydrogen concentration, remaining almost independent of the volume fraction of the δ hydride phase. In the α phase field, the addition of hydrogen decreased the shear and Young’s moduli and increased the bulk modulus, Lamé constant, and Poisson’s ratio. The Young’s and shear moduli decreased more rapidly with increasing temperature than in cubic phases. By contrast, Poisson’s ratio increased with temperature. In the β phase field, the temperature dependence of the elastic constants was weak. However, alloying with hydrogen increased the shear and Young’s moduli, decreased Poisson’s ratio, but did not appreciably affect the bulk modulus and Lamé constant. The different effects of hydrogen on the elastic constants of alpha and beta titanium are interpreted in terms of the influence of dissolved hydrogen on the stability of the hexagonal close-packed (hcp) and body-centered cubic (bcc) lattices in the vicinity of the α-to-β transformation. The present results are also used to help account for the effect of hydrogen concentration on the mechanical properties of Ti-H alloys.     

Single-crystal elastic constants of Fe-15Ni-15Cr alloy

Resonant ultrasound spectroscopy (RUS) and pulse-echo (PE) superposition techniques have been used to determine the three independent elastic-stiffness constants C₁₁, C₁₂, and C₄₄ as a function of temperature for single crystals of 70Fe-15Ni-15Cr alloy. The values of the elastic moduli determined using RUS and PE are in very good agreement within the range of uncertainties. This particular ternary composition of Fe, Ni, and Cr undergoes an fcc-bcc structural phase transformation near 190 K resulting in a low-temperature ferromagnetic phase. The Debye characteristic temperature was determined to be 447 K from PE and 451 K from RUS measurements. The Zener elastic anisotropy A=2C₄₄/(C₁₁−C₁₂) is nearly constant: A=3.53±0.16 in Fe-Ni-Cr alloys with similar compositions. For these alloys, only small variations are observed in the Grüneisen parameter, γ≈2.08, and in the Poisson ratio, v _[hkl]=0.293±0.013.     

Deformation of semi-solid Sn-15 Pct Pb alloy

The rheological behavior of semisolid Sn-15 pct Pb alloy was studied using a parallel-plate viscometer. Small nondendritic and dendritic semisolid samples of the alloy were deformed under a constant load at initial pressures up to 232 kPa (33.6 psi) and at fractions solid from 0.15 to 0.60. Strain-time data for the nondendritic material obey the non-Newtonian, two-parameter, Ostwald-de-Waele, power-law model,i.e. μ = mγ ⁿ⁻¹, where μ is viscosity γ shear rate andm andn are constants. For fractions solid above about 0.30, the following empirical equation relates viscosity, shear rate and fraction solidμ = a exp (bf_s) γ^(cf _s ^+d) 0.3 <f _s < 0.60 wheref _s is fraction solid anda, b, c, d are constants. The nondendritic alloy deformed homogeneously without cracking to very large strains (up to 80 pct). Dendritic alloys required much higher loads and cracked easily. For the nondendritic alloys the forging pressures to obtain 50 pct compression were of the order of 7 to 70 kPa (1 to 10 psi) for fractions solid under 0.55 and 172.5 to 207 kPa (25 to 30 psi) for fraction solid of about 0.60. For the dendritic alloys, the forging pressure required to achieve 10 pct compression is about 85 kPa at a fraction solid of 0.35 and increases rapidly with increasing fraction solid.     

Standard gibbs energies of formation of the carbides of vanadium by emf measurements

The relative partial molar Gibbs energies of vanadium in the vanadium-carbon system have been determined for the V-C alloys containing 36.7, 41.2, 43.1, 44.8, 45.5, 46.8, 50.5, and 54.0 at. pct carbon by using galvanic cells of the type (−) V, VF₃, CaF₂ // CaF₂ // CaF₂, VF₃, ‘V-C’ (+) The measurements were carried out in the temperature range of 816 to 1008 K. The relative partial molar Gibbs energies of carbon have been calculated in the same composition range. The relative integral molar Gibbs energy in the VC single-phase region can be expressed asG ^M = −98,850 + 72,242X_C + (24.81 −37.23X _C)TJ/mol The standard Gibbs energies of formation of V₄C_3-x and V₂CC can be represented as ΔG° = −67,208 + 9.37T J/mol of V_0.60C_0.40 and ΔGδ = −62,581 + 7.10T J/mol of Va66Co.34 respectively.     

The effects of silicon on the reaction between solid iron and liquid 55 wt pct Al−Zn baths

The effect of various silicon levels on the reaction between iron panels and Al-Zn-Si liquid baths during hot dipping at 610°C was studied. Five different baths were used: 55Al−0.7Si−Zn, 55Al−1.7Si−Zn, 55Al−3.0Si−Zn, 55Al−5.0Si−Zn, and 55Al−6.88Si−Zn (in wt pct). The phases which formed as a result of this reaction were identified as Fe₂Al5 and FeAl3 (binary Fe−Al phases with less than 2 wt pct Si and Zn in solution),T1, T2, T4, T8, andT _5H (ternary Fe−Al−Si phases), andT _5C (a quaternary Fe−Al−Si−Zn phase). Compositional variations through the reaction zone were determined. The phase sequence in the reaction zone of the panel dipped for 3600 seconds in the 1.7 wt pct Si bath was iron panel/(Fe₂Al₅+T ₁)/FeAl₃/(T _5H+T _5C)/overlay. In the panel dipped for 1800 seconds in the 3.0 wt pct Si bath the reaction zone consisted of iron panel/Fe₂Al₅/(Fe₂Al₅+T ₁)/T ₁/FeAl₃/(FeAl₃+T ₂)/T _5H/overlay. In the panel dipped for 3600 seconds in the 6.88 wt pct Si bath the phase sequence was iron panel/Fe₂Al₅/(Fe₂Al₅+T1)/(T1+FeAl3)/(T1+T2)/T2/T8/T4/overlay. The growth kinetics of the reaction zone were also studied. A minimum growth rate for the reaction zone which formed from a reaction between the iron panel and molten Al−Zn−Si bath was found in the 3.0 wt pct Si bath. The growth kinetics of the reaction layers were found to be diffusion controlled in the 0.7, 1.7, and 6.88 wt pct Si baths, and interface controlled in the 3.0 and 5.0 wt pct Si baths. The presence of the interface between theT2/T5H, Fe2Al5/T ₁, orT ₁/FeAl₃ phases is believed responsible for the interface controlled growth kinetics exhibited in the 3.0 and 5.0 wt pct Si baths.     

Elastic constants of face-centered cubic and L1<Subscript>2</Subscript> Ni-Si alloys: Composition and temperature dependence

The adiabatic elastic stiffness constants C _ij of Ni-Si single-crystal solid-solution alloys of two slightly different compositions, 10.78 and 11.17 at. pct Si, were measured over the temperature range from 20 °C to 900 °C using the rectangular parallelepiped resonance method. The isotropic elastic constants of the polycrystalline ordered intermetallic compound Ni₃Si containing 23 at. pct Si were also measured over this temperature range. Values of the C _ij for Ni₃Si were estimated from the data on the polycrystalline alloy, as well as from published data in the literature on isomorphous ternary ordered intermetallic compounds containing different amounts of Si. Using measured values and previously published data, the stiffness constants of Ni₃Ti were estimated; these are the only available data on this alloy. The estimated single-crystal elastic constants of Ni₃Si, as well as the experimentally measured bulk modulus, are considerably smaller than published values calculated from first-principles methods. The same is true for the C _ij of Ni₃Ti, but the discrepancies are smaller.     

A TEM study of CVD TiC x /Ti(C,N) x coating on WC-Co cemented carbide

The cross-sectional microstructure of TiC _x /Ti(C,N) _x (0<x<1) coating deposited on WC-8 wt pct Co-4 pct TaC-6 wt pct TiC cemented carbide by chemical vapor deposition (CVD) has been investigated by transmission electron microscopy (TEM) and electron energy loss spectroscopic analysis (EELS), and the interfacial structures between TiC _x coating and the substrate were emphasized. TEM studies showed that a multilayer structure can be delineated in the coating. Selected area electron diffraction (SAED) and EELS analysis revealed that it consists mainly of a TiC _x layer and a Ti(C,N) _x layer. Additionally, two sublayers were found, one between the TiC _x and Ti(C,N) _x layers and the other between the substrate and the TiC _x layer. A <111> preferred orientation formed in the TiC _x layer, whereas <100> preferred orientation formed in the Ti(C,N) _x layer. Grains in the sublayers, in general, oriented randomly. The changes in composition of the coating were measured qualitatively by the EELS analysis. Microelectron diffraction (γ-Diff) analysis and centered dark-field (CDF) technique were used to investigate the interfacial structure between the TiC _x and the γ-phase (TaC or TiC in the substrate), and two kinds of coherent structure were revealed. According to these findings, the formation of the CVD TiC _x coating is possibly affected by the carbon supply from the component phases in the substrate.     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

Effect of Sn on microstructure and mechanical properties of Ti-base dendrite/ultrafine-structured multicomponent alloys

Ti_57−x Cu₁₅Ni₁₄Sn_4+x Nb₁₀ (x = 0, 5, or 10) alloys were prepared by copper mold casting. At Sn = 4 at. pct, a dendrite/ultrafine-structured multicomponent alloy was obtained, which exhibits 1271 MPa yield strength, 77 GPa Young’s modulus, and 2 pct plasticity at room temperature for 3-mm-diameter samples. The cooling rate significantly affects the as-cast microstructure and the mechanical properties. For 5-mm-diameter samples, the alloy exhibits 1226 MPa yield strength, 63 GPa Young’s modulus, and 2.5 pct plasticity. At Sn = 9 at. pct, Ti-, Sn-, and Nb-rich particles precipitate primarily. This near-hypereutectic alloy composition leads to the precipitation of intermetallics, which deteriorate the mechanical properties and result in the coexistence of ductile and brittle fracture mechanisms. At Sn = 14 at. pct, the alloy composition is completely in the intermetallic region, thus inducing the formation of Ti₂Cu, Ti₂Ni, and Ti₃Sn intermetallics. The alloy becomes very brittle because the intermetallic compounds dominate the fracture process.     

Production of Ni3Ti?TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi−TiC (stoichiometric) or Ni₃Ti−TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3≤x≤0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi; i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti−TiC _x composites are produced. Ignition, combustion and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented. Guglielmo Gottoli, formerly Graduate Research Assistant, Metallurgical and Materials Engineering Department, Institute for Space Resources, Colorado School of Mines     

Temperature and composition dependence of the elastic constants of Ni3Al

The stiffness constants, c _ij , of monocrystalline Ni₃Al of three different compositions, 23.2, 24.0, and 25.0 at. pct Al, were measured over the temperature range from 300 to 1100 K using the rectangular parallelepiped resonance (RPR) method. The bulk modulus, as well as the shear modulus, Young’s modulus, and Poisson’s ratio for randomly oriented polycrystalline stoichiometric Ni₃Al, were derived from the stiffness constants. The data indicate that c ₄₄ is essentially independent of composition, decreasing slightly with increasing temperature for all three alloys. The values of c ₁₁ and c ₁₂, however, decrease with increasing aluminum content, the difference being small at room temperature but becoming larger at higher temperatures. We find that c ₁₁ and c ₁₂ are not as sensitive to aluminum concentration as is implied by previous results. A comparison of different shear moduli of Ni₃Al and the saturated Ni-Al solid solution in equilibrium with it indicates that the ordered phase is generally elastically stiffer than the solid solution over the range of temperatures at which coarsening of the Ni₃Al precipitate has been heavily investigated.     

Production of Ni3Ti?TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi−TiC (stoichiometric) or Ni₃Ti−TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3≤x≤0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi;i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti−TiC _x composites are produced. Ignition, combustion and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented. Guglielmo Gottoli, formerly Graduate Research Assistant, Metallurgical and Materials Engineering Department, Institute for Space Resources, Colorado School of Mines     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Influence of grain size and stacking-fault energy on deformation twinning in fcc metals

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as (σ _tw −σ ₀)/G=C(d/b)^A, where σ _tw is the deformation twinning stress, σ ₀ is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger’s vector, and C and A are constants determined to have values of 0.0004 and −0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.     

Calorimetric and emf studies on liquid Li-Sn alloys

By means of high temperature calorimetry the mixing enthalpies ΔH of liquid Li-Sn alloys have been measured; however, due to experimental problems they were determined only forx _Li = 0.01 to 0.5 andx _Li = 0.87 to 0.99. The range of temperatures studied was 691 to 938 K. High compound forming tendency in Li-Sn is reflected by a triangular-shaped relation for ΔH vs x _Li. The extrapolated maximum of this plot is about −40 kJ mol⁻¹ close to Li₄Sn. Using the concentration cell Bi(l)Li₃Bi(s)| LiF-LiCl|Li-Sn(l) the emf was measured as function of temperature (775 to 906 K) atx _Li = 0.1 to 0.603 enabling calculations of partial thermodynamic data for lithium in liquid Li-Sn solutions. Integral enthalpies calculated from partial enthalpies of lithium correspond well to the calorimetrically obtained integral mixing enthalpies in the concentration range where both emf and calorimetric data were obtained. The extrapolated maximum of ΔH from calorimetric studies and minimum of integral excess entropies from emf measurements correlate well with results of structure measurements and of other structure sensitive properties. All this experimental information indicates a maximum chemical short range order close to the composition Li₄Sn.     

An investigation of high temperature thermodynamic properties in the Pt-Zr and Pt-Hf systems

High-temperature thermodynamic properties of Pt−Zr alloys containing 2 to 25 at. pct Zr and Pt−Hf alloys containing 20 to 25 at. pct Hf have been measured over the temperature range 1100 to 1400 K by a galvanic cell technique using a thoria-based electrolyte. Activities of Zr and Hf show large negative deviations from Raoult's Law; at 1300 K and 23 at. pct Zr of Hf, for instance,a _Zr=6.5×10⁻¹⁶ anda _Hf=7.9×10⁻¹⁷. Correlation of emf results with X-ray phase data enables calculation of standard free energies of formation of the intermetallic compounds ZrPt₅, ZrPt₃, and HfPt₃. At 1300 K ΔG _f ⁰ (ZrPt₅) =−92,680 cal/mole; ΔG _f ⁰ (ZrPt₃)=−91,740 cal/mole; and ΔG _f ⁰ (HfPt₃)=−97,350 cal/mole. The high stabilities of phases in the Pt−Ti, Pt−Zr, and Pt−Hf systems verify the predictions of the Engel-Brewer correlation. The large negative entropies of formation of TiPt₃, ZrPt₃ are discussed. Applications including side reactions in fuel cells and thermocouple systems are mentioned. P. J. MESCHTER, formerly a Graduate Student at the University of Pennsylvania This paper is based upon a dissertation submitted by P. J. Meschter in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Pennsylvania.