Predicted volume change behavior accompanying the solidification of binary alloys

For the volume changes accompanying solidification, distinctions are made between the volume changeβ _Mfor the whole freezing process, the volume changeβ _mfor liquid entrapped within the freezing zone, and the localized volume changeβ _Taccompanying the liquid-solid phase transformation at a given temperature. The first volume change is important in mold design, while the latter two are important factors in the formation of casting defects such as shrinkage pores, solidification cracks, and inverse segregation. Values ofβ _M, β_M, andβ _mare deduced for equilibrium conditions in the representative alloy systems Al-Cu, Bi-Sb, Fe-C and Pb-Sn. While the volume changeβ _Mmay vary only moderately with alloy composition,β _mis a strong function of composition and of the temperature of enclosure. The isothermal volume change,β _T, equal to the relative density difference between solid and liquid, varies during the freezing process and is strongly dependent upon composition. Isothermal volume changes and hence density differences as large as 20 pct are deduced for some Bi-Sb and Pb-Sn alloys.     

Melting of metals during rapid bulk heating

The melting of metals is studied during electric-current pulse heating at high rates (10⁶−10⁹ K/s). This process is found to be nonequilibrium: it proceeds with the overheating of the onset and end of melting and is nonisothermal. The experimentally measured enthalpies of nonequilibrium melting are used to calculate the nonisothermicity of nonequilibrium melting δ T and nonisothermicity factor δ T/T _m , where δT reflects the increase or decrease in the temperature of the end of nonequilibrium melting with respect to equilibrium melting temperature T _m . For many metals with a melting temperature of 500–3695 K, melting factor of nonisothermic melting δT/T _m ranges from 0.8–4.6% to −(0.2–5.1)% and depends on the relation between homogeneous and heterogeneous nucleation. The nonisothermicity of melting is confirmed by direct temperature measurements during melting of hafnium and zirconium. The contribution of the energy consumed for heating of a two-phase solid metal-melt system during nonisothermal melting in the course of pulsed heating to the melting heat is estimated. The relaxation times of equilibrium melting of some metals are estimated using nonequilibrium pulsed measurements. The role of the melting nonisothermicity factor in increasing the melting temperature of a graphite sample placed in a closed volume and heated with an electric current pulse at a rate of 5 × 10⁹ K/s is determined.     

Evaluation of hydrogen-trap binding enthalpy II

Influence of hydrogen diffusion parameters, heating conditions, and geometry of specimen on the H _B−T_p relation is theoretically estimated, where H _b is the hydrogen-trap binding enthalpy and T _p is the temperature of hydrogen release peak in elevated temperature hydrogen evolution experiments. If we write activation enthalpy for lattice diffusion of hydrogen as H _L, then theoretical analysis predicts that H _B+H_L+ΔH is substantially linearly related to thermal energy k _BT at hydrogen release peak temperature, T _p, as H _B+H_L+ΔH=γk_BT_p in a temperature region T _p>T₀+ΔT, below which deviation from linearity of the relationship takes place; ΔT mainly depends on the value of H _L. In the preceding expression, T ₀ is the initial temperature of heating; excess energy ΔH is found to be a constant, 0.09 eV, for a fixed value of H _L, 0.082 eV, for lattice diffusion of hydrogen in body-centered-cubic iron. The influence of diffusion and trap parameters, i.e., pre-exponential factor of hydrogen diffusivity, trap site density, parameter of trapping, heating rate, and specimen size, enters into the single parameter γ in the preceding expression.     

Growth of δ′ on dislocations in a dilute Al-Li alloy

The quantitative results of δ′ growth kinetics on dislocations in an Al-2.27 wt pct Li alloy demonstrate that at temperatures greater than 0.5 of the homologous melting temperature, T _m , volume diffusion is the dominant mechanism. However, a small contribution, approximately one atom in 300, is made by pipe diffusion through the dislocation. This can be established by careful examination of dislocation climb associated with precipitate growth. An analysis based on the δ′ growth kinetics and diffusion equation gives an activation energy of 0.56 eV for Li pipe diffusion. At temperatures <0.5 T _m , δ′ precipitate growth is faster when associated with dislocations, and here, pipe diffusion is necessary to account for the kinetics observed. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

Evaluation of hydrogen-assisted cracking behavior of low-alloy steel in the range 95 °C to 350 °C

In this report, hydrogen-assisted cracking (HAC) behavior of low-alloy steel was evaluated using a constant elongation rate tensile test, and the temperature and crack tip strain rate effects were observed. It was found that temperature dependence of the threshold condition (C _σm ^c ) of HAC above about 100 °C followed the relation C _σm ^c = Kexp(−41,300/Rr) whereK is a constant andT is absolute temperature. The relationship between HAC growth rate and crack tip strain rate was established as almost linear, irrespective of temperature and hydrogen concentration at the crack tip. Hydrogen heat release tests were also performed. From these tests, formation and growth of microcracks which are trap sites of hydrogen were thought to be the mechanism of HAC in the steel. From this mechanism, HAC behavior of the low-alloy steel could be qualitatively explained.     

Evaluation of hydrogen-trap binding enthalpy l

In order to evaluate hydrogen-trap binding enthalpy H_s from the elevated temperature hydrogen evolution peaksT _P, theH _B-T _P relation was analytically deduced in low hydrogen concentration approximation. The plot of H _B againstT _P revealed that in aT _P regionT _P ≧T _O + ΔT whereT _O is the initial temperature in heating and ΔT ≈ 80 K, H _B + 0.11 eV is substantially proportional tok _B T _P ,i.e., H _B + 0.11 eV = γ(N, ώ _O )k_B T _P for fixed geometrical and heating conditions, and the proportionality factor γ(N, ώ _O ) is influenced by hydrogen trap densityN and parameter of release ώ _O defined in the text. Thus for exact and precise determination of H _B parameter of release ώ _O as well as hydrogen trap densityN generally play an important part. In the present theory it is assumed that a specimen contais two kinds of deep traps (irreversible in the diffusion process at ambient temperature) and does not contain shallow traps (reversible at ambient temperature). The theory was applied to steels which vary in sulfur contents. The steels produced hydrogen release peaks at slightly different temperatures ranging from 588 K to 613 K, but the hydrogen-trap binding enthalpies H _B for these steels are shown to be substantially the same:H _P = 0.86 ± 0.005 eV.     

Rapidly solidified P/M X2020 aluminum alloys

X2020 aluminum alloys were produced with variations in the Li/Cu ratio by the ultrasonic gas atomization process. In alloy 68 (Al-4.9Cu-l.2Li) and 69 (Al-4.4Cu-l.55Li) alloys, the Θ′ and T₁, phases are dominant with evidence of the T_B phase. In the 70 (Al-3.5Cu-2.8Li) alloy, the δ′ phase is dominant with a trace of T₁. It was found that Θ′ andT ₁ are effective strengtheners whereas δ′ provides excellent fatigue crack initiation resistance. Overall results indicate that the fracture behavior of three RS-PM X2020 alloys is closely related to alloy production route as well as to the phases present in the alloys. Formerly Research Assistant, Massachusetts Institute of Technology.     

Evaluation of hydrogen-trap binding enthalpy II

Influence of hydrogen diffusion parameters, heating conditions, and geometry of specimen on the H _B-T_P relation is theoretically estimated, where H _B is the hydrogen-trap binding enthalpy and T _P is the temperature of hydrogen release peak in elevated temperature hydrogen evolution experiments. If we write activation enthalpy for lattice diffusion of hydrogen as H _L, then theoretical analysis predicts that H _B+H _L+ΔH is substantially linearly related to thermal energy k _BT at hydrogen release peak temperature, T _P, as

Phase Equilibria and Phase Transition of the Ni–Fe–Ga Ferromagnetic Shape Memory Alloy System

Phase equilibria and martensitic and magnetic transitions of the β (B2 and L2₁) phase in the Ni–Fe–Ga system were investigated. The b phase was found to be in equilibrium with the γ (A1 structure) or γ′ (L1₂ structure) phase. The Curie temperature, T _c , equilibrium temperature, T _o 5 (Ms + Af)/2, martensitic transition starting temperature, M _s , and reverse transition finishing temperature, Af , of the β single–phase alloys were sensitive to the Fe and Ga compositions. The Fe substitution for Ni decreased and increased the T _o and T _c , respectively. The Ga substitution for Ni or Fe decreased both the T _o and T _c . The entropy change accompanying the reverse martensitic transition showed compositional dependence due to the magnetic contribution. The saturation magnetization I _s of the Ni–Fe–Ga system showed a strong dependence on the magnetic valence Z _M . The Is values of the Ni–Fe–Ga alloys annealed at 1023 K showed the same Z _m dependence as other ferromagnetic shape memory alloy (FSMA) systems. This article is based on a presentation made in the symposium entitled "Phase Transformations in Magnetic Materials," which occurred during the TMS Annual Meeting, March 12-16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS–MPMD and ASMI–MSCTS Phase Transformations Committee.     

A simple relationship between the temperature dependence of the density of liquid metals and their boiling temperatures

For elemental metals, other than those in Groups VIII and IIB, there exists a simple rela-tionship between the temperature dependence of the liquid density, Λ [=(δD/δT)p], and the boiling temperature,T _B . It is shown that the fractional change in density between 0 K andT _B appears to be constant;i .e ., AT _B /d00 =-0.23. D00 is a scale factor defined by Doo =D _M - ΛT_m, whereD _m is the density of the liquid at the absolute melting temperature, T_M. This relationship has been used to estimate A for eight high melting point elements for which no experimental data exist.     

Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni alloy

Data on the temperature and composition dependence of the magnetic moment and Curie temperature of several Fe-Ni-Co and Fe-Ni-Mn alloys have been obtained. The temperature dependence of the magnetization was obtained for each alloy from 298 to 873 K, following the magnetization change through the transformation from martensite to austenite. The effect of cobalt and manganese additions to an Fe-29.9 at. pct Ni alloy on the reverse transition temperature,A _s , the Curie temperature,T _c , and the saturation magnetization at absolute zero, ρ_so, has been determined, Values forA _s , T _c , and ρ_so were obtained by fitting a Brillouin function to the respective contributions of austenite and martensite to the total magnetization. This technique represents a very sensitive method of obtaining transition temperatures and the respective amounts of each phase present in the alloys. A theoretical prediction of ρ_so andT _c was in agreement with the experimentally determined values.     

Influence of rare-earth metals on the high-temperature strength of Ni<Subscript>3</Subscript>Al-based alloys

The influence of the content of reaction- and surface-active alloying elements (rare-earth metals (REMs)) and the method of their introduction into cast high-temperature γ′-Ni₃Al-based intermetallic alloys, which are thermally stable natural eutectic composites, on their structure-phase state and the mechanical properties is studied. The life of low-alloy heterophase γ′ + γ cast high-temperature light Ni₃Al-based alloys is shown can be increased at temperatures exceeding 0.8T _m (T _m is the melting temperature of Ni3Al) due to additional stabilization of the single-crystal structure of these alloys with submicron and nanometer-sized particles of the phases formed by refractory and active REMs. It is also shown that stage-by-stage fractional introduction of all components into alloys during vacuum induction melting with allowance for their reaction activities (most refractory metals are introduced in the form of low-melting-point master alloys at the first stage of vacuum induction melting, and lanthanum is introduced with a master alloy in the optimal contents of 0.1–2 wt % into the charge of VKNA-1V and VKNA-25 alloys at the final stage) leads to the formation of a modified structure stabilized by nanoprecipitates of nickel and aluminum lanthanides and the phases formed by refractory metals. This method increases the life of VKNV-1V-type alloys (0.5 wt % Re) at 1000–1200°C by a factor of ∼1.7 and that of VKNA-25-type alloys (1.2 wt % Re and Co) by a factor of ∼3.     

Ferromagnetic bulk amorphous alloys

This article reviews our recent results on the development of ferromagnetic bulk amorphous alloys prepared by casting processes. The multicomponent Fe-(Al,Ga)-(P,C,B,Si) alloys are amorphized in the bulk form with diameters up to 2 mm, and the temperature interval of the supercooled liquid region before crystallization is in the range of 50 to 67 K. These bulk amorphous alloys exhibit good soft magnetic properties, i.e., high B _s of 1.1 to 1.2 T, low H _o of 2 to 6 A/m, and high μ _e of about 7000 at 1 kHz. The Nd-Fe-Al and Pr-Fe-Al bulk amorphous alloys are also produced in the diameter range of up to 12 mm by the copper mold casting process and exhibit rather good hard magnetic properties, i.e., B _r of about 0.1 T, high H _o of 300 to 400 kA/m, and rather high (JH)_max of 13 to 20 kJ/m³. The crystallization causes the disappearance of the hard magnetic properties. Furthermore, the melt-spun Nd-Fe-Al and Pr-Fe-Al alloy ribbons exhibit soft-type magnetic properties. Consequently, the hard magnetic properties are concluded to be obtained only for the bulk amorphous alloys. The bulk Nd- and Pr-Fe-Al amorphous alloys have an extremely high T _x/T_m of about 0.90 and a small ΔT _m(=T _m−T _x) of less than 100 K and, hence, their large glass-forming ability is due to the steep increase in viscosity in the supercooled liquid state. The high T _x/T_m enables the development of a fully relaxed, clustered amorphous structure including Nd-Nd and Nd-Fe atomic pairs. It is, therefore, presumed that the hard magnetic properties are due to the development of Nd-Nd and Nd-Fe atomic pairs with large random magnetic anisotropy. The Nd- and Pr-based bulk amorphous alloys can be regarded as a new type of clustered amorphous material, and the control of the clustered amorphous structure is expected to enable the appearance of novel functional properties which cannot be obtained for an ordinary amorphous structure. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Electron Microscopy Studies of Charge Density Wave (CDW) Transitions in Ti58.7Ni37.5Al3.8 Alloy

Electron diffraction and microscopy and electrical resistancevs temperature measurements of a Ti_58.7 Ni_37.5Al_3.8 alloy between room temperature and liquid nitrogen temperature have been carried out. On cooling, the increase in electrical resistance and the appearance of 1/3(110)- and 1/3(111)-type superlattice reflections are interpreted to be due to the formation of three-dimensional CDW’s. The transformation behavior, structural and microstructural changes of the present alloy are similar to those of the “premartensitic” behavior of a Ti₅₀Ni₄₇Fe₃ alloy which undergoes “normal-to-incommensurate” and “incommensurate-to-commensurate” CDW transitions, as reported earlier. The substitution of Al for Ni in TiNi and the nonstoichiometry of the present alloy apparently cause the incommensurate phase to exist over a larger temperature range. Formerly with the University of Illinois     

Glass-Forming Ability and Competitive Crystalline Phases for Lightweight Ti-Be–Based Alloys

The glass-forming ability (GFA) for the Ti-Be–based alloys in the Ti-Be-Zr ternary system is systematically studied. It was found that the best GFA obtained at a composition of Ti₄₁Be₃₄Zr₂₅ (at. pct) in the Ti-Be-Zr ternary system, and the bulk-metallic-glass (BMG) rod samples with a diameter of 5 mm were fabricated by Cu-mold casting. The competitive crystalline phases around the composition of the best GFA materials were determined by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The GFA of the ternary alloys was further improved by an addition of 4 at. pct vanadium (V). The largest supercooled liquid region, ΔT _x (ΔT _x  = T _x − T _g , T _g is the glass-transition temperature, and T _x the crystallization temperature), in the ternary alloy system reaches about 110 K (110 °C) for the Ti₃₅Be₃₂Zr₃₃ alloy.     

Synthesis of new bulk metallic glassy Ti60Al15Cu10W10Ni5 alloy by hot-pressing the mechanically alloyed powders at the supercooled liquid region

The mechanical alloying method has been used to fabricate multicomponent Ti₆₀Al₁₅Cu₁₀W₁₀Ni₅ glassy alloy powders, using a low-energy ball-milling technique. The glassy powders that were obtained after 720 ks of milling have a sphere-like morphology with an average particle size of 0.38 μm in diameter. This new glassy alloy exhibits a glass transition temperature (T _g) at 733 K. It crystallizes at a crystallization temperature (T _x ) of 804 K through a single sharp exothermic peak, with an enthalpy change of crystallization (ΔH _x ) of −5.20 kJ/mol. The supercooled liquid region before crystallization ΔT _x of the obtained glassy powders shows a large value (71 K). The reduced glass transition temperature (ratio betweenT _g and liquidus temperatures,T ₁(T _G /T ₁) was found to be 0.46. The synthetic glassy powders were uniaxial hot-pressed into consolidated round objects with large dimensions (20 mm in diameter × 30 mm in height) in an argon gas atmosphere at several temperatures with a pressure of 936 MPa. The samples that were consolidated at the temperature range of 755 to 775 K (within the ΔT _x region) are fully dense (∼99.85 pct) and maintain the chemically homogeneous glassy structure. These hot-pressed glassy samples exhibit excellent mechanical properties for Ti-base metallic glasses. They have high Vickers microhardness values, in the range between 8.0 and 8.2 GPa. They also show high fracture strength (2.28 GPa) with an extraordinarily high Young’s modulus of 153 GPa. Neither yielding stress nor plastic strain could be detected for this glassy alloy, which shows an elastic strain of 1.39 pct.     

Composite materials with an intermetallic matrix based on nickel and titanium monoaluminides hardened by oxide particles or fibers

Hardening phase/intermetallic matrix pairs are chosen for composite materials (CMs) intended for long-term high-temperature operation. These materials must have high and stable mechanical properties during a long time at high temperatures and loads. The compatibility of the physicochemical and mechanical properties of CM components is estimated to choose hardening phase/intermetallic matrix pairs in which the matrix is represented by an alloy based on NiAl or TiAl monoaluminide and the hardening phase is a refractory thermodynamically stable oxide of a Group III transition metal M ₂O₃. The following two schemes are used to perform hardening of a CM with a matrix consisting of a TiAl or NiAl alloy by the most thermodynamically stable interstitial phases, i.e., refractory oxides, at temperatures higher than the operating temperature (T _op) of the IMM. The first scheme consists in creating Al₂O₃/TiAl CMs hardened by continuous single-crystal sapphire fibers using the impregnation of a bundle of single-crystal fibers with a matrix melt followed by directional solidification. The TiAl-based matrix in these CMs serves as a binder connecting oxide phase fibers and preventing them from fracture due to high adhesion forces between oxide fibers and the matrix and a high fiber/matrix interface strength. In the second scheme, Y₂O₃/NiAl CMs are produced by powder metallurgy methods, which include severe deformation by extrusion accompanied by the formation of deformation texture and subsequent recrystallization annealing. In these CMs, disperse refractory oxide particles stabilize grain boundaries in a recrystallized matrix material and lead to the formation of directional structures with coarse elongated grains and a low fraction of transverse boundaries. Al₂O₃/TiAl CMs containing 20–25 vol % hardening single-crystal sapphire Al₂O₃ fibers can operate at temperatures of 1000–1050°C (∼0.7T _m of matrix), which is 250–300°C higher than the maximum values of T _op of a TiAl-based matrix and 400-450°C higher than the maximum values of T _op of a Ti-based matrix. An Y₂O₃/NiAl composite with a directionally recrystallized structure of a NiAl-based matrix hardened by 2.5 vol % Y{ia2}O₃ particles can be recommended for operation at temperatures of 1400–1500°C ((0.8–0.9)T _m of matrix), which are higher by 100–400°C than not only T _op but also T _m of Ni superalloys.     

The transition from gray to white cast iron during solidification: Part III. Thermal analysis

In Part I of this work, an analytical model was derived based on the theory for the transition from gray to white cast iron during solidification. Accordingly, in this part of the work, analytical expressions were found that can be related to the chill width in wedge-shaped castings (or the critical pin diameter and critical wall thickness below which the chill is expected) with temperature parameters that are commonly measured by thermal analysis. Among these parameters are ΔT _c =T _m −T _c and ΔT _sc =T _s −T _c , where T _m is the minimal solidification temperature for graphite eutectic, T _c is the transformation temperature for cementite eutectic, and T _s is the equilibrium solidification temperature for graphite eutectic. In particular, it was found that the analytical model agrees well with the experimentally measured temperatures. Measured values from thermal analysis for the excess-temperature range (ΔT _c ) and of the ΔT _c /ΔT _sc ratios were found to be in good agreement with the theoretical predictions for the chill of cast iron. In addition, it was found that the predictions from the proposed analytical expressions are similar to those obtained from statistical analyses of the experimental outcome.     

Combined hardening of alloy Ti-6Al sheet workpieces during reversible hydrogen alloying

The effect of the initial hydrogen concentration, warm rolling, and vacuum annealing conditions on the formation of the phase composition, structure, and mechanical properties of rolled sheet workpieces made of a Ti-6Al α alloy is studied. When the initial hydrogen concentration increases to C _Hⁱⁿⁱ = 0.3–0.9%, the grain size decreases and the phase composition of the alloy is complicated. In the grain size range 27–5 μm, the yield strength of the alloy obeys the Hall-Petch relation with the lattice friction stress σ _i = 662 MPa. When the initial hydrogen concentration increases, the grain-boundary hardening intensity and the yield strength increase. At an average α grain size of 5 μm, the yield strength increases from 770 MPa in the alloy with C _Hⁱⁿⁱ = 0.004% to 970 MPa in the alloy with C _Hⁱⁿⁱ = 0.7%. The maximum yield strength (σ_y = 1064 MPa) is obtained for the alloy with C _Hⁱⁿⁱ= 0.5% after vacuum annealing at 650°C. The conditions and contributions of solid-solution hardening, grain-boundary hardening, precipitation hardening induced by the formation of the α₂ phase, and strain hardening to the total hardening of the alloy are considered.     

High temperature creep in a semi-coherent NiAl-Ni2AlTi alloy

Creep experiments have been made on a Ni-Ti-Al alloy, which has a microstructure consisting of a distribution of semi-coherent NiAl(β) precipitates with a Ni₂AlTi(β′) Heusler phase matrix. The creep strength of this bcc type structure alloy is at least comparable with that of the nickel-base superailoy MARM-200 for values ofT/T _m in the range 0.68 to 0.82. Quantitative electron microscope experiments show that both undissociated α₀〈110〉 dislocations, and paired α₀〈100〉 dislocations coupled by a sublattice A.P.B. exist within the β′ phase;α ₀ is the lattice parameter of a bcc cell of which the large Ni₂AlTi unit-cell is composed. The sublattice A.P.B. is a crystallographic fault created by wrong bonds between atoms on the Al-Ti sublattice. Theoretically the energy γ of a sublattice A.P.B. is shown to be minimum on {100}, and the experimental value for γ on {100} is ~40 mJ/m₂.