Comparison of the fatigue and fracture of α+β and β titanium alloys

The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical processing, maximum K _Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue crack initiation and propagation resistance (especially against small fatigue cracks).     

Ordering and phase separation in the ternary Ag-Cu-Zn β-phase alloys

The microstructure of the ternary Ag_xCu_55−xZn₄₅ and Ag_xCu_53−xZn₄₇ β-phase alloys was studied by means of electron microscopy. The range of temperature and composition where the mottled and modulated structures appeared was investigated, and the composition dependence of the spinodal decomposition temperature T_s was also estimated. The composition dependence of T_s had a maximum at about Ag 22 at.%. The critical temperature of the CsCl(B2)-type superstructure T_c decreased with increase in Ag content, but was constant in a range of composition where the phase separation was observed. The precipitation of α-phase (f.c.c.) was observed even in the quenched specimens, that suggests the phase separation of β-phase at a high temperature. The composition dependence of T_c and T_s, was calculated theoretically. The occurrence of the ordering and the spinodal decomposition in the ternary Ag-Cu-Zn β-phase alloys is discussed briefly in terms of the difference of ordering energy that is negative in Ag-Zn and Cu-Zn pairs and positive in Ag-Cu pair.     

Application of physical–empirical models to calculate a fragment of the phase diagram and the physical properties of bcc Fe–Cr alloys: II. Calculation of phase boundaries,spinodal, and the temperature dependence of the heat capacity of an alloy

The contributions at a temperature of 500 and 600 K of the chemical, elastic, vibrational, magnetic, electronic, and configurational energies to the Gibbs energy of mixing of bcc alloys without regard for the contribution of a short-range order are calculated as functions of composition and temperature using physical–empirical models. The temperature dependences of the heat capacity of an alloy in both one- and twophase states are calculated. The heat capacity jumps calculated for alloys of various compositions can be used to estimate the equilibrium solubility boundaries of Fe–Cr alloys, which can hardly be found from experimental data because of the slow diffusion processes that occur when an equilibrium state is reached. The calculated solubility boundary of bcc solid solutions and the spinodal and the heat capacity of Fe–Cr alloys are compared with the experimental data and the calculation results obtained in other works. The agreement and discrepancy between these data are discussed.     

Grain coarsening in FeNiCr alloys and the influence of second phase particles

The effects of second phase particles, e.g. M₂₃C₆, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M₂₃C₆ carbides varied, since Fe, Ni and Mo entered the M₂₃C₆ unit cell; and complex carbides such as (Cr₁₅Fe₄Ni₂Mo₂)C₆ were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.     

Formation regularities of grain-boundary interlayers of the α-Ti phase in binary titanium alloys

The microstructure of polycrystalline alloys of titanium with chromium (2, 4, and 5.5 wt %), cobalt (2 and 4 wt %), and copper (2 and 3 wt %) is investigated. Series of prolonged isothermal annealing of these materials are performed in a temperature range from 600 to 850°C (in vacuum). Annealing temperatures fall in two-phase regions α(Ti,Me) + β(Ti,Me) of phase diagrams Ti–Cr, Ti–Co, and Ti–Cu. Temperature dependences of the fraction of grain boundaries β(Ti,Me)/β(Ti,Me) completely “wetted” by interlayers of the second solid phase α(Ti,Me) and average contact angle are measured. The results of microstructural investigations showed that the type and concentration of the second component in the alloy strongly affect the formation of equilibrium grain-boundary interlayers. A nonmonotonic temperature dependence of the fraction of grain boundaries completely wetted by interlayers of the second solid phase in the absence of ferromagnet–paramagnet phase transformations in the volume is revealed for the first time.     

On the f.c.c. → h.c.p. phase transformation in high manganese-iron alloys

Electron transmission microscopy was done on the Fe alloys with 29.8 and 37.2 at.% Mn. with the γ→ϵ transformation induced by all three driving forces: temperature change, cold deformation and high-pressure soaking (up to 9.0 GPa (90 kbar)). Contrary to the well established data, the presence of the ϵ phase after cooling was observed at Mn concentrations not less, than 37at.%. It was proved by direct observation that high-pressure soaking and cold deformation strongly affect phase equilibria in Fe-Mn alloys: this effect is dependent on the Mn concentration, decreasing with it. The familiar orientation relationships between γ and ϵ phases, namely (1̄11)_γ ∥(0001)_ϵ, and [101̄]_γ ∥[112̄0]_ge, are observed in all obtained structures independently of the kind of driving force applied to promote the γ→ϵ transformation. It is proposed to explain the unusual behaviour of Fe-Mn alloys (limited growth of the ϵ martensitic phase upon cooling in the vicinity of M^γ→ϵ_s. very small sizes of the ϵ particles) by a model which takes into account the antiferromagnetic ordering in both phases. It is shown that this explanation is consistent with experimental results.     

The effect of conditions of preparation on the strength of WC — Co hard alloys and the relationship between the strength of these alloys and their structure and composition

Conclusions This paper presents a review of investigations which elucidated the effect of the technological conditions of obtaining the initial tungsten powder and the grinding and sintering conditions on the strength of a hard alloy during bending. An analysis of the results of these investigations shows that the differences in the strength of alloys differing in carbide grain size is primarily due to various manufacturing conditions and not to differences in grain size.On applying various methods of regulating the size of the carbide grain, sharply different changes in alloy strength are observed. The influence of the size of the grain was not noted, since it was suppressed by the greater effect of the technological factors.On excluding the effect of changing the sintering temperature (and other variable technological factors) the dependence of the alloy strength on the cobalt content changes its form: the maximum strength between 15 and 25% cobalt, noted in many researches, is not observed in this case. The strength increases continually up to 50% cobalt.The dependence on grain size with various methods of regulating carbide dispersity, and the dependence of the strength on the cobalt content on excluding the effect of the technological factors differ substantially from the regularities described by Gurland [1].The theoretical theses of Gurland are ungrounded, inasmuch as the strength relations are at variance with them, when the effect of the technological factors is excluded.The relations actually existing may be explained on the basis of the skeleton structure of the carbide and cobalt phases.     

Friction properties of composites of the system TiN−AlN. I. Effect of structure and phase composition of friction and wear of materials of the system TiN−AlN

The tribological properties of ceramic composites of the system TiN?AlN are studied in the concentration range 10–90% AlN. It was found that materials which contain 25, 50, and 75% AlN have a low friction coefficient. Their rate of wear when paired with steel is neglible—2.9–6.0 μm/km (the wear rate of a steel-steel 024 couple is 1000 μm/km). Thin oxide films are formed during friction at high velocities and pressures. These films have relatively high adhesion with respect to materials of the TiN?AlN system and relatively low adhesion with respect to steel. The films may act as a solid lubricant, thus reducing the friction coefficient and wear. This is particularly true of the materials 25% TiN?75% AlN and 50% TiN?50% AlN.     

C-Ni and Al-C-Ni phase diagrams and thermodynamic properties of C in the alloys from 1550 °C to 2300 °C

The C-Ni and Al-C-Ni phase diagrams were determined by chemical analysis of alloys saturated with carbon within sealed graphite crucibles. The solubility of carbon in nickel over the temperature range 1550 °C to 2300 °C is given by log (at. pct C)=2.0376−1874.68/T, where T is temperature in kelvin. Isothermal sections for the ternary system were determined at intervals of 150 °C over the range of temperatures investigated. The univariant points on the 1700 °C, 1850 °C, and 2000 °C isotherms were determined by metallographic examination of rapidly cooled alloys to be about 67.2Al-1.1C-31.7Ni, 70.3Al-2.3C-27.4Ni, and 82.5Al-7.0C-10.5Ni, respectively, where all concentrations are in atomic percent. Graphite, Al₄C₃ (decomposition temperature 2156 °C), and AlNi (decomposition temperature 1638 °C) were the only solid phases observed within the temperature range investigated. The excess partial Gibbs energy for dissolved C, , in liquid Al-C-Ni solutions in equilibrium with C, as calculated from the experimental solubilities and thermodynamic data on Al-Ni, is

Cu-C and Al-Cu-C phase diagrams and thermodynamic properties of c in the alloys from 1550 °C to 2300 °C

The Cu-C and Al-Cu-C phase diagrams were determined at 1550 °C to 2300 °C by chemical and X-ray diffraction analyses of alloys saturated with carbon within sealed graphite crucibles. Isothermal sections for the ternary system were determined at intervals of 150 °C over the range of temperatures investigated. The univariant points in atomic percent on the 1700 °C, 1850 °C, and 2000 °C isotherms are 70.7Al-28.9Cu-0.4C, 74.4Al-24.0Cu-1.6C, and 78.3Al-17.0Cu-4.7C, respectively, as determined by metallographic examination of rapidly cooled alloys. Graphite and Al₄C₃ (decomposition temperature 2156 °C) were the only solid phases observed at these temperatures. The excess partial Gibbs energy for dissolved carbon in the liquid Al-Cu-C solutions in equilibrium with C, as calculated from the experimental solubilities, isˉ G _c ^e = - RT lnx =y ²[176,860 - 55.42T - (224,200 - 110.84T)x] +z ²[237,000 - 48.61T] +yz[320,510 - 36.77T + (30,180 + 35.10T)z + (51,570 - 74.13T)yz + (246,400 - 88.04T)yz ² - 60,000], J/g atom where R is the gas constant,T is the temperature in K, andx, y, andz are the atomic fractions of C, Al, and Cu, respectively. The equation also is a good approximation for liquid solutions in equilibrium with A1₄C₃.     

Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a review of the literature

Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum alloys. Recently, there has been a shift in understanding of the grain-refinement paradigm from purely being concerned with the nucleation event, called here the “nucleant paradigm,” to also being concerned with the effect of solute elements, or, the “solute paradigm,” on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement.     

Segregation and order in binary substitutional alloys—II. Ground state phase stability diagrams

A general method to determine phase diagrams and local order from the electronic theory of alloys is presented; we report a first numerical application of this method to the study of binary substitutional transition metal alloys using a canonical tight binding model for the “d” band structure. The relevant physical quantities (chemical, elastic…energies), which are required in such calculations are discussed in detail. Thus the results about the concentration dependence of the pair interactions and the energy of the disordered state are discussed. The stability of the homogeneous ordered structures, against a mixing of two phases is investigated and the general trends about segregation, miscibility and order are then reported. Finally the ground state phase stability diagrams of a series of systems corresponding to physical situations are presented.     

Plastic flow of Fe-binary alloys—II. Application of the description to the ductile-brittle transition

This is the second of a two-part series designed to elucidate the role of dislocation dynamics and particle-nucleated cleavage in the ductile-brittle transition temperatures (DBTT) of Fe-binary alloys. Detailed measurements of slow-bend and impact transitions demonstrate that additions of Ni slightly increase the cleavage fracture stress and effective fracture surface energy of iron while Si additions provide the opposite trend. With the establishment of 19 flow and fracture parameters, a semiempirical DBTT model evolves which compares favorably to observed transition temperatures under slow notch bend and impact conditions for Fe, Fe-Ni and Fe-Si. The model clarifies how reinforcing effects of solid solution softening and an increased cleavage fracture stress leads to a monotonically decreasing transition temperature with increasing Ni additions. In addition, it demonstrates how opposing effects of softening and a lowered cleavage fracture stress leads to a minimum in the transition temperature with increasing Si additions.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Formation of the structure and phase composition of the Ti–Al–Ta-based materials

The experiments on the fabrication of materials based on the Ti–3Al–0.5Ta and 3Ti–2Al–Ta systems by self-propagating high-temperature synthesis (SHS) are performed. The influence of the composition of the initial mixture, dispersity of powders, and preliminary mechanical activation on the phase composition and structure of the SHS product is investigated. The optimal ratio between the mechanically activated and initial powder in a mixture for the synthesis of materials is determined. The dependence of the structure of final products on the structure of initial powders is established. The use of porous tantalum leads to the formation of the intermetallic matrix based on titanium aluminide with the uniform distribution of Ta particles. It is noteworthy that tantalum powders of both studied series (which differ by dispersity and morphology) partially reacted already at the stage of mechanical activation with the formation of the Al₂Ta phase. It is shown that aluminum plays the leading role in processes of mechanical activation in Ti–Al–Ta reaction mixtures. Indeed, a considerable rise of unreacted tantalum particles in the microstructure of sintered samples is observed with a decrease in the amount of aluminum in the reaction mixture.     

Effect of coherency stresses on α + α2 phase equilibria in TiAl alloys

Experiments were performed on two TiAl alloys to examine recent theoretical predictions concerning the characteristics of coherent two-phase equilibria in binary alloys. Equilibrium phase compositions in both the coherent and incoherent α + α₂ states at 725 and 800°C were determined by analytical electron microscopy. The results provide experimental evidence for the following predictions: (1) tie-line ends (equilibrium phase compositions) need not coincide with phase boundaries in the temperature-composition phase diagram and (2) the tie-line ends change with alloy composition at constant temperature. The experimental results have been compared with theoretical calculations of α + α₂ phase equilibria incorporating both elastic and ordering effects.     

Constitutive modeling and characterization of the flow behavior of semi-solid metal alloys slurries—I. The flow response

We present an internal variable constitutive model for semi-solid metal alloy slurries in this two-part paper. The first part discusses the constant internal structure flow equation and the second part derives the structure evolution equation. The model is valid for low (0.1) to moderate solid fractions (0.5-0.6). We also present a set of experiments on two alloy systems, Sn15% Pb and Al7% Si0.6% Mg, performed on a computer controlled, high temperature Couette rheometer to determine the model parameters. We independently validate the internal variable model through separate, primarily hysteresis, experiments.