Precipitation hardening in nickel-copper base alloy monel K 500

The occurrence of a significant amount of age hardening, due to the precipitation of the γ′ phase, has been demonstrated in the nickel-copper base alloy MONEL K 500. The micro-structure of the precipitation-hardened and deformed alloy has been examined in peak-aged underaged and overaged conditions. An attempt has been made to compare the observed increments in strength in these three aged conditions to those predicted on the basis of relevant theoretical models.     

On the effect of stress on nucleation and growth of precipitates in an Al-Cu-Mg-Ag alloy

A study has been made of the effect of an externally applied tensile stress on Ω and Θ′ precipitate nucleation and growth in an Al-Cu-Mg-Ag alloy and a binary Al-Cu alloy which was used as a model system. Both solutionized and solutionized and aged conditions were studied. The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). The volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutionized samples aged under stress, precipitation occurs preferentially parallel to the stress axis. A threshold stress has to be exceeded before this effect can be observed. The critical stress for influencing the precipitate habit plane is between 120 and 140 MPa for Ω and between 16 and 19 MPa for Θ′ for the aging temperature of 160 °C. The major effect of the applied stress is on the nucleation process. The results are discussed in terms of the role of the lattice misfit between the matrix and the precipitate nucleus.     

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

The precipitation of the γ^’ (Ll₂) and γ^" (DO₂₂) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ^"particles have always been found to precipitate on γ^’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ^’ particles coated on their six faces with a shell of γ^" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained. This paper is based upon a thesis submitted by R. COZAR in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Nancy.     

Phase transformations and tensile properties of Ti-10V-2Fe-3AI

The effect of heat treatment on the microstructure of the metastableα-Ti alloy Ti-10V-2Fe-3Al has been studied using light and electron metallography, analytical electron microscopy, and X-ray diffraction. A survey of the effects of microstructure on tensile properties of the alloy also has been conducted. It has been found that the alloy contains inclusions which are rich in Ti, S, Si, and P. The alloy has been shown to form ω-phase both athermally and isothermally. The isothermal ω can have either an ellipsoidal or a cuboidal morphology. The reasons for this are enumerated. The formation of α-phase has been studied, and three distinct modes of formation are described. A stress induced orthorhombic martensite also has been observed. The effect of this stress induced product on tensile behavior is discussed. The relative roles of inclusions and α-phase precipitates in the tensile fracture also have been examined. Formerly at Carnegie-Mellon University, is now at Brown, Boveri & Cie, AG, Baden, Switzerland,     

Physical metallurgy of a Ti-5%Ta-1.8%Nb alloy

An alloy of Titanium with 5% Tantalum and 1.8% Niobium has been developed which possesses high corrosion resistance in highly oxidising environments. The microstructural basis that enabled design of optimum thermo-mechanical treatments has been established for this alloy. The classification of the alloy, transformation temperatures and different types of phase transformations were evaluated for the first time by experimental methods like metallography and calorimetry and empirical methods. Systematic microstructural modifications were introduced by thermo-mechanical treatments to improve corrosion resistance and mechanical properties. The Ti-5Ta-1.8Nb alloy exhibited interesting texturing behaviour. Deformation and transformation textures exhibited during unidirectional cold rolling and subsequent β→α+β transformation were studied using XRD and Electron Back Scatter Diffraction techniques. The cross section of a wire drawn specimen exhibited (1 0–1 0)_α texture while a cold rolled specimen showed (0 0 0 2)_α deformation texture along the length — width direction. The transformation texture by itself was found to be dependent on the type of deformation texture, (1 1 −2 0)_α in cold rolled and (1 1 −2 2)_α texture in the case of wire drawn alloy. A new method has been proposed to determine theoretical misorientation angle and axis between variants of hcp a product transforming from a parent bcc crystal, obeying Burgers Orientation Relationship. The role of variant selection mechanisms in the final texture of the alloy has been demonstrated by comparison of texture maps obtained by X-ray Diffraction with those computed.     

Deformation and fracture behavior of Ni-Mo-Al (γ/γ′-α)in situ composite

Tensile properties for Directionally Solidified (DS) eutectic alloy of a nominal composition Ni-33 Mo-5.7 Al (weight percent) have been investigated both at room temperature and the elevated tem-peratures. The microstructure-mechanical property relationship has been studied for the alloy both in the as-DS and heat-treated conditions. Changes in the yield strength, the work hardening behavior, and the fracture morphology have been explained in terms of the microstructural changes due to the heat treatment. The yield drops observed have been attributed to the microdebonding due to the possible segregation of impurities at the fiber-matrix interface, and partly to the strain aging. The deformation mechanism has been identified to be cutting of γ′ particles. On leave from defence Metallurgical Research Laboratory, Hyderabad-500258, India.     

Phase transition in an Fe-23.2Al-4.1Ni alloy

The phase transition in an Fe-23.2 at. pct Al-4.1 at. pct Ni alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, the microstructure of the alloy is a mixture of (A2 + B2) phases. When the as-quenched alloy is aged at temperatures ranging from 500 °C to 1050 °C, the phase transition sequence is found to be (A2 + B2∗) → (B2 + B2∗) → B2 → A2, where B2∗ is also a B2-type phase. It is worthwhile to note that the coexistence of two kinds of ordered B2 phase has not previously been observed by other workers in the Fe-Al-Ni ternary alloy system.     

Decomposition of the metastable beta phase in the all-beta alloy Ti-13V-11Cr-3Al

The phase transformations which occur in the all-β titanium alloy Ti-13V-llCr-3Al at temperatures below ∼ 500°C have been studied by transmission electron microscopy, X-ray diffraction, and electrical resistivity techniques. The decomposition of the metastable β phase has been found to proceed through a transition reaction leading to the formation of two bcc phases rather than through the precipitation of the ω phase as has been previously reported. An interpretation of the observed decomposition sequence, based on thermodynamic reasoning, is presented. The decomposition characteristics of the Ti-13V-llCr-3Al alloy have been compared with those of a Ti-13V-llCr alloy to determine the influence of aluminum on the transformation behavior.     

Effect of additional hydrogen alloying on the formation of the phase composition and structure of a Ti-8.3Al-2.1Mo-2.2Zr-0.2Si titanium alloy

The effect of additional hydrogen alloying on the formation of the phase composition and structure of a titanium alloy with a high aluminum content is studied. It is shown that an α₂ + β structure can be developed under certain temperature-concentration conditions. The phase composition-hydrogen concentration-hydrogenation annealing temperature diagram has been constructed. It shows a change in the phase composition of a Ti-8.3Al-2.1Mo-2.2Zr-0.2Si alloy as a function of the hydrogenation annealing temperature and the hydrogen content after cooling to a normal temperature at a rate higher than the first critical rate.     

Differential scanning calorimetry study and computer modeling of <Emphasis Type="Italic">β</Emphasis> ⇒ <Emphasis Type="Italic">α</Emphasis> phase transformation in a Ti-6Al-4V alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.     

Microstructure refinement of NiCoCrAIY alloys by laser surface melting

The surface of a cast γ + β Ni-20Co-19Cr-24Al-0.2Y (at. pct) alloy is melted with 75 MW/m² laser pulses for five different laser irradiation times and two different alloy surface conditions (different initial alloy reflectivity). Detailed analysis of the chemical composition, microstructure, and phase constitution of the laser-melted and original alloy reveals that after laser surface melting (1) the average size of the β dendrites is much smaller, (2) the Y is distributed more homogeneously as smaller NiY-rich precipitates along γ/β phase and γ/γ grain boundaries, (3) the alloy contains a higher volume fraction of the β phase, (4) the β phase contains less Al and more Cr and has a lower lattice parameter, and (5) the γ phase contains more Al and less Cr and has a higher lattice parameter. Based on both experimental results and model calculations, it is shown that the maximum depth of the melt pool increases with increasing laser irradiation time and decreasing initial alloy reflectivity. The arm spacing of the secondary β dendrites, which also increases with increasing laser irradiation time, is related to the average alloy cooling rate according to the power of −1/3, in agreement with theories on solidification.     

Characterisation of solution annealed VT-14 titanium alloy using eddy current based electrical resistivity measurements

This paper discusses characterisation of microstructures in Ti-4.5Al-3Mo-1V (VT-14) alloy specimens solutionized in the temperature range of 923K–1373K, using room temperature electrical resistivity measurements. Using phase angle of impedance change in eddy current coil as a parameter, calibration curve between resistivity and phase angle has been established using three reference specimens of known resistivity. The changes in the electrical resistivity of the specimens have been correlated with optical microscopy investigations. The electrical resistivity of unstable β phase has been compared with α and α′ phases and possible reasons for higher resistivity of β phase and lower resistivity of α′ have been given based on the rule of mixtures and scattering and mobility of electrons. The electrical resistivity of α′ martensite phase of the chosen VT-14 alloy has been determined as 1.04 αΩ-m. The studies reveal that it may be possible to identify the β transus temperature of α+β titanium alloys from the electrical resistivity data.     

Application of the metalhydrogen equilibration for determining thermodynamic properties in the ti-cu system

A technique has been proposed for determining thermodynamic activities in a binary alloy from measurements of the pressure of hydrogen gas equilibrated with the alloy and the hydride of one of the components. The technique is similar to a method by equilibration of oxygen gas with an alloy and the oxide of a component. With the use of this technique, activities of the components in the Ti-Cu system at 773 have been measured. From the measured activities, the standard free energies, enthalpies, and entropies of formation for intermetallic compounds “TiCu₄”, “TiCu”, and “Ti₂Cu” have been determined. formerly Senior Student at Tokyo Institute of Technology.     

Influence of microstructure on tensile and creep properties of a new castable TiAl-based alloy

Snecma Motors has been working on the development of γ-TiAl low-pressure turbine blades, including manufacturing optimization, castability evaluation of a selected alloy called G4, and heat-treatment optimization of mechanical and physical properties. The objective of this study was to evaluate microstructure variability regarding casting conditions and aluminum content. The response of cast microstructures to hot isostatic pressing (hipping) and subsequent heat treatments was determined and quantified using tensile and creep testing. Such investigations helped define an optimized heat treatment. Tensile and creep property assessment has shown a high-temperature potential for G4 alloy with respect to other γ alloys. The G4 alloy also appears to be more creep resistant than conventional nickel-based superalloys on a specific basis. The enhanced creep properties under the optimized low-temperature treatment are mainly attributed to solid solution strengthening with Re, W, and Si elements and precipitation hardening with primary β phase decorating the primary dendrites.     

Phase transformation of stainless steel during fatigue

Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous. Formerly a Research Assistant     

Compatibility of deformation in two-phase Ti-Al alloys: Dependence on microstructure and orientation relationships

A two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures. These consisted of lamellar and equiaxed distributions of y grains in which the α₂ phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between γ/γ and γ/α₂ grains have been measured, and these have been used to understand their effect on the compatibility of deformation across adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy (TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.     

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ^′ and carbides), alloy B with double aging treatment (spherical γ^′ of 18-nm diameter and M₂₃C₆), and alloy C with another double aging treatment (γ^′ of size 35 nm, MC and M₂₃C₆). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ^′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ^′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ^′ and consequent softening. Coarser γ^′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.     

The effect of aging on the tensile mechanical properties of high-purity binary Ni-10, -20, and-30 Wt pct Cr alloys

The variation of the tensile mechanical properties and hardness (at 25‡C) of three Ni-Cr alloys has been determined as a function of aging time (up to 4 months) in the range 290 to 530‡C. Aging has a negligible effect on the 10 pct Cr alloy, and only a slight effect on the 20 pct Cr alloy. However, the 30 pct Cr alloy showed a marked sensitivity to aging; for example, at 479‡C the yield strength doubled after about 1 month, then decreased slightly at 4 months. The effect in the 30 pct Cr alloy is due to the formation of the Ni₂Cr superlattice.     

Sliding wear response of a zinc-based alloy compared to a copper-based alloy

In the present investigation, an attempt has been made to study the wear response of a zinc-based alloy at different sliding speeds and pressures. The wear behavior of the zinc-based alloy has been compared to that of a conventionally used copper-based alloy against an EN24 steel disc. Operative wear mechanisms have also been discussed through the examination of wear surfaces, subsurface regions, and debris particles. The results obtained revealed that the wear response of the zinc-based alloy is much better than the conventionally used copper-based alloy at all the speeds; the latter suffered from “material chipping off,” leading to its higher wear rates. The wear characteristics of the specimens have been explained on the basis of their specific microstructural features.     

The effect of Cu additions on the precipitation kinetics in an Al-Mg-Si alloy with excess Si

The microstructural evolution during age hardening of a Cu-bearing Al-Mg-Si alloy has been investigated by the three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques, in order to clarify the effect of Cu on the initial age-hardening response. After 30 minutes of artificial aging at 175 °C, the alloy shows a significant increase in hardness. The TEM observations have revealed that very fine, needle-shaped β″ precipitates are formed in addition to spherical Guinier-Preston (GP) zones, whereas only the spherical GP zones are observed in the Al-Mg-Si ternary alloy using the same aging condition. The number density of the precipitates is significantly affected by the preaging conditions. The 3DAP analysis shows that the distribution of Cu atoms is uniform after 30 minutes of artificial aging at 175 °C, whereas Cu atoms are incorporated into the needle-shaped β″ precipitates after 10 hours of aging at 175 °C. Based on these microanalytical results, the effect of Cu additions on the age-hardening response of Al-Mg-Si alloys is discussed.