Internal friction in diamagnetic copper and lead polycrystals in a DC magnetic field

The amplitude-dependent internal friction of diamagnetic copper and lead polycrystals 99.9% pure has been studied experimentally in dc magnetic fields with an induction 0 ≤ B ≤ 0.75 T. In a dc magnetic field B = 0.1 T at room temperature, the sign inversion (from plus to minus) of internal-friction increments ΔQ ^?1(B) = Q ^?1(B) ? Q ^?1(0) is revealed for two copper samples. The copper contained impurity-pinned dislocations at ΔQ ^?1(B) > 0 and impurity-free dislocations at ΔQ ^?1(B) < 0. The positive sign of ΔQ ^?1(B) for the internal friction in copper is due to a decrease in the potential-barrier height U _b for dislocation motion in impurity-containing metals. The negative sign of ΔQ ^?1(B) for the internal friction in copper is due to a simultaneous decrease in the barrier height U _b and width (activation volume Θ) during dislocation motion in impurity-containing metals. For a magnetic field B = 0.1 T, the sign of ΔQ ^?1(B) in lead is positive at temperatures below 70°C and the sign of ΔQ ^?1(B) is negative at temperatures above 70°C. For lead, the sign inversion of ΔQ ^?1(B) is explained by the dissolution of dislocation impurity atmospheres at temperatures higher than 70°C.     

Internal friction due to thermoelastic martensitic transformation

A physical model and phenomenological treatment are developed to derive an expression for internal friction associated with the thermoelastic martensitic transformation. Special attention is given to the effect of pseudoelastic hysteresis(h), which has never been previously addressed. The amplitude dependence of the internal friction is discussed in various ranges of stress amplitude with respect to the critical stresses (σ_cl, σ_c2) for stress-induced martensitic transformation. In the case where the applied stress amplitude is much higher than the critical stresses, the internal friction, due to stress-induced martensitic transformation, is found to be linearly dependent upon the reciprocal of the stress amplitude, in good agreement with the experimental results.     

The mechanisms of anneal hardening in Cu-Al alloys

The anneal hardening behavior of a Cu-16 at. pct. Al alloy was investigated by tensile testing, measurement of Young’s modulus and electrical resistivity, and electron microscopy. It was shown that three main stages of annealing processes can be distinguished. Stage 1 atT ≤ 150°C is characterized by vacancy annihilation and short-range diffusional processes giving rise to three successive, low maxima in yield and tensile strength. Substages in this temperature range are also evident from resistivity measurements. Stage 2 at 150 ≲ 275°C is essentially associated with solute segregation to dislocations giving rise to the main increase in flow stress. In Stage 3 at 725 ≤T ≤ 350°C recrystallization takes place.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5 · 10⁻⁵ s⁻¹≤ε≤1 · 10⁻² s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε≲10⁺⁴ s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600 °C because of dynamic recovery and interlamellar separation. The plastic strain energy-stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy-stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600°C because of dynamic recovery and interlamellar separation. The plastic strain energy—stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy—stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Damping properties of austenitic stainless steels containing strain-induced martensite

Damping properties of two austenitic stainless steel grades, EN 1.4318 and EN 1.4301, were investigated. The test materials were cold rolled to different reductions and damping capacity was measured as a function of temperature with an internal friction method. Microstructures of the test materials were studied by means of X-ray diffraction (XRD) and magnetic measurements. The results showed that damping capacity of the studied materials depended on the amounts of strain-induced ε- and α′-martensite phases. At temperatures around 0 °C, highest damping capacity was achieved with cold rolling reduction of 10 to 15 pct. This behavior is related to the existence of ε-martensite and stacking faults. Internal friction peak due to α′-martensite phase was present at the temperature of 130 °C. Strain aging heat treatment at 200 °C for 20 minutes decreased the damping capacity in the entire studied temperature range.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5·10⁻⁵ s⁻¹≤ε≤1·10⁻²s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε<~10 ⁺⁴s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled ‘Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Influence of small additions of antimony on the texture and magnetic properties of isotropic high-alloy electrical-engineering steel

The influence of small additions of antinomy (0.004–0.015% Sb) on the texture and magnetic properties of isotropic electrical-engineering steel with ∼3% Si and no more than 0.006% C is studied. The texturing of the final steel is improved, with faster development of favorable orientations (hk0) and a smaller proportion of orientations “220”〈uvw〉. Steel modified with antimony is characterized by smaller magnetic losses (P _1.5/50 ≤ 2.7 W/kg), without loss of magnetic induction: B ₂₅₀₀ = 1.550–1.556 T. The asymmetry is acceptable: ΔP _1.5/50 = 9–11%; ΔB ₂₅₀₀ = 0.07–0.08 T.     

A 500 °C isothermal section for the Al-Au-Cu system

The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This article addresses this omission by presenting an isothermal section at 500 °C, derived with the aid of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), metallography, and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500 °C, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500 °C is not ruled out, and the diagram presented is, therefore, applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary β phase with a nominal stoichiometry of AlAu_2−x Cu_1−x (0≤x≤1) was confirmed, and its phase field at 500 °C was determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the γ-Al₄Cu₉ phase extends deep into the ternary and, in the vicinity of the commercially interesting 18-carat line, appears to exist in a ternary ordered form, designated here as γ ₂ .     

Internal friction in ferrous martensites

Internal friction measurements were made at 1 Hz in the temperature range of 25 to 500°C on quenched, tempered and cold-worked Fe-Ni-C martensites. The alloys, which contained 15 to 30 wt pct Ni and 0.1 to 1.0 wt pct C, andM _s temperatures <0°C, transformed to martensite with a twinned (259)_γ habit and exhibited a relaxation peak at ~160°C. These results could be contrasted with those for Fe-C martensites, which form above room temperature, have predominantly dislocated (111)_γ or (225)_γ habits, and exhibit an internal friction peak at about 250°C. The literature on substructures, tempering and internal friction of all ferrous martensites and cold-worked ferrites was utilized in the interpretation of the 160°C peak. Several dissimilarities between the 160°C peak and the 250°C peak in Fe-C martensites or the cold-work peak in ferrite were noted such that models of dislocation-interstitial interaction for these peaks could not explain the 160°C peak. It was concluded that the 160°C peak is associated with the stress-induced motion of twin boundaries containing mobile carbon atoms. Such a mechanism was shown to be consistent with the present experimental observations and all other available data.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

Phase transitions in the magnetic superconductor (Dy<Subscript>0.8</Subscript>Y<Subscript>0.2</Subscript>)Rh<Subscript>4</Subscript>B<Subscript>4</Subscript>

A new (Dy_0.8Y_0.2)Rh₄B₄ superconductor (the superconducting transition temperature is T _c ≈ 5.5 K), which has an inherent magnetic subsystem whose properties are determined by the crystal structure of the superconductor, is synthesized at a high pressure (∼8 GPa) and t ≈ 1800°. The magnetic sublattice of the (Dy_0.8Y_0.2)Rh₄B₄ compound is found to substantially affect its superconducting properties and, in a number of cases, to lead to their anomalous variations, namely, to the absence of the traditional Meissner effect and an anomalously abrupt increase in magnetic induction B _k2 (upper critical field) upon a transition of the magnetic subsystem into the antiferromagnetic state. Upon cooling from 250 to 1.6 K, the (Dy_0.8Y_0.2)Rh₄B₄ compound undergoes a number of phase transformations, namely, a paramagnet-ferrimagnet transition at a Curie temperature T _C ≈ 30 K, a superconducting transition at T _c ≈ 5.5 K against the background of a ferrimagnetic order, and a ferrimagnet-antiferromagnet transition (the Neel temperature is T _N ≈ 2.8 K) in the retained superconducting state.     

Mechanical relaxation due to hydrogen—solute complexes in dilute Ni-Ti-H alloys

Dilute nickel alloys charged with hydrogen are known to exhibit an internal friction peak, which is attributed to the reorientation of hydrogen—solute atom pairs. In order to determine the geometrical configuration of the pair, we have studied anisotropy of the relaxation strength in dilute Ni-Ti-H alloy single crystals by low-frequency internal friction measurements. The orientation dependence of the peak height suggests that the defect symmetry is 〈111〉 trigonal. One of the possible configurations is the Ti-H pair in which hydrogen occupies the second neighbour octahedral sites around the substitutional solute atom. The peak has been found to be suppressed by an external magnetic field, indicating that magneto-elastic effects play an important role.     

Measurement and analysis of plane-strain work hardening

A new technique for measurement of plane-strain work hardening has been developed which uses tensile loading and computer analysis for interpretation, and which eliminates the experimental uncertainties of large strain gradients, friction, and out-of-plane bending inherent in the usual plane-strain deformation modes. Plane-strain and tensile work-hardening curves have been measured for 2036-T4 aluminum alloy using several types of sheet specimens. The work-hardening rate in plane strain is lower than that in uniaxial tension. In each case a Voce-type empirical work hardening law represents the data well. Hill’s theories cannot account for these data because the isotropic hardening assumption is violated. A method of analysis was introduced to determine Hill’s newm parameter as a function of strain andm was found to vary from 1.6 to 2.0 in the strain range 0.02 ≤ ε ≤ 0.18.     

The effect of a locked screw dislocation on a pile-up of screw dislocations in a two phase system

The influence of an internal stress field of a locked screw dislocation of Burgers vector mb on a pile-up of screw dislocations in a two-phase system composed of two welded elastic half-planes is analyzed using the method of continuously distributed dislocations. The distribution function, the number of dislocations in the pile-up and the τ_yZ component of stress field on they =o plane are obtained. The results are discussed and applied to the fracture behavior of two-phase systems,     

Phase relations in the Mo-Si-C system relevant to the processing of MoSi2-SiC composites

Phase relations in the Mo-Si-C system were evaluated at 1200 °C and 1600 °C within the composition range delimited by the phases Mo₅Si₃, MoSi₂, Mo_≤5Si₃C_≤1, and SiC. The evaluation included estimation of possible equilibria from known thermodynamic data of the binary phases as well as experimental work. For the experimental evaluation, high-purity powders were hot-pressed, heat-treated, and characterized by X-ray diffraction and electron microprobe analysis. It is shown that MoSi₂ is in equilibrium with Mo_≤5Si₃C_≤1 at 1600 °C, as previously established by Nowotnyet al, (Monatsh. Chem., 1954, vol. 85, pp. 255-72), and at 1200 °C, in contrast to the Mo₅Si₃-SiC equilibrium reported by van Looet al. (High Temp.- High Press., 1982, vol. 14, pp. 25-31). The thermodynamic estimation suggests that these phase relations should extend to lower temperatures in the range of compositions investigated. Thus, the third phase in silicon-lean MoSi₂-SiC composites should be the Nowotny phase (Mo_≤5Si₃C_≤1) instead of Mo₅Si₃. The Gibbs free energy of formation at 298 K of the idealized compound Mo₅Si₃C is estimated as -40.2 kJ/mol.     

Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys

Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered fee γ-Ni, which contains precipitates of γ′-Ni₃Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains. The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the high ductility.     

Analysis of grain growth in a two-phase gamma titanium aluminide alloy

Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in the temperature range 1200 °C to 1320 °C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, T _α=1300 °C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p=2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200 °C≤T≤1300 °C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p≈2.6 could satisfactorily account for the experimental results.     

Mechanisms of internal friction in a cu-zn-ai shape memory alloy

The internal friction in a Cu-Zn-AI shape memory alloy has been studied in various microstructural states in a wide range of stress amplitude. Simultaneous measurements of the corresponding shape change indicate correlations between internal friction and the deformation mechanisms. It is found that the observed stress-amplitude dependence of the internal friction cannot be represented by a single formalism. Rather, in different ranges of stress amplitude, where the internal friction mechanisms are different, separate formalisms are applicable. The comparison of the experimental results with a previously developed theory is presented.