Uniaxial Tensile and Simple Shear Behavior of Resistance Spot-Welded Dual-Phase Steel Joints

Small test coupons were machined from single spot welds in a dual-phase steel (DP600) to investigate deformation and failure of weld joints in both tension and shear. Quasi-static () testing was conducted in a miniature tensile stage with a custom image acquisition system. Strain accumulation in each weld was analyzed where fracture occurred, which was typically outside the fusion zone. A few shear test coupons that failed in the fusion zone were found to have the same spheroidal defects noted in previous work, and thus, severely limited weld strength and ductility. A novel strain mapping method based upon digital image correlation was employed to generate two-dimensional deformation maps, from which local stress-strain curves to failure were computed. As an important first step toward incorporation of material models into weld simulations, a preliminary finite element analysis of a tension test successfully reproduced the experimental results with material models for the base, heat-affected, and fusion zone materials generated from prior work.     

High-Temperature Deformation Behavior of Ti-6Al-4V Alloy without and with Hydrogenation Content of 0.27 wt.%

Isothermal compression of Ti-6Al-4V alloy without and with hydrogenation content of 0.27 wt.% was carried out on Gleeble-1500D thermal simulation machine at deformation temperature between 760 and 1000 °C and strain rate from 0.001 to 1 s⁻¹. The experimental results show that hydrogenation can decrease the deformation temperature or increase the strain rate of Ti-6Al-4V alloy. The apparent activation energy was determined to be 667 kJ mol⁻¹ for isothermal compression of the Ti-6Al-4V alloy without hydrogenation content of 0.27 wt.% in the α + β phase region (760-960 °C), and this value was about 655 and 199 kJ mol⁻¹ for the alloy with 0.27 wt.% of hydrogenation content in the α + β phase region (760-840 °C) and β phase region (840-960 °C), respectively. Constitutive equation was developed for the high-temperature deformation of Ti-6Al-4V alloy both without and with hydrogenation content of 0.27 wt.%.     

On the Change in Work Hardening Characteristics of Molybdenum Polycrystals Due to Natural Aging

Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then aged for 6 months at room temperature were deformed in tension at various strain-rates in the range 2.1 × 10⁻⁴ to 4.2 × 10⁻³ s⁻¹ till fracture. It is found that natural aging of the annealed specimens for 6 months leads to 20-30% reduction in the yield stress (YS), 18-22% reduction in the ultimate tensile strength (UTS), and 72-76% reduction in the ductility, i.e. the tensile strain ε_max corresponding to UTS, depending on the value of [\upvarepsilon \dot] \dot{\upvarepsilon } in the tensile strain-rate range referred to. Data analysis in terms of the kink-pair nucleation model of flow stress shows that the reduction in YS of the aged Mo specimens is a consequence of lowering of the Peierls energy per interatomic spacing along the length of screw-dislocation segments trapped in the Peierls valleys on the migration of point defects to the dislocation cores during the course of natural aging. The reduction in UTS and ε_max is attributed to the variation in the relative contribution of the processes of dislocation multiplication and annihilation together with the reduction in the Peierls stress of the metal.     

Ternary Diffusion in a RuAl-NiAl Couple

A ternary diffusion couple assembled with NiAl and RuAl disks and annealed at 1100 °C was examined by scanning electron microscopy and analyzed for concentration profiles by electron microprobe analysis. Complete mutual solid solubility with continuous variations in compositions was observed between the binary B2 aluminides. Ternary interdiffusion coefficients were determined with the aid of a program called MultiDiFlux over two composition ranges, one Ru-rich and the other Ni-rich, within the diffusion zone. The interdiffusion coefficient, varies little with variation in composition, but the interdiffusion coefficient, decreases by an order of magnitude from the Ni-rich region to the Ru-rich region. is larger than in the Ni-rich region by an order of magnitude. The cross coefficients, and , are both positive. is comparable in magnitude to the main coefficient in the Ni-rich region; hence, Ni interdiffusion flux is enhanced down a Ru concentration gradient but decreased against it. Similarly, Ni interdiffusion is reduced down Al gradients. Characteristic depth parameters calculated for Ni and Ru are larger on the NiAl side than on the RuAl side. Approximate calculations of cumulative intrinsic diffusion fluxes past a Kirkendall plane suggest that the atomic mobility of Ni is larger than that of Ru.     

Partial Thermodynamic Properties of <Emphasis Type="Italic">γ</Emphasis>′-(Ni,Pt)<Subscript>3</Subscript>Al in the Ni-Al-Pt system

Measurements were made to determine how Pt influences the partial thermodynamic properties of Al and Ni in γ′-(Ni,Pt)₃Al and liquid in the Ni-Al-Pt system. The activities of Al and Ni were measured with a multiple effusion-cell mass spectrometer (multi-cell KEMS). For a constant X _Al = 0.24, adding Pt, from X _Pt = 0.02-0.25, reduces a(Al) almost an order of magnitude, from 2 × 10⁻⁴ to 2 × 10⁻⁵, at 1560 K. This occurred for of −203 ± 10 kJ mol⁻¹ and the a(Al) decrease was due to increasing from −60 to −40 J mol⁻¹ K⁻¹ with Pt addition. The large negative and indicate Al-atoms are highly ordered in γ′-(Ni,Pt)₃Al. Nickel activity, a(Ni), remained essentially constant, ∼0.7, indicating an increasing ternary interaction between Ni-atoms and (Al + Pt)-atoms with Pt addition, where γ_Ni increased from about 0.7 to 1.2. This is supported by in the range 6.1-7.1 ± 1.5 kJ mol⁻¹ at 1520 K, and a positive , which suggest disorder on the Ni-lattice. For a consistent X _Al = 0.27, adding Pt, from X _Pt = 0.10–0.25, also reduces a(Al) but only by a factor of about 3, while a(Ni) remained essentially constant, with γ_Ni increasing from about 0.7 to 0.95. A dramatic change in the mixing behavior was observed between the and 0.27 series of alloys, where and are seen to increase about 50 kJ mol⁻¹ and 20 J mol⁻¹ K⁻¹ at T = 1566 K, respectively. In contrast, decreased about 16 kJ mol⁻¹ at T = 1520 K and changed from a positive to a negative value. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by the Alloy Phase Committee of the joint EMPMD/SMD of The Minerals, Metals, and Materials Society (TMS), held in San Antonio, Texas, March 12–16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany.     

Hot deformation behavior of Mg-7.22Gd-4.84Y-1.26Nd-0.58Zr magnesium alloy

The behavior evolvement of Mg-7.22Gd-4.84Y-1.26Nd-0.58Zr(GWN751K) magnesium alloy during the hot deformation process was discussed.The flow stress behavior of the magnesium alloy over the strain rate range of 0.002 to 2.000 s-1 and in the temperature range of 623 to 773 K was studied on a Gleeble-1500D hot simulator under the maximum deformation degree of 60%.The experimental results showed that the relationship between stress and strain was obviously affected by strain rate and deformation temperature.The flow stress of GWN751K magnesium alloy during high temperature deformation could be represented by the Zener-Hollomon parameter in the hyperbolic Arrhenius-type equation.The stress exponent n and deformation activation energy Q were evaluated by linear regression analysis.The stress exponent n was fitted to be 3.16.The hot deformation activation energy of the alloy during hot deformation was 230.03 kJ/mol.The microstructures of hot deformation were also influenced by strain rate and compression temperature strongly.It was found that the alloy could be extruded at 723 K with the mechanical properties of σ0.2 = 260 MPa,σb = 320 MPa,and δ = 18%.     

Superplastic characteristics of fine-grained 7475 aluminum alloy

A 7475-aluminum alloy was subjected to a thermomechanical heat treatment that resulted in a final recrystallized grain size on the order of 10 μm. Tensile specimens of dimensions 10 × 4 × 2.3 mm were machined such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at high temperatures in the range of 773 to 803 K at different cross-head speeds corresponding to initial strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Elongations of several hundred percent were observed at strain rates of <10⁻³ s⁻¹. The correlation between flow stress and strain rate suggests that the strain rate sensitivity m is close to 0.5 at the lower strain rates. The value of m decreases to ≈0.2 at high strain rates. The decrease in m suggests a transition in the rate-controlling process from superplastic deformation (m ≈ 0.5) to dislocation creep (m ≈ 0.2) with increasing strain rate. The calculated activation energies in the two deformation regions are consistent with the suggested rate-controlling processes.     

Superplastic Behavior of Al-4.5Mg-0.46Mn-0.44Sc Alloy Sheet Produced by a Conventional Rolling Process

This article describes the superplastic behavior of the Al-4.5Mg-0.46Mn-0.44Sc alloy. The investigated alloy was produced by casting and was conventionally processed to form a sheet with a thickness of 1.9 mm and an average grain size of 11 μm. The superplastic properties of the alloy were investigated using a uniaxial tensile testing with a constant cross-head speed and with a constant strain rate in the range 1 × 10⁻⁴ to 5 × 10⁻² s⁻¹ at temperatures from 390 to 550 °C. The investigations included determinations of the true-stress, true-strain characteristics, the maximum elongations to failure, the strain-rate sensitivity index m, and the microstructure of the alloy. The m-values determined with the strain-rate jump test varied from 0.35 to 0.70 in the temperature interval from 390 to 550°C and strain rates up to 2 × 10⁻² s⁻¹. The m-values decreased with increased strain during pulling. The elongations to failure were in accordance with the m-values. They increased with the temperature and were over 1000%, up to 1 × 10⁻³ s⁻¹ at 480 °C and up to 1 × 10⁻² s⁻¹ at 550 °C. A maximum elongation of 1969% was achieved at an initial strain rate of 5 × 10⁻³ s⁻¹ and 550 °C. The results show that the addition of about 0.4 wt.% of Sc to the standard Al-Mg-Mn alloy, fabricated by a conventional manufacturing route, including hot and cold rolling with subsequent recrystallization annealing, results in good superplastic ductility.     

Thermodynamic Interactions of Si and Ti in Liquid Cu

The activity coefficients of titanium in liquid Cu-Ti at 1623 and 1673 K were measured by equilibrating the liquids with Ti₃O₅ in a oxygen partial pressure controlled by C(s)/CO(g) equilibrium. Furthermore, the thermodynamic interaction parameter of silicon on titanium and the self-interaction parameter of titanium in liquid Cu-Ti-Si at 1773 K were determined by equilibrating the 58 mass% TiO₂-42 mass% CaF₂ slag with Cu-Si-Ti liquids. And the interaction parameters e_\textTi^\textTi e_{\text{Ti}}^{\text{Ti}} and e_\textTi^\textSi e_{\text{Ti}}^{\text{Si}} obtained using a multiple regression were as large as −69.32 and 15.44 respectively. Based on the above determined value of e_\textTi^\textTi e_{\text{Ti}}^{\text{Ti}} , the relationship between Henrian constant of titanium in liquid Cu-Ti melt, \upgamma_{\textTi(\texts)}⁰ \upgamma_{{{\text{Ti}}({\text{s}})}}^{0} , from 1473 to 1923 K was evaluated, and is expressed as:

Hot deformation mechanisms in Ti-5.5Al-1Fe alloy

The mechanisms of hot deformation in the alloy Ti-5.5Al-1Fe have been studied in the temperature range 750 to 1150 °C and with the true strain rate varying from 0.001 to 100 s⁻¹ by means of isothermal compression tests. At temperatures below β transus and low strain rates, the alloy exhibited steady-state flow behavior, while, at high strain rates, either continuous flow softening or work hardening followed by flow softening was observed. In the β region, the deformation behavior is characterized by steady-state behavior at low strain rates, yield drops at intermediate strain rates, and oscillations at high strain rates. The processing maps revealed two domains. (1) In the temperature range 750 to 1050 °C and at strain rates lower than 0.01 s⁻¹, the material exhibits fine-grained superplasticity. The apparent activation energy for superplastic deformation is estimated to be about 328 kJ/mole. The optimum conditions for superplasticity are 825 °C and 0.001 s⁻¹. (2) In the β region, a domain occurs at temperatures above 1100 °C and at strain rates from 0.001 to 0.1 s⁻¹ with its peak efficiency of 47% occurring at 1150 °C and 0.01 s¹. On the basis of kinetic analysis, tensile ductility, and grain size variation, this domain is interpreted to represent dynamic recrystallization (DRX) of β phase. The apparent activation energy for DRX is estimated to be 238 kJ/mole. The grain size (d) is linearly dependent on the Zener-Hollomon parameter (Z) per the equation

The adiabatic correction factor for deformation heating during the uniaxial compression test

The isothermal uniaxial compression test is a common method to determine the flow stress of metals. For accurate flow stress data at strain rates >10⁻³ s⁻¹, the data must be corrected for flow softening due to deformation heating. The first step in the correction is to determine the increase in temperature. An adiabatic correction factor, η, is used to determine the temperature between strain rates of 10⁻³ to 10¹ s⁻¹. The adiabatic correction factor is the fraction of adiabatic heat retained in the workpiece after heat loss to the dies, η=(ΔT _ACTUAL)/(ΔT _ADIABATIC), where ΔT _ADIABATIC=(0.95 f σdɛ)/(ρC _p ). The term η is typically taken to be constant with strain and to vary linearly (0 to 1) with log ( ) between 10⁻³) and 10¹ s⁻¹. However, using the finite element method (FEM) and a one-dimensional, lumped parameter method, η has been found to vary with strain, die and workpiece thermal conductivities, and the interface heat-transfer coefficient (HTC). Using the lumped parameter method, an analytical expression for η was derived. In this expression, η is a function of the die and workpiece thermal conductivities, the interface heat-transfer coefficient, workpiece heat capacity, strain, and strain rate. The results show that an increase in the HTC or thermal conductivity decreases η.     

Constitutive relationship of IN690 superalloy by using uniaxial compression tests

The hot deformation behavior of IN690 superalloy was characterized in a temperature range of 1273-1473 K and a strain rate range of 0.01-10 s-1 using uniaxial compression tests on process annealed material.The constitutive relations between flow stress and effective strain,effective strain rate as well as deformation temperature were studied.It can be concluded that the flow stress significantly reduces with the deformation temperature of IN690 superalloy increasing.Whereas,there is a significant increase of flow stress when the strain rate increases from 0.1 s-1 to 10 s-1.Based on the hyperbolic-sine Arrhenius-type equation,a constitutive equation considering compensation of strain was developed.The activation energy and the material constants(Q,n and ln A) decrease as the deformation strain increases.The strain dependent term is successfully incorporated in the constitutive equation through a quartic equation.A good agreement between the experimental data and the predicted results has been achieved,indicating that the proposed constitutive equation and the methods of determing the material constants are suitable to model the high temperature deformation behavior of IN690 superalloy.     

Interdiffusion in <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript>-Ni<Subscript>3</Subscript>Al Alloyed with Re

Ternary interdiffusion in L1₂-Ni₃Al with ternary alloying addition of Re was investigated at 1473 K using solid-to-solid diffusion couples. Interdiffusion flux of Ni, Al, and Re were directly calculated from experimental concentration profiles and integrated for the determination of average ternary interdiffusion coefficients. The magnitude of main interdiffusion coefficients and was determined to be much larger than that of the main interdiffusion coefficient A moderate tendency for Re to substitute for Al sites was reflected by its influence on interdiffusion of Al, quantified by large and positive coefficients. Similar trends were observed from ternary interdiffusion coefficients determined by Boltzmann-Matano analysis. Profiles of concentrations and interdiffusion fluxes were also examined to estimate binary interdiffusion coefficients in Ni₃Al, and tracer diffusion coefficients of Re (5.4 × 10⁻¹⁶ ± 2.3 × 10⁻¹⁶ m²/s) in Ni₃Al.     

Yield Stress of 21-6-9 Stainless Steel Over Very Wide Ranges of Strain Rates and Temperatures

The yield strength of solution-annealed 21-6-9 austenitic stainless steel was determined over a wider temperature range (−195 to 1100 °C) and strain rate (10⁻⁴ to 10⁴ s⁻¹) than has been previously reported. The most noteworthy characteristic of the variation of yield stress with temperature was the dramatic decrease in yield strength from −195 to 300 °C. The strain-rate sensitivity exponent, m, was determined using strain-rate change tests. m dramatically increases at about 850 °C with increasing temperature and m is approximately independent of strain (structure). Hopkinson split-bar tests from ambient temperature to 750 °C indicate that the strain-rate sensitivity of 21-6-9 is not strongly influenced by the over eight orders of magnitude change in strain rate. This suggests that the mechanism(s) of plastic flow at the higher rates is similar to that at lower rates. This contention was corroborated by transmission electron microscopy. The yield stress shows grain-size dependency.     

Chemical Changes at the Interface Between Low Carbon Steel and an Al-Si Alloy During Solution Heat Treatment

The aim of this work was to characterize the chemical changes during solid state solution heat treatment of a metallurgically bonded steel/Al-Si interface. For this purpose, low carbon steel plates covered with the A-S7G03 aluminium alloy (7 wt.% Si, 0.3 wt.% Mg analogous to A356) were prepared by dip coating, water-quenching to room temperature and reheating in the solid state at 480-560 °C for 3-160 h. Upon reheating at 535 °C, a reaction layer was observed to grow at the interface between steel and the iron-saturated Al-Si alloy. As long as an intimate contact could be maintained, the total thickness, x, of the reaction layer increased with time, t, according to a nearly parabolic growth law x ² = K·t − b. At 535 °C, the value of the growth constant was K = 4.045 × 10⁻¹⁴ m² s⁻¹. This constant was found to be thermally activated [K = K ₀ exp(−Q/RT)] with K ₀ = 4.37 × 10⁻⁴ m² s⁻¹ and Q = 153 kJ mol⁻¹. The whole chemical interaction process was controlled by solid state volume diffusion and the reaction layer sequence corresponded to a diffusion path in the Al-Fe-Si phase diagram. A striking feature of the reaction process is the unbalanced diffusion of aluminium atoms through the reaction zone which rapidly results in the formation of Kirkendall voids. As these voids coalesce, solid state diffusion becomes more and more difficult and the steel/alloy bond gets weakened. Oxidation appears to be an aggravating factor, where applicable.     

Interdiffusion of Bi in Liquid Sn

The diffusion coefficients for the interdiffusion of Bi in liquid Sn were determined using the thin layer, long capillary technique. The effect of 0.5, 0.8, and 1.6 mm diameter capillaries on the apparent diffusion coefficient was investigated. For any temperature up to 600 °C all three diameter capillaries yielded similar interdiffusion coefficients. At 700 °C, the 1.6 mm diameter capillaries yielded concentration-penetration profiles that exhibited considerable scatter and abnormally high diffusion coefficients, indicative of the presence of convective mixing. At 800 °C, both the 0.8 and 0.5 mm capillaries yielded considerable scatter in the concentration-penetration profiles and abnormally high diffusion coefficients. Because of agreement of the results for both the 0.5 and 0.8 mm capillaries up to and including 700 °C (and the 1.6 mm capillaries up to 600 °C), it was concluded that the interdiffusion coefficients obtained are accurate for the temperature range 300 to 700 °C with no significant contribution from buoyancy driven convection, capillary wall effects and/or Marangoni convection. The interdiffusion coefficient for the diffusion of Bi in liquid Sn can be represented by: D_\textBi^\textSn = { 3.4 ±0.6 ×10^-8 } { exp [ - ( 13,600 ±1200/RT ) ] } \textm²/\texts. D_{\text{Bi}}^{\text{Sn}} = \left\{ {3.4 \pm 0.6 \times 10^{-8} } \right\}\,\left\{ {\exp \,\left[ { - \left( {13,600 \pm 1200/RT} \right)} \right]} \right\}\,{\text{m}}^{2}/{\text{s}}. The results show that Bi diffuses in Sn at a slower rate than does Sn itself. In addition, it is possible that the interdiffusion coefficient may not be predicated on simple temperature dependence in at least some binary liquid metal systems, and that the diffusion coefficient may be affected by liquid/liquid phase transformations occurring at some temperature within the liquid.     

Gibbs Energy of Formation of MnO: Measurement and Assessment

Based on the measurements of Alcock and Zador, Grundy et al. estimated an uncertainty of the order of ±5 kJ mol⁻¹ for the standard Gibbs energy of formation of MnO in a recent assessment. Since the evaluation of thermodynamic data for the higher oxides Mn₃O₄, Mn₂O₃, and MnO₂ depends on values for MnO, a redetermination of its Gibbs energy of formation was undertaken in the temperature range from 875 to 1300 K using a solid-state electrochemical cell incorporating yttria-doped thoria (YDT) as the solid electrolyte and Fe + Fe_1 − δO as the reference electrode. The cell can be presented as

The positive strain-rate dependence of ductility in a 50Mo-50Re alloy

This paper is a review of the recent studies of deformation and fracture behaviors, especially the anomalous strain-rate effect on plasticity of a 50Mo-50Re alloy. The ductility of this alloy was found to increase with increase in strain rate within the range of 10⁻⁶ s⁻¹ to 1 s⁻¹ at room temperature in air. The fracture surfaces in the alloy changed from brittle to ductile in nature with increasing strain rate. A damage-toughening phenomenon was also observed in this alloy.     

Phase Equilibria in the System Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-CoO and Gibbs Energy of Formation of Ca<Subscript>3</Subscript>CoAl<Subscript>4</Subscript>O<Subscript>10</Subscript>

An isothermal section of the system Al₂O₃-CaO-CoO at 1500 K has been established by equilibrating 22 samples of different compositions at high temperature and phase identification by optical and scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy after quenching to room temperature. Only one quaternary oxide, Ca₃CoAl₄O₁₀, was identified inside the ternary triangle. Based on the phase relations, a solid-state electrochemical cell was designed to measure the Gibbs energy of formation of Ca₃CoAl₄O₁₀ in the temperature range from 1150 to 1500 K. Calcia-stabilized zirconia was used as the solid electrolyte and a mixture of Co + CoO as the reference electrode. The cell can be represented as: From the emf of the cell, the standard Gibbs energy change for the Ca₃CoAl₄O₁₀ formation reaction, CoO + 3/5CaAl₂O₄ + 1/5Ca₁₂Al₁₄O₃₃ → Ca₃CoAl₄O₁₀, is obtained as a function of temperature: /J mol⁻¹ (±50) = −2673 + 0.289 (T/K). The standard Gibbs energy of formation of Ca₃CoAl₄O₁₀ from its component binary oxides, Al₂O₃, CaO, and CoO is derived as a function of temperature. The standard entropy and enthalpy of formation of Ca₃CoAl₄O₁₀ at 298.15 K are evaluated. Chemical potential diagrams for the system Al₂O₃-CaO-CoO at 1500 K are presented based on the results of this study and auxiliary information from the literature.