Minimum Strain Rate Research Articles

Grain Boundary Morphology and Its Effect on Creep of TiAl Alloys

Three kinds of microstructures with different grain boundary morphologies and their creep properties of a Ti-47Al-2Nb-2Mn+0.8 vol%TiB 2 alloy are investigated. Tensile creep tests and microstructural examinations indicatethat a stabilized fine-grained fully lamellar (FGFL) microstructure with relatively smooth grain boundaries shows inferior creep resistance. A stabilized fully lamellar (FL) microstructure with well-interlocked grain boundaries and wider lamellar spacing yields reduced minimum strain rate and extended creep rupture life. Furthermore, a nearly lamellar microstructure (NL) with well-interlocked grain boundaries exhibits better creep resistance than the stabilized FGFL microstructure though it has four times wider lamellar spacing and 15 vol% equiaxed y grains at the grain boundaries, but worse creep resistance than the stabilized FL microstructure. Examinations to the deformed microstructures show that grain boundary instability involving spheroidization of the lamellae is a major microstructural degradation process, resulting in fine globular regions at the grain boundaries. Voids develop along the grain boundaries, particularly in the fine globular regions, leading to intergranual fracture. It is suggested that grain boundary sliding (GBS) is operating in the stabilized FGFL microstructure, and promotes mutually with the grain boundary instability during subsequent creep deformation, resulting in increased minimum strain rate and shortened tertiary stage. The well-interlocked grain boundary inhibits the onset of GBS and enhances the grain boundary stability effectively. These results demonstrated that the grain boundary stability has a great effect on creep behavior of TiAl Alloys.

Microstructure stability during creep deformation of hard-oriented polysynthetically twinned crystal of TiAl alloy

The hard-orientated polysynthetically twinned (PST) crystal with the lamellar plates oriented parallel to the compression axis was deformed at 1150 K under the applied stress of 158 to 316 MPa. Microstructural changes were examined quantitatively for the PST crystal during creep deformation. In the as-grown PST crystal of the present study, proportions of α2/γ, true twin, pseudotwin, and 120 deg rotational fault interfaces were 12, 59, 12, and 17 pct, respectively. After creep deformation, lamellar coarsening by dissolution of α2 lamellae and migration of γ/γ interfaces were observed. The acceleration of creep rate after the minimum strain rate in the creep curve was attributed to the lamellar coarsening and destruction of lamellar structure during the creep deformation. Thirty-two percent of α2/γ interfaces, 51 pct of true twin interfaces, 74 pct of pseudotwin interfaces, and 80 pct of 120 deg rotational faults disappeared after 4 pct creep strain at 1150 K. The α2/γ interface was more stable than γ/γ interfaces during the creep deformation. The pseudotwin interface and 120 deg rotational fault were less thermally stable than the true twin interface for γ/γ interfaces.

Creep rupture of lead-free Sn-3.5Ag-Cu solders

Creep-rupture properties of lead-free Sn-3.5Ag-based alloys with varying amount of Cu were investigated using rolled and heat-treated dog-bone-shaped specimens. Nominal compositions of added copper were 0 wt.%, 0.5 wt.%, 0.75 wt.%, 1.0 wt.%, and 1.5 wt.%. During creep tests, the matrix hardness dropped significantly, and the minimum strain rates (\(\dot \varepsilon _{min} \)) were lowest for the 0.75Cu specimens. The stress exponents (n) of \(\dot \varepsilon _{min} \) were usually around 4, with the exception of the 0.5Cu and 0.75Cu alloys, which showed somewhat higher values of n. Fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities in the matrix except the 1.5Cu specimens, which showed cavity nucleation at brittle Cu6Sn5 particles.

Creep Deformation of Lead-Free Sn–3.5Ag–Bi Solders

Creep rupture properties of lead-free Sn–3.5Ag-based alloys with varying amounts of Bi were investigated using dog-bone-shaped rolled and heat-treated bulk specimens. Nominal compositions of Bi additions were 0, 2.5, 4.8, 7.5, and 10 wt%, respectively. The minimum strain rates (\\dotεmin) were lowest for the 2.5Bi specimens. The stress exponents (n) of \\dotεmin were usually around 4±0.6, with the exception of the 10Bi alloys, which showed n∼2. Additions of Bi reduced the creep ductility of Sn–3.5Ag-based ternary alloys, and fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities on the grain boundaries except for the Bi-free alloy which had cavities in the grains. Subsequent AES analyses revealed that Bi segregation to grain boundaries facilitated the cavity nucleation, thereby increasing the propensity for the brittle creep rupture.

Pressure–temperature effects on the compressive behavior of laboratory-grown and iceberg ice

The compressive behavior of freshwater ice subjected to a nominally constant compressive stress of 15 MPa while under confinement is investigated. Cylindrical ice specimens 155 mm in length and 70 mm in diameter were machined from polycrystalline ice made with ultra pure water and from genuine iceberg ice. The confining pressures ranged from 10 to 65 MPa. Test temperature varied from about −5 to −26 °C. The activation energy for the laboratory ice is determined at five distinct levels of hydrostatic pressure: 15, 35, 55, 65 and 70 MPa. It displays a substantial increase above 55 MPa. A similar increase in minimum strain rate is observed, which becomes more significant at higher temperatures. Our data are consistent with those of other investigators obtained from constant strain rate experiments. The iceberg ice tends to show a higher scatter than the laboratory ice, and is generally more compliant. Some implications for the behavior of this material upon fast compressive loading are discussed.

Creep behavior of Ti–6Al–2Sn–4Zr–2Mo: I. The effect of nickel on creep deformation and microstructure

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal ∼1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in α-titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the α-phase. We suggest that trace levels of Ni in the α-phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.

Rupture time analyses of the Sn–3.5Ag solder alloys containing Cu or Bi

The creep rupture properties of Sn–3.5Ag based ternary alloys with varying amounts of Cu or Bi were investigated using rolled and heat-treated bulk specimens. Nominal compositions of the third element were 0% ( Base), 0.5, 0.75, 1.0 and 1.5% Cu and 2.5, 4.8, 7.5 and 10% wt% Bi, respectively. The alloys generally showed the secondary and tertiary creep characteristics only, and the minimum strain rates ε ̇ min were lowest for the 0.75 Cu specimens, and highest for the 10 Bi specimens. The stress exponents ( n) of ε ̇ min were usually around 4 ± 1, with the exception of the 0.5 Cu and 0.75 Cu alloys with n=6.0∼7.6, and the 10 Bi alloy which showed n ~ 2. Fractographic analyses revealed typical creep rupture by the nucleation and growth of cavities in the matrix for the Base and Cu-containing alloy except the 1.5 Cu specimens, which showed cavity nucleation by the cracking of brittle Cu 6Sn 5 particles. On the other hand, all the Bi-containing alloys showed cavity nucleation on grain boundaries due to the segregation of Bi to the β-Sn grain boundaries. Subsequent rupture time analyses showed that creep rupture by necking could predict rupture times of the Base and the Cu-containing alloys reasonably, and that the model based on the continual nucleation of cavitating facets could explain the rupture times of the Bi-containing alloys reasonably well. The Kachanov equation made reasonable predictions of the rupture time for all cases.

Effect of stress axis orientation on the creep deformation behavior of Ti–48Al polysynthetically twinned (PST) crystals

The creep deformation behavior of PST crystals Ti–48Al was investigated at 1150 K under 100–400 MPa. Two hard orientations with the lamellar plates parallel or perpendicular to the compression axis and a soft orientation with the lamellar plates oriented 35° to compression axis were compared to investigate the effect of the orientation of lamellar interface on the creep behavior. The soft PST orientation exhibited the highest creep rates and larger primary creep strain than those of the hard PST orientations. The stress exponents were evaluated as 7.0 in the hard PST orientations for the stress dependence of the minimum creep rates. In the soft PST orientations, the stress exponent increased with decreasing stress from 4 at high stress regime to 8 at low stress regime. The primary creep strain increased with decreasing stress. The rotation of lamellar played an important role to accommodate the creep deformation in the soft PST orientation. The acceleration of creep rate after the minimum strain rates were attributed to the microstructural softening at strain concentrated regions. Those are kinking and dynamic recrystallization along the shear band in the hard PST orientations and recrystallization along the region at which angle of lamellar boundaries abruptly change in soft PST orientation.

Mechanisms and effect of microstructure on creep of TiAl-based alloys

Transmission Electron microscopy studies on crept samples of an equiaxed Ti–48Al alloy deformed to strains near the minimum strain rate show a microstructure dominated by unit 1/2[110] type dislocations. These dislocations are pinned by jogs of varying heights. The jogged-screw model is adopted, where the rate controlling step is assumed to be the non-conservative dragging of the jogs along the length of the screw dislocations. The presence of tall jogs and the existence of a stress-dependent upper bound to the height of tall jogs which can be dragged have been incorporated into the model. These modifications lead to excellent agreement with experimental data. The evolution of the creep curve is also qualitatively predicted. In contrast, lamellar structures show a highly inhomogeneous deformation behavior with dislocation activity in lamellae above a critical thickness and negligible activity below that limit. This limit is related to the minimum stress required to cause channeling of dislocations. The observation of jogged segments in the thicker lamellae suggests that a modification of the jogged-screw velocity law could be used by incorporating an effective stress approach. Qualitative analysis of a probable method of evaluating the creep rate is discussed. Finally, microstructural changes during the aging process in K5 and K5SC alloys have also been studied. Aging causes the dissolution of metastable α 2. The effect of Si and C on the aging behavior and precipitation of the silicide and carbide particles during aging and following creep is discussed. These results suggest that microstructural stability is critically important in order to achieve the highest possible creep strengths.

Tensile creep behavior of a ytterbium silicon oxynitride–silicon nitride ceramic

Tensile creep behavior of hot pressed silicon nitride on the Si 3 N 4 –Yb 4 Si 2 O 7 N 2 tie line was investigated at temperatures of 1300 and 1400 °C under an applied stress of 125 to 200 MPa. During the tests, the creep strain increased with time and the creep rate monotonically decreased both with time and strain. On the basis of minimum strain rates, the stress exponents for 1300 and 1400 °C were determined to be 3.1 and 1.7, respectively. All the specimens tested at 1400 °C lead to failure while exhibiting a large scatter in the time-to-failure data. The activation energy was determined to be 879 kJ/mol from a comparison between creep rates at different temperatures. The creep mechanism is discussed on the basis of the creep parameters and creep damage observation.

Factors contributing to creep strengthening in discontinuously-reinforced materials

There have been several experimental investigations of the creep behavior of materials with discontinuous reinforcement. For these materials, logarithmic plots of the steady-state or minimum strain rate against the applied stress usually reveal significant curvature such that the stress exponent, determined from the slope of the line, increases with decreasing stress. Plots of this nature are usually interpreted by invoking a threshold stress, σ o and replacing the applied stress, σ, with an effective stress, defined as ( σ− σ o). This paper examines the implications of this approach using published creep data for several aluminum-based materials. It is shown that the introduction of an effective stress leads to a stress exponent that is similar to that observed in the unreinforced matrix material but, nevertheless, the creep rates in the reinforced materials are often significantly slower than in the matrix. This difference is examined with reference to the occurrence of load transfer and substructure strengthening.

The localization of strain and c-axis evolution in anisotropic ice

AbstractUsing a series of combined compression–simple-shear experiments, it has been possible to investigate strain distributions and crystal-orientation fabrics related to varying layer orientation in ice. A variety of flattening strains accompanied by layer buckling, simple shear and the development of a lenticular layering are produced in anisotropic ice masses. In samples where the creep curve has only just reached a minimum strain rate, the c-axis preferred orientation is similar to that in the starting material, with specific c-axis concentrations affected by the extent of preserved host grains. At shear strains where γ ≤ 1, it was found that the c-axis preferred orientations were highly variable depending on the magnitude of strain, strain distribution and upon the modification and degree of rotation of initial c-axis preferred orientation. However, once recrystallization dominates in high-strain zones (γ ≥ 1), there is a rapid development of an asymmetric two-maxima fabric with little evidence of any contributions from inherited fabric elements. The final c-axis pattern is asymmetric with respect to the direction of shortening, with a strong maximum at ∼80° to the shear zone, with a sense of asymmetry in the direction of the shear, and a secondary maximum lying at ∼50° to the plane of shearing.

Creep Properties of Austenitic Stainless-Steel Weld Metals

The creep behavior of two austeitic stainless-steel weld metals was investigated. Two AISI 316L stainless-steel base plates were welded together using the submerged arc-welding process. Creep tests were carried out on the welds at constant load, over a stress range of 100 to 400 MPa, and in the temperature range of 600 to 700 °C. The relationships between stress and minimum secondary creep rate at a constant temperature were obtained with Norton’s law. The results showed that AISI 347 weld metal presented a higher creep resistance with lower values of the minimum strain rate, and, consequently, it exhibited a longer life before rupture than AISI 316L weld metal. However, this weld metal showed a lower ductility value than AISI 316L weld metal. The weld-metal microstructure survey, performed before and after the creep testing, has shown different amounts of delta ferrite, which was strongly dependent on time, temperature, and stress level.

Adiabatic shear in annealed and shock-hardened iron and in quenched and tempered 4340 steel

Adiabatic shear localization is a catastrophic failure mechanism which can occur in ductile metals under high strain rate loading. This mechanism is driven by a thermal instability process in which rapid temperature rise due to plastic work couples with thermal softening to cause uniform deformation to collapse into narrow bands of intense shear within which material ductility is exhausted. Adiabatic shear localization is studied in three ferrous metals: annealed Armco and as-received Remco iron, both of which are high purity alpha iron; shock-hardened Remco iron; and 4340 steel quenched and tempered to a range of hardness levels. Using a compressive split-Hopkinson bar, punching-shear experiments were performed at room and elevated initial temperatures at shear strain rates of up to 45 000 s−1. Optical and scanning electron microscopy was performed on the deformed shear specimens to determine the extent of shear localization and mode of failure. Experimental evidence showed that the tempered 4340 steels were susceptible to localization through adiabatic shear banding; however, as-received and shock-hardened Remco iron and annealed Armco iron were not. Finite element simulations of the experiments were performed utilizing a user material subroutine developed as part of this research. This constitutive routine incorporates two adiabatic shear failure criteria, namely (i) maximum shear stress with a minimum critical shear strain rate and (ii) flow localization. These criteria proved to be capable of predicting the onset of an instability; however, the deformation which follows the instability was not predicted well.

自然乾燥状態および湿潤状態における登別溶結凝灰岩のクリープ変形とクリープ寿命

Not only axial strain but also lateral strain were measured during creep tests on Noboribetsu welded tuff under air-dried and water-saturated conditions in which the water contents of specimens were regulated about 0.3% and 17%, respectively. Creep rupture strength under water-saturated condition was smaller than that under air-dried condition. The difference in them was comparable to that in uniaxial compressive strength under both conditions. The axial and lateral strain rates decreased linearly with time in a log-log plot in the primary stage. They increased until a rupture occurs after the strain rates had reached the minimum magnitude. Creep failure time tended to increase as the "decrease rates of the strain rates" in the primary stage increased. Creep failure time and the life after the minimum strain rate increased as the minimum magnitude in the strain rates decreased. In the tertiary stage, the strain rates were inversely proportional to residual life. These relationships were represented by the same curves regardless of the water content. Therefore, the creep failure time would be successfully estimated from the behaviors of either axial or lateral strain rate regardless of the water content by using the experimental results. Thin sections of the specimens that had experienced various levels of the creep deformation were prepared and observed under a microscope. Nucleation of cracks and pores that distributed along the loading axis in the matrix were observed for specimens that had been just before the rupture.

Tensile creep behavior in an advanced silicon nitride

Abstract Tensile creep behavior and changes in the microstructure of the advanced silicon nitride, SN 88M, were studied at temperatures from 1250 to 1400°C to reveal the creep resistance and lifetime-controlling processes. Assuming power law dependence of the minimum strain rate on stress, stress exponents from 6 to 8 and an apparent activation energy of 780 kJ/mol were obtained. Extensive electron microscopy observations revealed significant changes in the crystalline secondary phases and creep damage development. Creep damage was classified in two groups: ‘inter-granular’ defects in the amorphous boundary phases, and ‘intra-granular’ defects in silicon nitride grains. The inter-granular defects involved multigrain junction cavities, two-grain junction cavities, microcracks and cracks. The intra-granular defects included broken large grains, small symmetrical and asymmetrical cavities, and crack-like intragranular cavities. Cavities are generated continuously during the whole deformation starting from the threshold strain of ∼0.1%, and they contribute linearly to the tensile strain. Cavities produce more than 90% of the total tensile strain, and it is concluded that cavitation is the main creep mechanism in silicon nitride ceramics. The multigrain junction cavities are considered to be the most important for generating new volume and producing tensile strain. The Luecke and Wiederhorn (L&W) creep model, based on cavitation at multigrain junctions according to an exponential law, was proven to correspond to the stress dependence of the minimum strain rate. A qualitative model based on the L&W model was suggested and expanded to include intragranular cavitation. The basic mechanisms involve a repeating of the sequence grain boundary sliding (GBS) ⇒ cavitation at multigrain junctions ⇒ viscous flow and dissolution-precipitation. The oxynitride ⇒ disilicate secondary phase transformation also occurs during creep and may result in the reduction of the residual glass content. The suggested creep model considers the effects of phase transformation, oxidation, crystallization, etc. on the basic creep mechanisms due to changes of the properties of the residual glass. Control of the amount and viscosity of the residual glass provides the possibility for the reduction of cavitation and consequently, for more creep-resistant ceramics.

Description of Stress-strain Curves Based on Thermal Activation Models for a Ti-Fe-O Alloy at 77 to 296K with Strain Rates from 10-9 to 10-2sec-1.

Using two thermal activation models for dislocation motion, we analyzed experimental data for a Ti-Fe-O alloy. One is the Kocks-Mecking model and the other is the Ogawa model. These two models differ from each other in the determination of athermal stress component and the modeling of work hardening. The Kocks-Mecking model is found to describe well the measured flow curves in a temperature range between 77 and 296 K and in a strain rate range between 10 -5 and 10 -2 sec -1 . The so-called base curve is found to be a flow curve at the strain rate of approximately 10 -9 sec - 1 by the calculations using the Kocks-Mecking model. In actual, the strain rate of 10 -9 sec -1 is approximately the minimum strain rate obtained by the crosshead displacement dwell test. The Ogawa model was found to be insufficient to describe the above flow curves for the Ti-Fe-O alloy.

533 アルミニウム合金、A5083の高温環境下におけるクリープ挙動(OS 加工による材料の機能創製)

Steady state creep doesn't exist and the minimum strain rate exists for A5083 at a constant stress. The initial creep of A5083 differs from that of pure aluminum. The inverse transient creep is observed for A5083. We obtain the experiment equation, t^* = t^0σ^δexp(Q^*/RT), for the incubation time.

On apparent activation energies of creep in nickel-base superalloys

Generally, the steady state is well defined in the creep curve in pure metals and some single-phase alloys. In this case, the diagram log {dot {epsilon}} vs. {epsilon} (where {epsilon} is the plastic strain) shows an extensive range of strain where the steady-state creep rate is maintained approximately constant. However, in those materials which present an unstable substructure, such as the particle strengthened alloys, the strain rate reaches a momentary minimum value instead of a steady-state creep rate. In this way, the minimum creep rate is observed in a regime of unstable transition. In this work, a study of the apparent activation energy of creep for IN-X750 was performed, using the differential temperature step technique. The tests were carried out near the minimum strain rate in order to analyze the influence of the material substructure on the values of the apparent activation energy.

MHD-A: A Fluctuation Splitting Wave Model for Planar Magnetohydrodynamics

This paper describes a two-dimensional (2D) upwind residual distribution or fluctuation splitting (FS) scheme (MHD-A) for the numerical solutions of planar magnetohydrodynamics (MHD) equations on structured or unstructured triangular meshes. The scheme is second order in space and time, and utilizes a consistent 2D wave model originating from the eigensystem of a 2D jacobian matrix of the MHD flux vector. The possible waves existing in this wave model are entropy, magnetoacoustic, and (numerical) magnetic monopole waves; however, Alfven waves do not exist since the problem is planar.One of the important features of the method is that the mesh structure has no influence on propagation directions of the waves. These directions are dependent only on flow properties and field gradients (for example, it is shown that the magnetoacoustic waves propagate in the directions of maximum and minimum magnetic strain rates). The other feature is that no flux evaluations and no information from the neighboring cells are needed to obtain a second order, positive, and linearity preserving scheme.A variety of numerical tests carried out by the model on structured and unstructured triangular meshes show that MHD-A produces rather encouraging numerical results even though it is the first FS wave model ever developed for multidimensional MHD.