Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹ over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM _s ^σ temperature. At temperatures much greater thanM _s ^σ , austenite twinning will occur, while at temperatures close toM _s ^σ , bcc martensite will form.     

Martensite formation,strain rate sensitivity,and deformation behavior of type 304 stainless steel sheet

The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed. Formerly Visiting Scientist, Department of Metallurgical Engineering, Colorado School of Mines.     

Static strain-aging in nickel 200 between 373 and 473 K

Static strain aging was investigated in Nickel 200 using tensile specimens aged between 373 and 473 K. The kinetics of the lower yield stress return may be described in terms of either at ^1/7 or a logt relationship. The activation energy for the process was about 25,000 cal/mole. It is believed, as proposed by Rose and Glover and by Jenkins and Smith for austenitic stainless steels, that the aging phenomena are associated with carbon atom vacancy pairs which undergo Snoek ordering in the stress fields of dislocations. Macroscopic hardening probably results from the growth of atmospheres of pairs around the dislocations as a result of the drift of vacancies toward the dislocations. However, the growth of these atmospheres should be a slow process because the trapping of vacancies by the much less mobile carbon atoms seriously limits the number of free or mobile vacancies. Finally, since the number of vacancies which were formed by the prestrain deformation greatly exceeds the thermal equilibrium concentration of vacancies, there should be a slow loss of vacancies to sinks. This can explain the fact that at long times pronounced overaging is exhibited. where W. R. Cribb was formerly Graduate Assistant     

Effects of Vacancies on the Onset of Plasticity in Metals—An Atomistic Simulation Study

As strength of crystalline materials strongly depends on their ability to nucleate new dislocations or move the existing ones, a proper knowledge of factors that play a major role at the first stages of plastic deformation is highly desirable. However, fundamental understanding of the role of point defects, such as vacancies or inclusions, on the onset of plastic deformation in otherwise defect-free solids is still lacking. In this work, atomistic simulations are applied to study the role of vacancies in the inception of plasticity in metals. In particular, the effect of single vacancy as well as different vacancy distributions on the onset of plastic deformation in Ni single crystal during the nanoindentation test is explored. The combined effect of vacancy concentration and temperature on the onset of plasticity is also studied. The increase in vacancy concentration typically results in a decrease of load at the onset of plasticity. However, it is found that this is not always the case, and a particular location of one vacancy can be more important for the onset of plasticity than the higher total number of vacancies distributed in the crystal. It is also found that at higher temperatures, the effect of vacancy concentration on the load at the onset of plasticity is less pronounced than at low temperature.     

Development of Grain Boundary Precipitate-Free Zones in a Ni-Mo-Cr-W Alloy

In this study, the morphology and development of precipitate-free zones (PFZs) near grain boundaries (GBs) in low coefficient of thermal expansion (CTE) Ni-Mo-Cr-W alloys (based on Haynes 244) have been investigated as a function of thermal history and composition using electron microscopy techniques. It is shown that the formation of wide, continuous PFZs adjacent to GBs can be largely attributed to a vacancy depletion mechanism. It is proposed that variations in the vacancy distributions that develop after solution heat treatment (SHT) and subsequent quenching and aging greatly influence the development of the γ′-Ni₂(Mo,Cr) precipitates during the aging process and result in the development of PFZs of varying sizes. The relatively large (5 to 10 μm) PFZs are distinct from the smaller, more common PFZs that result from solute depletion due to GB precipitation that are typically observed after prolonged aging. During the course of this investigation, heat treatment parameters, such as aging time, SHT temperature, cooling rate after SHT, and heating rate to the aging temperature—all of which change vacancy concentration and distribution adjacent to GBs—were investigated and observed to have significant influence on both the size and morphology of the observed PFZs. In contrast to results from other Ni-based alloys studied previously, PFZ development in the current alloys was observed across a broad range of aging temperatures. This appears to be due to the high misfit strain energy of the γ′ precipitates, resulting in a nucleation process that is sensitive to vacancy concentration. It is also shown that a slightly modified alloy with higher Mo concentrations develops smaller, more typical PFZs; this is presumably due to an increased driving force for γ′ precipitation which overshadows the influence of misfit strain energy, thereby decreasing the sensitivity of precipitation on vacancy concentration.

     

Microstructure and Vickers-hardness of 20CrMnTiH steel during hot compression testing

The isothermal hot compression tests of 20CrMnTiH steel were carried out by using Gleeble-3500 thermo-simulation-machine at deformation temperatures ranging from 1123 to 1423?K and at strain rates ranging from 0.01 to 1?s^?1. The microstructural characteristics after hot compression under various deformation conditions were described with optical microscope and EBSD method, meanwhile the Vickers-hardness (HV) corresponding to the samples was measured. The strain-induced fine and uniform recrystallised grains instead of the original coarse microstructure and higher hardness were obtained with increasing strain rate and decreasing deformation temperature. The average misorientation angle got increased and an intense α-fibre texture was dominated with increasing temperature, and then the angle decreased when the temperature increase to 1423?K. By introducing Zener–Hollomon (Z) parameter, the relationship of recrystallised grain size (D) and HV under different Z values were generated. The relationship of 20CrMnTiH steel recrystallised grain size (D) and HV was formulated based on the analysis of the experimental data, which is in agreement with Hall–Petch relationship.     

Correlation between the cold-working and aging treatments in a Cu-15 Wt Pct Cr in situ composite

The cold-working and aging treatments are the two most important materials processing techniques used to produce the high strength and high conductive Cu in situ composites. In this work, we systematically investigated the relationship between these two techniques in a Cu-15 wt pct Cr in situ composite by means of the electrical conductivity measurement, hardness testing, tensile testing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In contrast to the solution-treated samples, the cold working significantly improves the electrical conductivity but only produces a moderate hardening effect during the subsequent aging treatment. This is attributed to the rapid precipitation of incoherent Cr phases due to the high vacancy density in the Cu matrix after the cold deformation. At high aging temperature, the hardness and tensile strength of the material decreases significantly due to a pronounced process of recovery and recrystallization in the Cu matrix, as well as the recovery in the Cr fibers. When aged for 1 hour, an optimum aging temperature of 715 K is recommended.     

Tensile properties of 0.05 to 0.20 Pct C TRIP steels

The uniaxial tensile properties of a series of TRIP steels of varying carbon contents and processing histories were determined over a wide range of test temperatures. The yield strengths at room temperature varied both with the deformation temperature (over the range 250° to 550°C) and with the carbon content (0.05 to 0.20 pct). Possible reasons for these variations are advanced. For all steels, the −100°C yield strengths were substantially lower than the 100°C yield strengths. The minima and maxima in the yield strengths vs temperatures curves were especially pronounced for the steels processed at the lowest deformation temperatures. Both the rate of work hardening and the elongation were influenced by the strain-induced austenite-to-martensite transformation. The rate of strain hardening and the rate of production of strain-induced martensite (per unit strain) increased with decreasing temperature. Formerly Graduate Student, University of California, Berkeley, Calif.     

Stress and electromigration in Al-lines of integrated circuits

The generation of tensile and compressive stress by the annihilation and production of vacancies which are subject to driving forces due to electromigration and due to the developing stress gradient is calculated. The rate of the stress changes is related to the deviation of the vacancy concentration from its equilibrium concentration. Depending on the magnitude of the rate constant different mechanisms of annihilation and production of vacancies (i.e. in the grain boundary itself, in adjacent grain boundaries or at dislocations) are covered. The resulting differential equations are solved numerically and analytically for some limiting cases. A quasi steady state concentration profile is established witin a short initial period of time, which is determined by one of the three processes diffusion, electromigration or rate of vacancy annihilation. During the quasi steady state the stresses increase linearly with time. When stresses are large enough to change the equilibrium vacancy concentration deviation from the linear increase occur and a rather long period follows where the true steady state is approached. If the time to failure, t_f, of an Al-line is defined as the time necessary to reach a critical stress, t_f is proportional to j⁻ⁿ where the current exponent changes from 1 at low critical stresses to 2 at higher stresses. The theoretical results are in excellent agreement with recent measurements of compressive stresses both with respect to the dependence on time and position.     

Contributions to the theory of jogged screw dislocation glide

The velocity of screw dislocations limited by the nonconservative motion of jogs is investigated. The vacancy concentration profile near a moving screw dislocation containing alternately signed jogs separated by a distance λb, whereb is the magnitude of the Burgers vector, has been calculated. It is shown that for λ > 20 the vacancy profile is essentially the same as that found previously for the case of a moving screw dislocation containing isolated jogs. It is shown that the jog-jog diffusion interaction does not change the stress dependence of the glide velocity as has been suggested. The steady state velocity of a gliding jogged screw dislocation responding to an effective stress τ_e is calculated using the quasiequilibrium approach to dislocation climb and the steady state vacancy concentration profile. It is shown that the glide velocity exhibits hyperbolic tangent stress dependence if the average vacancy concentration in the crystal equals the equilibrium vacancy concentration, C_o. If, however, the average vacancy concentration in the crystal follows the relation C_o cosh (λb ³ τ _e kT) during deformation, the glide velocity can be expressed as υ = πD_υb²C₀ sinh (λb ³ τ _e /kT whereD _υ is the diffusion coefficient for vacancies in the crystal. A model which suggests that the equilibrium vacancy concentration follows a relation similar to the hyperbolic cosine dependence is presented.     

Cold work hardening in stages IV and V of F.C.C. metals—II. Model fits and physical results

The paper presents a common concept for work hardening of f.c.c. metals at temperatures below 0.5 T_m up to very high strains. The concept considers statistical dislocation dynamics in terms of screw and edge dislocations and their specific interactions, and also allows for deformation-induced vacancies. By fitting the concept equations to experiment and using measured interaction parameters, very good agreement with strengthening data and dislocation densities of Cu and Al is achieved. The resulting fit parameters exhibit the expected order of magnitude and temperature dependence of dislocation storage and annihilation, for each dislocation type. The annihilation parameter of edge dislocations yields vacancy migration enthalpies of ≈0.2 eV for CU and Al indicating core migration of vacancies. From this the vacancy concentration is calculated for all strains and temperatures in question. Up to ≈0.3 T_m, the saturation concentration of vacancies decreases, but reincreases beyond this temperature following the thermal vacancy concentration.     

Analysis of the Plasticity-Enhancing Mechanisms in 12 pctMn Austeno-ferritic Steel by In Situ Neutron Diffraction

The tensile behavior of ductile ultra-high strength Fe-12 pctMn-0.3 pctC-2 pctAl austeno-ferritic steel was studied by in situ neutron diffraction measurement of the elastic lattice strains, dislocation density, stacking fault probability, and strain-induced transformation kinetics. Micro-yielding was observed in austenite, and the plastic deformation of ferrite remained very limited throughout the deformation. The analysis identified three contributions to the strain hardening: twinning-induced plasticity, transformation-induced plasticity, and the accumulation of a high density of geometrically necessary dislocations accommodating the strain mismatch at the phase boundaries.     

A point defect model for fatigue crack initiation in Ni3Al+B single crystals

Transmission electron microscopy of boron-doped Ni₃Al single crystals, oriented for single slip and cyclically deformed at room temperature, revealed a high density of dislocation dipoles and point defect clusters. Observations of circular perfect dislocation loops, Frank loops, vacancy tetrahedra and spherical voids provide evidence of vacancy condensation during fatigue cycling at room temperature. It is suggested that lattice misfit develops between persistent slip bands (PSB) and matrix as a result of the generation and coalescence of excess vacancies in PSBs. The misfit strain at PSB/matrix interfaces is considered to increase with increasing cumulative plastic strain. Together with SEM observations of surface topography, it is suggested that fatigue damage in Ni₃Al single crystals is initiated by the formation of microvoids (microcracks) at PSB/matrix interfaces. The microvoids (microcracks) break down the coherency of the PSB/matrix interfaces and thereby relieve the accumulated misfit strain at the interfaces. A model of fatigue crack initiation based upon a surface energy criterion is proposed.     

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

     

Deformation behavior of an Al-3.37 Wt Pct Li alloy

Al-3.37 wt pct Li alloy was deformed by differential strain rate and constant initial strain rate test techniques to investigate deformation and failure behavior over the strain rate range of 10^-5 to 10^-2 s^-1 and the temperature range of 22 °C to 580 °C. Flow stress first increases then decreases with an increase in test temperature, whereas ductility shows a sigmoidal relationship with the test temperature. The maximum ductility of about 80 pct is obtained at intermediate strain rate and 550 °C. Failure is noted to occur by cavity interlinkage and crack formation. Strain rate sensitivity (m) and activation energy (Q) for deformation are determined to be 0.04 to 0.13 and 96.2 to 157.4 kJ/mol, respectively. Toward lower test temperatures, both them andQ are found to have lower values. Deformation at high temperature is suggested to be controlled by dislocation climb. However, under non-steady-state conditions due to cavitation,m andQ both vary with strain. Formerly B. Tech. Final Year Student, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay.     

The effect of small plastic deformation and annealing on the properties of polycrystals: Part II. Theoretical model for the transformation of nonequilibrium grain boundaries

The nonequilibrium grain boundary state which has a high energy state is the result of absorption of a certain density of extrinsic grain boundary dislocations (EGBD’s). The equilibrium of such a boundary occurs by annealing at higher temperatures. A model has been proposed in this paper which assumes that the equilibrium of a nonequilibrium grain boundary involves the annihilation of EGBD’s by climbvia lattice diffusion of vacancies at the triple points. Due to the stress field of the EGBD’s, there is a vacancy concentration gradient around the triple points. The profile of the vacancy concentration gradient is derived by assuming a steady state flux of vacancies. Using this vacancy concentration profile, the expressions for the rate of climb of EGBD’s are derived. The proposed model predicts that the time required for the equilibration of nonequilibrium grain boundaries is dependent not only on the annealing temperature but also on the initial density of EGBD’s and the boundary length (which is related to the grain size). It has also been shown that the equilibrium behavior predicted by our model is in good agreement with the experimental results obtained for 316L stainless steel.     

Steady state vacancy concentration around a plastic crack

The vacancy chemical potential associated with the crack region and with the lattice dislocations representing the plastic zone are identified in terms of the energy of the dislocation configuration. In order to determine the steady state vacancy concentration, the configuration is considered as made up of several internal sources of stress. The diffusion equation under steady state is solved for a crystal containing each source of stress. Further, superposition of the concentration of vacancies around each source is used to determine the total concentration of vacancies for small scale deformation at the crack tip. On the other hand, for large scale deformation at the tip, matching boundary conditions are applied to determine the concentration in each region containing an internal source. Both the discrete dislocation and the single lattice dislocation representations of the crack are employed to determine the crack growth rate. The results are used to emphasize the influence of the plastic zone on the crack growth rate by vacancy diffusion mechanism. Formerly Assistant Professor, Department of Engineering Science and Mechanics, Tennessee Technological University. Cookeville, TN 38501.     

The deformation of silicon at high temperatures and strain rates

Polycrystalline and 〈100〉 single crystalline semiconductor grade silicon samples have been subjected to uniaxial compression at strain rates from 10⁻⁵ to 12 s⁻¹ at temperatures ranging from 1100 to 1380 °C. Both intrinsic and p-type polycrystalline material and p-type single crystalline material were tested. Except at the highest temperature and lowest strain rate, no steady state deformation was observed for the polycrystalline material. In all other cases strain hardening was observed which increased with increasing strain rates. The polycrystalline material could be compressed by as much as 50 pct at 1380 °C and a strain rate of 7 s⁻¹ without cracking. An axial stress of approximately 170 MPa produces a strain rate of 5 s⁻¹ at 1380 °C. The stress necessary to produce a given strain rate increases rapidly with decreasing temperature while the ductility rapidly decreases. A preliminary forming limit diagram has also been determined for the polycrystalline material at 1380 °C. The deformation rate-controlling process in the polycrystalline material at high stresses could be the production of vacancies on jogged dislocations. Formerly with the Department of Materials Science and Engineering, University of Pennsylvania     

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.