Stress-Assisted and strain-induced martensites in FE-NI-C alloys

A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withM _s temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowM _s except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}_γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite. Formerly a graduate student at Stanford University     

Transformation behavior of TRIP steels

True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above M_s ^σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below M_s ^σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below M_s ^σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above M_s ^σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above M_s ^σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below M_s ^σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above M_s ^σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below M_s ^σ.     

Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10_-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.     

Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10⁻³ to 1 s⁻¹. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10⁻²s⁻¹, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10⁻¹s⁻¹ for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.     

Thermal gradients,strain rate,and ductility in sheet steel tensile specimens

The effect on ductility of strain rate and thermal gradients arising from deformation is examined in tensile specimens of 1008 AK steel. The total elongatione _tot is taken as the measure of ductility, since it reflects changes in the strain hardeningn and strain-rate sensitivitym. Tensile specimens are pulled to failure in 23 °C air, at initial strain rates from 10⁻³ to 10⁻¹ s⁻¹, with thermocouples recording temperature along the 50.8 mm gauge section. The maximum temperature is ∼110 °C just prior to failure at the highest rate. Thee _tot, however, remains fairly constant with rate at ∼40 pct. When thermal gradients are prevented by immersing the specimens in circulating water at 23 °C,e _tot, increases with rate to a maximum of ∼54 pct at 10⁻¹ s⁻¹. Direct measurements of isothermal values ofm at 23, 60, and 90 °C show thatm increases with rate.e _tot, therefore, would be expected to increase with rate. Since under nonisothermal conditionse _tot does not change, it appears thatm and thermal gradients are competing influences on ductility at higher rates. Enhanced ductility in stampings should be possible by suppressing gradients, either by controlling die temperature or by heat transfer properties of a lubricant.     

Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M _s ) temperatures. The NiTi-I alloy (M _s ≈+27 °C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of “stress-assisted” martensite. The NiTi-II alloy (M _s ≈−45 °C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of “strain-induced” martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M _s temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10⁻⁴s⁻¹ to 400 s⁻¹. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

The shape memory effect and superelasticity in two-phase polycrystalline α/β brasses

The shape memory effect (SME), superelasticity (SE), and cyclic deformation behavior of two-phase α/β brasses have been investigated at various temperatures, using tensile tests andin situ optical microscopic observations. The morphology and characteristics of the (thermoelastic) martensitic transformation and the mechanism of the SME are similar to those for single-phase β-brass, but the amount of irrecoverable strain is larger in the two-phase alloys due to plastic deformation of the α particles. After unloading and heating, the slipbands in the discrete a particles remain, whereas the martensite almost disappears; thus, the higher the volume fraction of α particles, the larger the amount of irrecoverable strain. The deformation behavior of alloy A at temperatures above the martensite start (M_s) temperature (with 26 pct α phase) is dominated by deformation of the α phase, so complete SE cannot be obtained after cyclic deformation, both at room temperature and at -40 °C. While in alloy B (containing 15 pct α phase), the deformation behavior is dominated by the formation of stress-induced martensite (SIM). The α particles are deformed before SIM formation on loading at room temperature, but on the contrary, SIM forms before the α particles are deformed on loading at -40 °C (>M_s). Complete SE can be obtained in alloy B after cyclic deformation at room temperature to a given strain but does not occur at -40 °C because the a particles are deformed along with the growth of pre-existing SIM under larger strain during cycling at this temperature.     

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.     

High-temperature deformation properties of austenitic Fe-Mn alloys

The influence of the Mn content on the hot deformation properties of austenitic binary Fe-Mn alloys containing 1 to 20 mass pct Mn has been investigated for the first time. The influence of the Mn content on the constitutive equations was determined for the temperature range of 950 °C to 1250 °C and the strain rate range of 0.1 to 2 s⁻¹. The activation energy for hot working increased with increasing Mn content, and dynamic recrystallization was observed for all the Fe-Mn binary alloys. The Mn was found to delay dynamic recrystallization. An increase in the Mn content resulted in an increase of both the peak stress σ _p and the corresponding peak strain ε _p , thus revealing the pronounced influence of Mn on the hot deformation process.     

Elevated temperature strength of fine-grained INCONEL alloy MA754

Elevated temperature tensile and creep-rupture tests were performed on INCONEL alloy MA754 in an as-rolled, fine-grained condition. Tensile tests were performed at 25 °C, 800 °C, 900 °C, and 1000 °C; creep-rupture tests were performed at 800 °C, 900 °C, and 1000 °C. the elevated temperature strength in the fine-grained condition was approximately 25 pct of that the coarse-grained, annealed condition. While good ductility was observed in tensile tests at a nominal strain rate of 1 × 10⁻³s⁻¹, ductility in creep-rupture tests was very low, with failure elongations less than 5 pct and no reduction in area. Creep deformation appeared to occur primarily by cavity formation and growth.     

Shear localization-martensitic transformation interactions in Fe-Cr-Ni monocrystal

A Fe-15 wt pct Cr-15 wt pct Ni alloy monocrystal was deformed dynamically (strain rate ∼10⁴ s⁻¹) by the collapse of an explosively driven thick-walled cylinder under prescribed initial temperature and strain conditions. The experiments were carried out under the following conditions: (a) alloy in austenitic state, temperature above transformation temperature; (b) alloy in transformed state; and (c) alloy at temperature slightly above M _s , propitiating concurrent shear-band propagation and martensitic transformation. The alloy exhibited profuse shear-band formation, which was a sensitive function of the deformation condition. Stress-assisted and strain-induced martensitic transformation competes with shear localization. The alloy deformed at a temperature slightly above M _s shows a significantly reduced number of shear bands. The anisotropy of plastic deformation determines the evolution of strains and distribution of shear bands. The different conditions showed significant differences that are interpreted in terms of the microstructural anisotropy. Calculated shear-band spacings based on the Grady-Kipp (GK) and Wright-Ockendon (WO) theories are compared with the observed values. The microstructure within the shear bands was characterized by transmission electron microscopy. Regions of sub-micron grain sizes exhibiting evidence of recrystallization were observed, as well as amorphous regions possibly resulting from melting and rapid resolidification. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10⁻⁶ and 2×10⁻³ s⁻¹ and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol⁻¹ for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10⁻⁴, where σ is the applied stress andE is the Young’s modulus.     

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     

Structure and mechanical properties of Fe−Cr−C−Co steels

As part of a continuing program concerning the microstructures and mechanical properties of steels in which particular attention is given to transformation substructures, the present work is concerned with martensite and bainite in Fe−Cr−C steels with and without cobalt. Although cobalt raises theM _s temperature it does not affect the extent of twinning for the same carbon level and so M _s temperature alone does not control transformation substructure. Thus cobalt is not effective in retaining dislocated martensite as carbon is increased and in this regard cobalt is not beneficial to toughness. TheM _s temperatures of the steels were relatively high and hence isothermal transformation yielded mixtures of bainites and tempered martensite depending on the temperature of transformation. The mechanical properties of the isothermally transformed steels were inferior to those of the tempered steels due to the interference of upper bainite or (tempered) martensite during the isothermal transformation. Thus, in the steels having highM _s temperatures the twinning tempered martensitic structure had relatively better mechanical properties compared to the isothermally transformed steels. Attempts to produce desirable autotempered structures by air cooling (single heat treatments) were not successful and did not improve the mechanical properties since the structure consisted of a mixture of bainite and martensite. This paper is based upon a thesis submitted by M. RAGHAVAN in partial fulfillment of the requirements of the degree of Master of Science at the University of California.     

The development of martensitic microstructure and microcracking in an Fe-1.86C alloy

The development of the martensitic microstructure in a 1.86 wt pct C steel has been followed by quantitative metallographic measurements over the transformation range of 0.12 to 0.50 fraction transformed (f). The transformation kinetics are described by the equationf = 1 − exp [−0.008 (M _s − T_q)] where M_s and T_q are the martensite start and the quenching temperatures respectively. Fullman’s analysis shows that the average volume per martensite plate decreases by almost an order of magnitude over the transformation range studied, but this decrease is less than that predicted by the Fisher analysis for partitioning of austenite by successive generations of martensite. Microcracking increases with increasingf up to 0.3, but does not increase forf above 0.3 where transformation proceeds by the nucleation of large numbers of small martensite plates. These observations indicate that a critical size of martensite plate is necessary to cause microcracking. Formerly Postdoctoral Fellow at Lehigh University