Anisotropy in impact behavior of Zr-2.5Nb pressure tube alloy

Zr-2.5Nb alloy tubes in cold worked and stress relieved (CWSR) condition serve as pressure boundary for hot coolant in Indian Pressurized Heavy Water Reactor (IPHWR). Due to both microstructural and crystallographic anisotropy, the mechanical properties in general and fracture behavior in particular are anisotropic for this material/component. In this work impact behavior of the pressure tube material was characterized over a range of temperature by impact test using specimens with crack growth direction along axial and transverse directions of the tubes. It has been found that both temperature and orientation have strong influence on the absorbed impact energy.     

Analysis of steady-state thermal creep of Zr-2.5Nb pressure tube material

The steady-state thermal creep rate in the axial and transverse directions of Zr-2.5Nb of pressure tubes, used in CANDU nuclear reactors, was determined. The data were obtained both from tensile samples having their tensile axes cut along the axial and transverse directions of the pressure tubes and from small-sized, thin-walled tubes, i.e., “mini” tubes stressed either in torsion or by internally pressurizing capsules manufactured from the mini tubes, or by additionally applying an external, axial load on these internally pressurized capsules. The temperature range of the data was from 373 to 596 K (100 °C to 323 °C) and the duration of the tests was from about 1500 hours to over 12,000 hours. The tests were carried out over a sufficiently long time for the creep rate to be measurable in the steady-state creep regime. It was found that the steady-state creep rate depends on stress in a nonlinear fashion and the stress exponent over the entire temperature range was about four. This value is consistent with the values measured earlier on other zirconium alloys. The activation energy Q was found to be about 21 and 10 kcal/mol for temperatures above and below 475 K (∼ 200 °C), respectively. These values are lower than those measured by other investigators on the same material at higher temperatures but similar to values found on other Zr alloys at low temperatures. It appears that Q is dependent on temperature and its value is consistent with the presence of dynamic strain aging (DSA). The results of this study were analyzed with a polycrystalline, nonlinear self-consistent model that take into account the crystallographic texture of the material. This model was used to derive the values of critical resolved shear stress (CRSS), which are consistent with prismatic, basal, and pyramidal glide. By using these values and the apparent temperature dependence of Q, it was shown that this model predicts well the steady-state creep rate over the entire temperature range and under very different stress states.     

Structural transformations during heating of a Zr-2.5% Nb alloy subjected to equal-channel angular pressing

The structure of a Zr-2.5% Nb alloy after equal-channel angular pressing (ECAP) at 690–700 K and annealing in the temperature range 670–1070 K is investigated. The structure of the Zr-2.5% Nb alloy deformed by ECAP is an irregular grain-subgrain oriented structure with an enhanced dislocation density, a cross-section of 30–150 nm of oriented structural elements, and an equiaxed-grain (subgrain) size of 50–200 nm. Heating after ECAP in the temperature range 720–770 K for 3–5 h is proposed for the formation of an ultrafine-grained equilibrium structure in the ECAP deformed Zr-2.5% Nb alloy. Heating of the Zr-2.5% Nb alloy after ECAP at 723 K for 5 h leads to the formation of a predominantly equiaxial submicrocrystalline structure with a grain size of 150–500 nm. Equal-channel angular pressing of the Zr-2.5% Nb alloy increases the yield strength to 622 MPa, which is higher than that in the as-delivered undeformed state by a factor of 1.6. In this case, the relative elongation decreases. Heating of the ECAP deformed Zr-2.5% Nb alloy at 723 K for 5 h decreases the yield strength to 504 MPa, but the relative elongation increases to 14%.     

Evolution of textures in zirconium alloys deformed uniaxially at elevated temperatures

Texture evolution inα-Zr due to uniaxial deformation at 923 to 1123 K was investigated in crystal-bar Zr and Zr-2.5Nb. The temperature range selected corresponds to the two-phase (α +β) field in the Zr-2.5Nb alloy. It was found that uniaxial compression causes a progressive rotation of the (0002) plane normals away from the compression direction and away from the compression plane. In the crystal-bar Zr, the compression texture consists of a [0001] fiber tilted 30 deg from the compression axis. By contrast, in Zr-2.5Nb, a [0001] fiber with an angular spread of 30 deg is obtained. The effect of theβ phase present in Zr-2.5Nb at the temperatures investigated was evaluated by testing a Zr-20Nb alloy in compression. The β-phase texture consisted of a weak 〈111〉-〈00l〉 double fiber. Comparison of this texture and the textures observed in Zr-2.5Nb indicates that theβ →α transformation takes place by the growth of pre-existing a grains and not according to the Burgers mechanism. This transformation has, therefore, no direct effect on the α-phase texture after cooling to room temperature from the (α +β) field. Uniaxial elongation by swaging of Zr-2.5Nb produces a dual fiber. Similar results are obtained in hot extruded rods. Modeling of the development of textures in the α phase was performed using linear programming and employing relaxed constraint (RC) models (“curling” for tension and ”pancake” for compression) implemented for hexagonal close-packed (hcp) grains. It is assumed that prismatic, basal, and 〈c +a〉 pyramidal slip were the active deformation modes at high temperatures. It is shown that these models reduce the activity of the pyramidal slip systems to realistic values, in contrast to the full constraint (FC) approach, where most of the deformation is accommodated by 〈c +a〉 slip. Microstructural evidence is presented regarding the occurrence of ”curling” during uniaxial elongation. Formerly Graduate Student with the Department of Metallurgical Engineering, McGill University     

Effect of process variables on texture and creep of pressurised heavy water reactor pressure tubes

Zr-2.5 wt%Nb pressure tubes are used for coolant channels of Pressurised Heavy Water Reactors (PHWRs). These pressure tubes are lifetime components of the reactor and have to sustain extremely harsh conditions of temperature, pressure and neutron irradiation during service. One of the major life limiting factors for pressure tubes is diametral creep. This causes dilation of the pressure tube leading to flow bypass and inefficient heat removal from fuel bundles. This underscores the importance of producing pressure tubes with higher creep resistance. The primary metallurgical parameters controlling the creep strength of Zr-2.5Nb pressure tubes are texture, microstructure, grain size, dislocation density and alloying additions. This includes the strengthening of beta phase due to niobium enrichment. Heat treated Zr-2.5Nb pressure tubes have been reported to have the highest creep strength. This paper briefly discusses the effects of various processing parameters on the microstructure and texture of pressure tubes, which enhance their creep resistance.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900°C and 1060°C and strain rates between 10⁻⁴ and 10⁻² s⁻². Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperatur and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy), were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of Mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallo graphic texture components determined for the B2 phase ((001) [100] and (−112) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900 °C and 1060 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and ( 12) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

Anisotropic threshold stress intensity factor,K IH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hybride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH, of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH, was evaluated as a function of orientation on the curved compactension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560°C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hydride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH , of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH , was evaluated as a function of orientation on the curved compact-tension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560 °C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the {10 7} pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the {10 7} hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

The effects of stress,temperature and hydrogen content on hydride-induced crack growth in Zr-2.5 Pct Nb

The velocity of hydride induced subcritical crack growth in Zr-2.5 pct Nb has been determined using the potential drop method for measuring crack extension. A revised picture of the two-stage, crack velocity-stress intensity relationship has been obtained with a threshold stress intensity of 6 MPa·m^1/2, independent of temperature. A consistent temperature dependence of the crack velocity has been determined for hydrided material but the velocity measurements in as-received material are unexpectedly high. A previous theoretical model has been improved. The improved model has provided a useful basis for explaining some of the present data which could not be rationalized in terms of the previous model. Criteria for the stepwise crack propagation behavior are discussed.     

Microsegregation of oxygen in Zr-2.5Nb alloy materials

Oxygen partitioning between the primary-a andβ phases during heating at an(α + β) temperature was investigated in a Zr-2.5Nb alloy containing about 1100 ppm (wt) oxygen. Standard pressure tubes used in CANDU reactors have been produced from this material by hot extrusion of billets after preheating in the (α + β) temperature range. Secondary ion mass spectroscopy (SIMS) was found most suitable for oxygen mapping and for determining quantitatively the extent of preferential oxygen enrichment of primary-a grains in pressure tube samples previously subjected to experimental preheat treatments at an (α +β) temperature of 870° for different soak times up to 24 hours. For the (α α β)-treated materials, the oxygen concentration in the primary-a grains increased with soak time, reaching up to about 8 times the concentration in the primary-β matrix region. Samples from two standard pressure tubes with different fracture toughness properties were also examined. Signi-ficant differences between these tubes were found in their primary-α grain size and in the levels of oxygen enrichment of the a grains, which could provide an explanation for the difference in the toughness of these tubes.     

Effect of precipitates on plastic anisotropy for polycrystalline aluminum alloys

The effects of crystallographic texture and precipitate distribution on macroscopic anisotropy in aluminum alloys were investigated. In order to simultaneously consider the effects of crystallographic texture and precipitate distribution on macroscopic anisotropy, predictions of plastic properties were carried out using an anisotropic yield function based on the material texture and a combined isotropic-kinematic hardening rule. The input to the model was a single stress-strain curve, the crystallographic texture, and the precipitate volume fraction, shape, and habit planes. It was shown that the kinematic hardening rule, which expresses a translation of the yield surface in stress space, was a function of all the parameters describing the precipitate distribution. The model was applied to the case of an extruded and recrystallized binary Al-3 wt pct Cu alloy deformed in uniaxial compression in different directions. Excellent agreement was observed between the experimental and predicted yield stress anisotropy and the specimen cross section shape anisotropy. Gaussian distributions of grain orientations around ideal texture components typical of aluminum alloys were generated using computer simulations. These textures were combined with the isotropic-kinematic hardening rule determined for the Al-3 wt pct Cu binary alloy to theoretically assess the influence of precipitates on the r-value (the width-to-thickness plastic strain ratio in uniaxial tension) and yield stress anisotropy for aluminum sheets. It was shown that, for these textures, the precipitate distribution had the effect of reducing plastic anisotropy, in agreement with the trends generally observed in practice.     

The origin of strain-rate sensitivity in OFHC copper

This investigation was conducted to determine the nature of the strain-rate sensitivity of OFHC copper at room temperature. In particular, the relative magnitudes of the dynamic contribution (particle inertia, suppression of thermal assistance, and so forth) and the nondynamic contribution (namely, the accelerated rate of strain hardening observed at high strain rates) to the strain-rate sensitivity were determined. Specimens were dynamically compressed using the Hopkinson pressure bar technique, and then were reloaded quasistatically to determine their respective yield strengths. The dynamic contribution to strainrate sensitivity was taken as the difference between the peak dynamic flow stress and the flow stress of the same specimen when reloaded quasistatically. The nondynamic contribution to strain-rate sensitivity of the flow stress was taken as the difference between the quasistatic flow stress in reloading of a specimen prestrained ε₀ dynamically and the flow stress at ε₀ for a sample deformed in uniaxial quasistatic compression. The room temperature dynamic flow-stress of OFHC copper, deformed at 500s⁻¹, was found to be 25 pct higher than the conventional quasistatic flow stress for this metal over a strain range of 0.08 to 0.20. The nondynamic contribution to strain-rate sensitivity was found to be about 60 pct of the total flow stress increase and has been attributed to a difference in strainhardening at different strain rates. Thus, it appears that in OFHC copper at room temperature and at strain rates of about 500s⁻¹, the nondynamic contribution to strain-rate sensitivity is more significant than the dynamic contribution.     

Texture and stress state dependent creep in Zircaloy-2

An experimental program was conducted to characterize creep strength differential (CSD) effects in Zircaloy-2 by the determination of differences in the magnitude of creep deformation in specimens under uniaxial tension and compression. The texture dependence of creep in these specimens was also characterized. Uniaxial thermal creep tests were conducted in tension and compression at 325 to 400 °C and 69 to 172 MPa on specimens taken from the three principal directions of textured Zircaloy-2 plate fabricated in both the recrystallized and the cold-worked, stress relieved condition. Stress relaxation tests were conducted at 400 °C on similar specimens to substantiate results of the thermal creep tests. Creep strength-differential effects were found in all three principal directions of both types of plate, and it appeared that the CSD was larger in the longitudinal than the normal direction. Creep strains in tension were as much as three times larger than those in compression. In addition, the relative magnitude of creep strains indicated that the tensile creep strength at 400 °C was proportional to the resolved fraction of basal poles in the test direction. Cold work attenuated this anisotropy, and “transitions’in some creep tests on recrystallized material reversed this anisotropy. Finally, the thermal creep data were used to construct creep loci which graphically illustrate these characteristics.     

Plastic behavior of 70/30 brass sheet

The plastic yield behavior of strip annealed 70/30 brass sheet has been investigated using several experimental techniques. Proportional path, stress-strain relations were measured in two strain states using a recently devised plane-strain test and a standard sheet tensile test. Based on these data, 70/30 brass exhibits a dramatic departure from Hill's plasticity models. Particularly notable is the lower work-hardening rate in plane strain. A second series of tests was carried out by deforming first in plane-strain tension and subsequently in uniaxial tension. The relative orientation of the principal strain directions in the two strain paths strongly affected the transient yielding behavior, but the original work-hardening pattern and plastic anisotropy were approached after an additional effective strain of ∼0.04. These observations are consistent with a two yield-surface model;i.e., one an underlying, proportional path yield surface and one an instantaneous, transient yield surface.     

The effect of hydrogen on the multiaxial stress-strain behavior of titanium tubing

The influence of internal hydrogen on the multiaxial stress-strain behavior of commercially pure titanium has been studied. Thin-walled tubing specimens containing either 20 or 1070 ppm hydrogen have been tested at constant stress ratios in combined tension and internal pressure. The addition of hydrogen lowers the yield strength for all loading paths but has no significant effect on the strain hardening behavior at strains ε ≥ 0.02. Thus, the hydrogen embrittlement of titanium under plain strain or equibiaxial loading is not a consequence of changes of flow behavior. The yielding behavior of this anisotropic material is described well by Hill’s quadratic yield criterion. As measured mechanically and by pole figure analysis, the plastic anisotropy changes with deformation in a manner which depends on stress state. Hill’s criterion and the associated flow rule do not describe the multiaxial flow behavior well because of their inability to account for changes of texture which depend on multiaxial stress path. Hence, a strain dependent, texture-induced strengthening effect in equibiaxial tension is observed, this effect having the form of an enhanced strain hardening rate. Formerly with Michigan Technological University     

<Emphasis Type="Italic">In Situ</Emphasis> Observation of Strain Evolution in Cp-Ti Over Multiple Length Scales

The strain evolution in polycrystalline CP-Ti strip under tension was studied in situ and at two length scales using Synchrotron X-ray diffraction. To establish the bulk material behavior, experiments were performed at the Australian Synchrotron facility. Because of the relatively large grain size, discontinuous “spotty” Debye ring patterns were observed, and a peak fitting algorithm was developed to determine the individual spot positions with the necessary precision for strain determination. The crystallographic directional dependence of strain anisotropy during the loading cycle was determined. Strain anisotropy and yielding of individual crystallographic planes prior to the macroscopic yield point were further clarified by in situ loading experiments performed at the Advanced Light Source (ALS). The deviatoric strain accumulation and plastic response were mapped on a grain-by-grain basis. The onset of microscopic yielding in the grains was identified and correlated with the relative orientation of the grains with respect to the loading direction.     

Short-range order effects on the tensile behavior of a nickel-base alloy

A nickel base weld filler metal alloy with nominal composition of 67 pct Ni, 20 pct Cr, 3 pct Mn, 3 pct Fe, and 2.5 pct Nb (Cb) is used to make austenitic-ferritic dissimilar metal joints. Tensile properties were determined for this alloy over the range 25 to 732°C at strain-rates of 3×10⁻⁶ and 3×10⁻⁴/s. Above about 450°C, both the yield strength and the ultimate tensile strength in the low strain-rate tests showed significant increases over the strengths at the higher strain-rate. The enhanced values for the yield strength persisted to the highest test temperature (732°C), whereas the ultimate tensile strength for the low strain-rate fell below the curve for the higher strain-rate at about 600°C. Above 600°C, the ultimate tensile strength dropped off rapidly and at 677°C approached the yield strength (i.e., the uniform elongation dropped to less than 1 pct). The strain-rate effects have been attributed to “K-state” formation, an effect that investigators have attributed to short range order in other Ni−Cr base alloys.     

Crack initiation at long radial hydrides in Zr-2.5nb pressure tube material at elevated temperatures

Crack initiation at hydrides in smooth tensile specimens of Zr-2.5Nb pressure tube material was investigated at elevated temperatures up to 300 °C using an acoustic emission (AE) technique. The test specimens contained long, radial hydride platelets. These hydrides have their plate normals oriented in the applied stress direction. Below~100 °C, widespread hydride cracking was initiated at stresses close to the yield stress. An estimate of the hydride’s fracture strength from this data yielded a value of ~520 MPa at 100 °C. Metallography showed that up to this temperature, cracking occurred along the length of the hydrides. However, at higher temperatures, there was no clear evidence of lengthwise cracking of hydrides, and fewer of the total hydride population fractured during deformation, as indicated by the AE record and the metallography. Moreover, the hydrides showed significant plasticity by being able to flow along with the matrix material and align themselves parallel to the applied stress direction without fracturing. Near the fracture surface of the specimen, transverse cracking of the flow-reoriented hydrides had occurred at various points along the lengths of the hydrides. These fractures appear to be the result of stresses produced by large plastic strains imposed by the surrounding matrix on the less ductile hydrides.     

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s^-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.