Tensile behavior of polycrystalline zirconium at 4.2°K

Tensile tests were conducted at 4.2°K on both longitudinal and transverse specimens machined from zirconium plate. In this plate the basal planes of the grains were strongly aligned parallel to and distributed nearly uniformly about the rolling direction. The metal had higher ductility at 4.2°K than at room temperature. Prestraining transverse specimens at 4.2°K produced higher room temperature damping capacity than prestraining at 77°K. All stress-strain curves for the 4.2°K tests exhibited characteristic sudden drops in the load. Transverse specimens showed a much higher incidence of twinning, greater strain hardening, more plastic deformation between serrations, and fewer serrations than longitudinal specimens. The experimental evidence is in agreement with Kula and DeSisto’s conclusions that twinning per se is probably not responsible for the discontinuous yielding. The present observations are also consistent with Basinski’s hypothesis that the load drops are related to thermal softening. A simple calculation shows that at temperatures near absolute zero, the heat generated in a specimen during deformation is much more than that necessary to cause an instability and consequent load drop. Hence it is concluded that all the heat generated is not retained in the specimen and a large fraction must be lost to the surroundings. An estimate of the magnitude of the temperature rise associated with a typical load drop was made.     

Elevated temperature plastic anisotropy of Ti-6AI-4V plate

The influence of temperature on the planar and normal anisotropy parameters (ΔR andR, respectively) for mill annealed, duplex annealed, and cross rolled Ti-6A1-4V plate was investigated from 25 to 704°C (77 to 1300°F). Both parameters were assessed in terms of the plastic strain ratio (R), ratio of width to thickness strain at maximum load (~0.065 longitudinal strain) in tensile specimens oriented at 0, 45, and 90 deg to the rolling direction, and correlated with texture and microstructure. With increasing temperature, plates characterized by alpha deformation type basal plane textures exhibited significantly larger anisotropy variations than plate with a beta transformation type texture. This behavior was related to the degree of textural randomness and to a thermally induced transition in primary deformation mode from twinning to slip. Depending on texture, the results strongly suggest that working temperature may be utilized advantageously to alter the plastic anisotropy of Ti-6A1-4V plate for improved formability in a given fabrication operation.     

Influence of initial texture on the microstructural instabilities during compression of commercial α-Titanium at 25 °C to 400 °C

Cylindrical specimens of textured commercial pure α-titanium plate, cut with the cylinder axis along the rolling direction for one set of experiments and in the long transverse direction for the other set, were compressed at strain rates in the range of 0.001 to 100 s~’ and temperatures in the range of 25 °C to 400 °C. At strain rates ≥ 1 s⁻¹ ’, both sets of specimens exhibited adiabatic shear bands, but the intensity of shear bands was found to be higher in the rolling direction specimens than in the long transverse direction specimens. At strain rates ⪯0.1s ⁻¹ the material deformed in a microstructurally inhomogeneous fashion. For the rolling direction specimens, cracking was observed at 100 °C and at strain rates ⪯0.1 s⁻¹. This is attributed to dynamic strain aging. Such cracking was not observed in the long transverse specimens. The differences in the intensity of adiabatic shear bands and that of dynamic strain aging between the two sets of test specimens are attributed to the strong crystallographic texture present in these plates.     

Elastic wave velocities in various alloy strips

The zeroth order plate mode shear wave velocity has been measured in thin strip specimens of Oriented Electrical Steel, Elinvar-Extra, and RMI 464 titanium alloy as a function of the angle between the propagation direction and the rolling direction. Also, the plane wave shear and longitudinal velocities have been measured along the normal to the rolling plane. The results in the Oriented Electrical Steel agree with the known (110)[001] texture. In the Elinvar-Extra there is a texture indistinguishable elastically from (100)[Oil], or partial (100)[011] superimposed on a random background. Different annealing temperatures following cold-rolling yield different degrees of anisotropy and different dependences (1/G)(dG/dT) of the shear modulus upon ambient temperature. At an annealing temperature of about 850‡C, (1/G)(dG/dT) evaluated near room temperature changes from positive to negative, and anisotropy becomes minimum. In the RMI 464 titanium alloy, the shear velocity anisotropy was only 1 pct in the rolling plane. Formerly with Bell Telephone Laboratories, Allen town, Pa.     

Notch Impact Behavior of Oxide-Dispersion-Strengthened (ODS) Fe20Cr5Al Alloy

In this article, tensile tests as well as LS and LT notched Charpy impact tests were performed at the temperature range between 77 K (?196 °C) and 473 K (200 °C) on an oxide-dispersion-strengthened (ODS) Fe20Cr6Al0.5Y₂O₃ hot-rolled tube. The absorbed energy values in the range of high temperatures of LS notched specimens are considerably higher than those of LT notched specimens; however, such values tend to converge as temperature increases. Ductile fracture on the normal planes to RD with delaminations parallel to the tube surface was observed in the temperature range between room temperature (RT) and 473 K (200 °C). Delaminations of crack divider type were observed in LT specimens, whereas delaminations of crack arrester type were observed in LS specimens. The yttria particles in the grain boundaries and the transverse plastic anisotropy are the possible reasons why the delaminations were parallel to the tube surface.     

The importance of mechanical twinning in the stress-strain behavior of swaged high purity fine-grained titanium below 424°K

Zone-refined iodide titanium specimens, swaged and annealed at different temperatures to give a range of grain sizes from 2.4 to 22 μ, were deformed to fracture between 77° and 424°K. A metallographic study of the fractured specimens revealed that mechanical twinning is an important factor in the plastic deformation of this grade of titanium at subambient temperatures. In all cases, the volume fraction of twins increased steadily with strain as well as with decreasing deformation temperature. At 77°K and at a grain size of 22 μ, the twin volume fraction was greater than 0.5 at a strain of 0.5. Furthermore, each of the 77°K stress-strain curves showed a linear work-hardening character with an associated high ductility similar to that originally reported by Wasilewski for commercial purity titanium. The nearly linear stress-strain curve in this latter case has been rationalized earlier in terms of deformation twinning. The present results are in good agreement with this point of view.     

Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and ELI Grades) and VT14

The effect of microstructural refinement and the β phase fraction, V _β, on the mechanical properties at cryogenic temperatures (up to 20 K) of two commercially important aerospace titanium alloys: Ti-6Al-4V (normal as well as extra low interstitial grades) and VT14 was examined. Multi-pass caliber rolling in the temperature range of 973 K to 1223 K (700 °C to 950 °C) was employed to refine the microstructure, as V _β was found to increase nonlinearly with the rolling temperature. Detailed microstructural characterization of the alloys after caliber rolling was carried out using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). Complete spheroidization of the primary α laths along with formation of bimodal microstructure occurred when the alloys are rolled at temperatures above 1123 K (850 °C). For rolling temperatures less than 1123 K (850 °C), complete fragmentation of the β phase with limited spheroidization of α laths was observed. The microstructural refinement due to caliber rolling was found to significantly enhance the strength with no penalty on ductility both at room and cryogenic temperatures. This was attributed to a complex interplay between microstructural refinement and reduced transformed β phase fraction. TEM suggests that the serrated stress–strain responses observed in the post-yield deformation regime of specimens tested at 20 K were due to the activation of \( \left\{ {10\bar{1}2} \right\} \) tensile twins.     

Effects of Rolling and Cooling Conditions on Microstructure and Tensile and Charpy Impact Properties of Ultra-Low-Carbon High-Strength Bainitic Steels

Six ultra-low-carbon high-strength bainitic steel plates were fabricated by controlling rolling and cooling conditions, and effects of bainitic microstructure on tensile and Charpy impact properties were investigated. The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites (GBs) and bainitic ferrites (BFs) were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K or 873 K (700 °C or 600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse GBs, while the steels cooled from 873 K (600 °C) contained a considerable amount of BFs having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy (USE). When the steels cooled from 673 K (400 °C) at a cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse GBs or BFs, while the slowly cooled specimens were composed mainly of acicular ferrites (AFs). Since AFs had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had a good combination of strength, ductility, and USE, together with very low energy transition temperature (ETT).     

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.     

Properties of TiAl sheet pack-rolled at temperatures below 1000°C

TiAl alloy specimens with microcrystalline (MC, grain size is 5 μm) and submicrocrystalline (SMC, grain size is 0.4 μm) structures were successfully pack-rolled to sheet with a thickness down to 0.4 mm in the temperature ranges of 800°C to 1000°C and 900°C to 1000°C, respectively. An 18/10 stainless steel was used as a rather inexpensive can material for pack-rolling. Unidirectional rolling and bidirectional cross-rolling were used. Because of a wider temperature range for pack rolling and a lower cost for production of the alloy preforms, the microcrystalline structure was found to be a better microstructural condition for the TiAl sheet rolling. The sheet produced by unidirectional rolling had an anisotropy of mechanical properties, i.e., strength was smaller and elongation larger in the rolling direction than in a transverse direction. The anisotropy decreased when the rolling temperature increased. The bidirectional rolling led to in-plane isotropic properties of the sheet. The produced sheet showed elongation of about 3 pct at room temperature, brittle-to-ductile transition in the temperature range of 750°C to 850°C, and superplastic behavior in the temperature range 900°C to 1000°C. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

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.     

Notched Tensile and Impact Fracture of Ti-15-3 Laser Welds

The notched tensile strength (NTS) and impact toughness of Ti-15V-3Cr-3Sn-3Al (β-type titanium alloy Ti-15-3) laser welds aged at temperatures ranging from 590 K to 866 K (317 °C to 593 °C) were determined, and the results were compared to those of unwelded Ti-15-3 plates aged at the same temperature. At a given aging temperature, α precipitates in welded specimens were finer and exhibited higher hardness than those in unwelded specimens. Among the tested specimens, the weld aged at 644 K (371 °C) was most susceptible to notch sensitivity. In those welds aged at or above 755 K (482 °C), the coarse columnar structure was prone to interdendritic fracture during notched tensile tests, which reduced the NTS of the weld relative to that of the unwelded plate aged at an equivalent temperature. Of the tested specimens, the weld that was not subjected to the postweld aging treatment possessed the highest impact toughness among the specimens.     

Transformation of retained austenite in carburized 4320 steel

A systematic study of stress-induced and thermal-induced transformation of retained austenite in carburized 4320 steel with an initial retained austenite of 35 pct has been conducted. The transformation was monitored by recording the change in volume of smooth fatigue specimens. Stress-induced transformation was studied by conducting monotonic and cyclic tests at temperatures in the range from 22 °C to 150 °C. The volumetric transformation strain was as large as 0.006 at 22 °C. The anisotropy of the transformation was such that the axial transformation strain component exceeded the diametral transformation strain component by a factor of 1.4. Thermal-induced transformation was investigated with temperature stepup tests in the range from 150 °C to 255 °C at constant stress (-500 MPa, 0 MPa, and 500 MPa) and with static tests where temperature was held constant at zero load. The maximum thermal-induced volumetric transformation strain of 0.006 was independent of stress. However, the anisotropy of the transformation strain components was dependent on stress direction and magnitude. An axial tensile stress increased the axial transformation strain relative to the diametral transformation strain. The influence of low-temperature creep(T = 150 °C) on the anisotropy of strains is noted. The differences between stress-induced and thermal-induced transformation mechanisms are discussed. Thermal-induced transformation primarily occurred at temperatures between 100 °C and 200 °C, with the rate of transformation increasing with temperature, while the stress-induced transformation primarily occurred at 22 °C, with the rate of transformation decreasing with increasing temperature. There was no stress-induced transformation above 60 °C.     

Dynamic strain aging and ductility minima in zirconium

Tensile tests using coarse grained zirconium specimens were conducted at two strain rates, differing by 3 orders of magnitude, between 77° and 1032°K. At each strain rate, peaks were observed when the flow stress was plotted against the temperature. The temperature corresponding to a given peak was observed to rise with increasing strain rate. A pronounced minimum in the strain rate sensitivity of zirconium near 675°K can be explained in terms of the strain rate dependence of these peaks. At each strain rate, the zirconium tensile specimens also showed a minimum elongation at the hardening peak temperature. Since the reduction in area did not pass through a corresponding minimum, the elongation minima do not reflect a true ductility loss. What actually takes place is an increased tendency to neck at the hardening peak temperature. This tendency to promote a neck can be rationalized in terms of variations in the strain rate sensitivity caused by dynamic strain aging. Former Graduate Student now Post Doctoral Worker and Professor     

Cleavage delamination in impact tested warm-rolled steel

Cleavage delaminations or separations have been observed on fracture surfaces of impact specimens made from rolled steel pipe and plate products. These delaminations appear as cleavage cracks lying parallel to the plane of rolling. Depending primarily on the test temperature and extent of rolling, four fracture modes have been reported: ductile, transgranular, ductile with cleavage delaminations, and transgranular with cleavage delaminations. A model is proposed which accounts semiquantitatively for rolling plane weakening with respect to test temperature, rolling strain, and rolling temperature. The four observed fracture modes and the manner of transition from one to another are delineated by the model.     

Deformation microstructures in titanium sheet metal

A metallographic study has been made of the microstructures produced by room temperature deformation of 0.6mm thick commercially pure titanium sheet metal in uniaxial, plane strain and biaxial tension. Deformation twinning becomes increasingly important as the deformation mode changes from uniaxial through plane strain to equibiaxial tension, and is more significant for strain transverse to the rolling direction than for strain in the longitudinal direction. In uniaxial tension, 1122 twins are dominant in longitudinal straining, while 1012 twins dominate in transverse straining. In plane strain and equibiaxial straining, 1012 twinning is suppressed and largely replaced by 1122 twinning. The observed changes in twin occurrence and type are attributed to the interaction of the imposed stress system and the crystallographic texture of the rolled sheet, which alters the distribution of the grain basal-plane poles with respect to the operative stress axes. In uniaxial tension parallel to the longitudinal direction, twins favored by ‘c’ axis compression are produced, while in the transverse direction twins favored by ‘c’ axis tension appear. In plane strain and biaxial tension the dominant stress is through-thickness compression, which produces twins favored by ‘c’ axis compression in nearly all cases. The alterations in twin orientation and numbers are associated with changes in stress-strain behavior. As twin volume fraction increases and twins are aligned more closely to the principal stress axis, the instantaneous work-hardening rate tends to stabilize at a nearly constant value over a large strain range. Formerly Chief Metallurgist, The APV Company.     

特厚板温度梯度轧制有限元模拟与实验研究

针对特厚板再结晶型轧制,板坯中心难以变形导致心部晶粒粗大的问题,使用Q345B钢,采用有限元方法建立了特厚板轧制的仿真模型,以研究在特厚板轧制过程中引入厚度方向上的温度梯度对钢板心部应变的影响,并与传统均温轧制进行对比,预测了两种温度场条件下奥氏体再结晶的晶粒尺寸.采用大试样平面应变实验对模拟结果进行验证.研究结果表明,温度梯度轧制有利于增加坯料心部应变量,最大增加了61.35%.计算和实验结果显示温度梯度轧制可以减小特厚板心部晶粒尺寸,晶粒度级别提高了一个等级,说明该工艺对提高特厚板中心区域性能有利.     

A study of the delamination behavior of a very low-carbon steel

Splitting,i.e., delamination behavior in Charpy specimens of low-carbon steel was found to be caused by cracks propagating between pancake grains parallel to the rolling plane of the as-rolled plate. The results suggest the mechanism is a decohesion of grain boundaries that is independent of the texture of the material. The number of splits increased with decreasing finishing temperature and impact test temperature. The energy absorbed in impact testing was inversely proportional to the number of splits. In specimens annealed after rolling splitting was evident until the material was almost completely recrystallized.     

TC4钛合金轧板的织构对动态力学性能影响

利用分离式Hopkinson压杆试验装置,对具有不同织构特征的TC4合金试样进行动态压缩试验,分析织构特征对钛合金轧板各方向动态力学性能的影响。结果表明,900℃轧制板材的主织构为{1219}<12391>±30°RD,织构强度为10.557,在φ1=15°时出现峰值,有一定的织构分散,其中晶面{1219}平行板材的轧面,与基面{0001}夹角26.6°,晶向由〈1010〉向〈6 331〉方向漫散;950℃轧板的主织构为{1219}〈5321〉±20°RD,织构类型与900℃轧板相似,但织构强度为6.387,相对900℃轧板较弱,晶向由〈7341〉向〈4311〉方向漫散,在φ1=35°出现峰值;1050℃轧制板材的主织构为{12 19}〈1010〉,织构比较集中,织构强度为15.333,晶向〈101-0〉平行板材的轧向,与c轴为90°夹角。950℃轧制的TC4板材,织构强度较弱,其轧向(RD)、横向(TD)、法向(ND)的动态流变应力和动态均匀塑性应变差别不明显。900℃和1050℃轧制的TC4板材,由于织构强度较高,轧板存在明显的各向异性:TD方向的动态流变应力最高,ND次之,RD最低;RD方向的动态均匀塑性应变最大,ND次之,TD最小。     

Effects of Annealing Treatment Prior to Cold Rolling on the Edge Cracking Phenomenon of Ferritic Lightweight Steel

Effects of annealing treatment from 923 K to 1023 K (650 °C to 750 °C) prior to cold rolling on the edge cracking phenomenon of a ferritic lightweight steel were investigated. The edge cracking was severely found in the hot-rolled and 923 K (650 °C)-annealed steels after cold rolling, whereas it hardly occurred in the 1023 K (750 °C)-annealed steel. As the annealing temperature increased, lamellar κ-carbides were dissolved and coarsened, and most of the κ-carbides continuously formed along boundaries between ferrite and κ-carbide bands disappeared. Microstructural observation of the deformed region of tensile specimens revealed that the removal of band boundary κ-carbides reduced the difference in tensile elongation along the longitudinal direction (LD) and transverse direction (TD), which consequently led to the reduction in edge cracking. The 1023 K (750 °C)-annealed steel showed fine ferrite grain size, weak texture, and decomposed band structure after subsequent cold rolling and intercritical annealing, because κ-carbides actively worked as nucleation sites of ferrite and austenite. The present annealing treatment prior to cold rolling, which was originally adopted to prevent edge cracking, also beneficially modified the final microstructure of lightweight steel.