Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys, part II: Creep deformation

This work focuses on the effect of the second phase on the ambient temperature creep deformation mechanisms of titanium alloys, using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V with Widmanstätten microstructures as the model systems. In Part I it was observed that the presence of a second phase can affect the tensile deformation behavior. Likewise, the creep deformation mechanisms of the two-phase alloys differ from the mechanisms of single-phase alloys. These α-β deformation mechanisms include twinning in fine grains of the α phase and stress-induced hexagonal martensite in the β phase of Ti-8.1 V. This is the first time that twinning in the α phase and stress-induced martensite in the β phase are reported as creep deformation mechanisms in an α-β titanium alloy. Several factors, including elastic interaction effects, shear stress due to deformation products in adjacent phases, and the stability of the β phase, affect the creep deformation mechanisms in these alloys. Models for the time-dependent growth of martensite are suggested. In addition, the difference between tensile and creep deformation in regard to accumulation of stresses to reach the critical stresses is described.     

Influence of the deformation mechanism on the anisotropy of the mechanical properties and workability of magnesium alloys

An experimental calculation method for the estimation of anisotropy is developed for semifinished sheet and pressed products made from magnesium alloys. The method makes it possible to calculate the anisotropy parameters from quantitative data on the texture and the relative values of the reduced critical shear stresses for the slip and twinning mechanisms operating in these alloys. The optimal alloying of magnesium alloys is shown to provide two methods for enhancing the deep drawing characteristics, namely, decreasing the intensity of the basal texture due to the formation of dispersed intermetallic compounds and increasing the compressive-strain resistance compared to tension due to a change in the deformation mechanism. Yttrium and neodymium are found to be most efficient in this respect, because they favor such a deformation mechanism that increases the Lankford coefficient by two to four times at the same texture in sheets. In addition, neodymium alloying weakens the intensity of the basal texture, which also favors an increase in the Lankford coefficient.     

Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys: Part I. Tensile deformation

The effects of α and β phase interactions on the room-temperature tensile and creep deformation behavior of α + β titanium alloys with Widmanst?tten microstructures were studied using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V as the model two-phase alloy systems. This article, Part I, deals with tensile deformation. It was found that when the α phase is present as thin (<10-μm) plates in the α + β alloys, significant twinning occurs. No significant twinning was observed in single-phase alloys with the same chemistry and similar grain size. Additionally, the β phase of Ti-8.1 V deforms by stress-induced hexagonal martensite (α′), while only twinning occurs in the single-phase β alloy with the same chemistry. Twinning in the α phase in association with stress-induced martensite (SIM) in the β phase was observed for the first time in a two-phase titanium alloy. This behavior is explained in terms of a number of factors including elastic interaction stresses between the α and β phases, coherency between the α phase and hexagonal martensite, and β phase stability.     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium.     

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium. Formerly a Graduate Student at Carnegie-Mellon University     

Mechanical Properties of Mg-Gd and Mg-Y Solid Solutions

The mechanical properties of Mg-Gd and Mg-Y solid solutions have been studied under uniaxial tension and compression between 4 K and 298 K (?269 °C and 25 °C). The results reveal that Mg-Gd alloys exhibit higher strength and ductility under tension and compression attributed to the more effective solid solution strengthening and grain-boundary strengthening effects. Profuse twinning has been observed under compression, resulting in a material texture with strong dominance of basal component parallel to compression axis. Under tension, twining is less active and the texture evolution is controlled mostly by slip. The alloys exhibit pronounced yield stress asymmetry and significantly different work-hardening behavior under tension and compression. Increasing of Gd and/or Y concentration leads to the reduction of the tension–compression asymmetry due to the weakening of the recrystallization texture and more balanced twinning and slip activity during plastic deformation. The results suggest that under compression of Mg-Y alloys slip is more active than twinning in comparison to Mg-Gd alloys.     

Deformation in wrought Mg−9Wt Pct Y

Considerable tensile-compressive yield strength anisotropy is normally associated with textured wrought magnesium alloys.¹ The ease of {1012} twinning is responsible for the lower compressive yield strengths of these materials. In Mg-9 wt pct Y, however, approximately equal tensile and compressive yield strengths of about 50 ksi have been previously reported.² This investigation was performed to study the deformation of wrought Mg-9 wt pct Y with the purpose of understanding its unusual isotropic behavior. It was found that almost no {1012} twinning occurs in this alloy, thus accounting for the absence of anisotropy. Initial plastic deformation both in tension and compression occurs almost entirely by slip, primarily on the basal plane. Subsequent deformation occurs by a combination of slip and {1121} twinning with short {1012} twins appearing only occasionally.     

Analysis of the Deformation Behavior of Magnesium-Rare Earth Alloys Mg-2 pct Mn-1 pct Rare Earth and Mg-5 pct Y-4 pct Rare Earth by In Situ Energy-Dispersive X-ray Synchrotron Diffraction and Elasto-Plastic Self-Consistent Modeling

The deformation behavior of the Mg-RE alloys ME21 and WE54 was investigated. Although both alloys contain rare earth elements, which alter and weaken the texture, the flow curves of the alloys deviate significantly, especially in uniaxial compression test. Apart from the higher strength of the WE54 alloy, the compression flow curve does not exhibit the typical sigmoidal shape, which is associated with tension twinning. However, optical microscopy, X-ray texture measurements, and EBSD analysis reveal the activity of tension twinning. The combination of in situ energy-dispersive X-ray synchrotron diffraction and EPSC modeling was used to analyze these differences. The investigation reveals that twin propagation is decelerated in the WE54 alloy, which requires a change of the twinning scheme from the ‘finite initial fraction’ to the ‘continuity’ assumption. Furthermore, an enhanced activity of the 〈c+a〉 pyramidal slip system was observed in case of the WE54 alloy.     

The Effect of Nd on the Tension and Compression Deformation Behavior of Extruded Mg-1Mn (wt pct) at Temperatures Between 298 K and 523 K (25 °C and 250 °C)

The tension and compression deformation behavior of extruded magnesium-1 wt pct manganese alloys with nominally 0.3 wt pct (MN10) and 1 wt pct neodymium (MN11) was studied over the temperature range of 298 K to 523 K (25 °C to 250 °C). Nd additions to Mg alloys tend to reduce the strong basal texture exhibited by conventional wrought Mg alloys and this work was intended to study the effect of Nd on the deformation behavior of Mg alloys. In situ tensile and compressive experiments were performed using a scanning electron microscopy, and electron backscatter diffraction was performed both before and after the deformation. A slip trace analysis technique was used to identify the distribution of the deformation systems as a function of strain, and based on this analysis and the texture of the undeformed samples, the critical resolved shear stress ratios between the deformation systems were estimated. In the case of MN11, the deformation behavior under tension at all temperatures was dominated by slip, while in compression, extension twinning was the major deformation mode. In tension at 323 K (50 °C), extension twinning, basal, prismatic 〈a〉, and pyramidal 〈c + a〉 slip were active in MN11. Much less extension twinning was observed at 423 K (150 °C), while basal slip and prismatic 〈a〉 slip were dominant and presented similar relative activities. At 523 K (250 °C), twinning was not observed, and basal slip controlled the deformation. With the reduction of Nd content, less slip deformation and more twinning were observed during the tensile deformation. However, like for MN11, the extent of twinning in MN10 decreased with increasing temperature and basal slip was the primary deformation mode at elevated temperatures. Extension twinning was the major deformation mode under compression for all test temperatures in MN10 and MN11. The tensile strength decreased with increasing temperature for both alloys, where MN10 was slightly stronger than MN11 at 323 K (50 °C), which was expected to be a result of the stronger basal texture exhibited by MN10 due to its lower Nd content. However, MN11 maintained its strength more at elevated temperatures compared with MN10, and this was explained to be a result of the greater Nd content.     

Substitutional alloying and deformation modes in high chromium ferritic alloys

The effect of microalloying additions of between 0.05 and 2 wt pct Ni, Ru, Nb, and Ti on the plastic deformation of alloys based on Fe-40 wt pct Cr has been studied. The dislocation sub-structures in the deformed and recovered conditions have been characterized for a series of isothermal annealing cycles. Unalloyed Fe-40Cr deforms at room temperature by mixed twinning and slip. Solute additions modify the operative dislocation configuration. The addition of Ni enhances the propensity for mechanical twinning and raises the cleavage resistance in accordance with the solid-solution softening phenomenon. Ruthenium advances a greater degree of cellularity and promotes restoration of the wrought alloy by recovery. Niobium and titanium act primarily as stabilizing agents for the residual interstitial elements. The formation of a recovered substructure favors the long-range tensile ductility. Enhanced toughness is, however, associated with a reduced degree of cellularity and restricted cross slip. I.M. WOLFF, formerly with the Department of Materials Engineering, University of Cape Town.     

Texture of the Mg-0.49% Al-0.47% Ca alloy after equal-channel angular pressing

Equal-channel angular pressing (ECAP) is used to refine grains and to change the texture of the initial pressed Mg-0.49% Al-0.47% Ca alloy rod in order to study the possibility of increasing the low-temperature ductility of the alloy. ECAP is performed at 300°C in six passes at a total true logarithmic strain ε = 6.8 according to route B _C . As a result, an ultrafine-grained structure with a grain size of 2–5 μm forms. The initial texture of the pressed rod is characterized by the [12 11] axial orientation parallel to the pressing direction. After ECAP, the texture changes its type and is characterized by a set of preferred orientations that represent basal planes located at an angle of 40°–50° with respect to the pressing direction. An analysis of the generalized Schmid factors, which were calculated for the main operating deformation systems with allowance for the critical shear stresses in them and the volume fractions of the preferred orientations, indicates that the texture caused by ECAP affects the decrease in the strength properties of the alloy measured at room temperature and the increase in the low-temperature ductility of the alloy.     

A taylor model based description of the proof stress of magnesium AZ31 during hot working

A series of hot-compression tests and Taylor-model simulations were carried out with the intention of developing a simple expression for the proof stress of magnesium alloy AZ31 during hot working. A crude approximation of wrought textures as a mixture of a single ideal texture component and a random background was employed. The shears carried by each deformation system were calculated using a full-constraint Taylor model for a selection of ideal orientations as well as for random textures. These shears, in combination with the measured proof stresses, were employed to estimate the critical resolved shear stresses for basal slip, prismatic slip, 〈c+a〉 second-order pyramidal slip, and { } twinning. The model thus established provides a semianalytical estimation of the proof stress (a one-off Taylor simulation is required) and also indicates whether or not twinning is expected. The approach is valid for temperatures between ∼150 °C and ∼450 °C, depending on the texture, strain rate, and strain path.     

Mechanisms of phase and structural transformations and texture formation in titanium alloy sheet semiproducts

Inverse pole figures are used to analyze texture formation in sheet semiproducts made from titanium alloys of various classes. The role of the mechanisms of phase and structural transformations in texture formation has been revealed. A VT6 alloy is used as an example to study the texture formation during hot, warm, and cold rolling of high-alloy α + β titanium alloys. Hot rolling followed by cooling is shown to form the β → α transformation texture. Warm or cold rolling of α + β titanium alloys leads to the (0001)〈hkio〉 deformation texture of the α phase, and subsequent annealing in the two-phase field results in the transformation texture with a strictly determined orientation distribution of α-phase crystallites due to the shear mechanism of α-phase nucleation. Specifically, the 〈10\(\overline 1 \)0〉 orientation is parallel to the rolling direction, the [0001] direction is parallel to the transverse direction, and the {11\(\overline 2 \)0} planes are parallel to the rolling plane. Thermohydrogen treatment in combination with rolling is shown to form nano-and submicrocrystalline structures with a virtually textureless α phase in VT6 sheet semiproducts. As a result, the anisotropy of the mechanical properties of the sheet semiproducts produced by this new combined hydrogen technology decreases sixfold.     

Role of Solute in the Texture Modification During Hot Deformation of Mg-Rare Earth Alloys

Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing, the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer alloys (≥0.17 at. pct) show the weakening effect, whereas the most dilute alloy (≤0.03 at. pct) does not. Electron backscattered diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation (GND) content in dilute, hot-rolled alloys contain primarily basal 〈a〉 dislocations. At higher concentrations, the dislocations are predominantly prismatic 〈a〉 type. This change in the GND content suggests a change in dynamic recrystallization (DRX) mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering. Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening phenomenon in Mg rare earth alloys.     

The Microstructural, Textural, and Mechanical Properties of Extruded and Equal Channel Angularly Pressed Mg-Li-Zn Alloys

The microstructural and textural evolution of the Mg-6Li-1Zn (LZ61), Mg-8Li-1Zn (LZ81), and Mg-12Li-1Zn (LZ121) alloys were investigated in the as-extruded condition and after being equal channel angularly pressed (ECAPed) for one, two, and four passes. The shear punch testing technique was employed to evaluate the room-temperature mechanical properties of the extruded and ECAPed materials. Microstructural analysis revealed that the grain refinement in both LZ61 and LZ121 alloys could be achieved after multipass ECAP through the continuous dynamic recovery and recrystallization process. For the LZ81 alloy, however, the occurrence of Li loss in the four passes of ECAP condition partly offsets the grain refining effect of the ECAP process by increasing grain size and volume fraction of the α phase. Textural studies in both LZ61 and LZ81 alloys indicated that the developed fiber texture after extrusion could be replaced by a typical ECAP texture, where the basal planes are mainly inclined about 45 deg to the extrusion axis. The increased volume fraction of the β phase in LZ81 significantly affected the α-phase texture by decreasing the intensity of the maximum orientations of the basal and prismatic planes in all deformation conditions, compared with the LZ61 alloy. It was also observed that the abnormal grain growth might be promoted by the strong texture developed in the extruded LZ121 alloy. This texture became more randomized when the number of ECAP passes increased. The SPT results showed that the shear yield stress, ultimate shear strength and normalized displacement in all studied alloys were improved through the grain refinement strengthening caused by ECAP. It was also established that increasing Li content decreased the shear strength and enhanced the shear elongation in all deformation conditions.     

热变形及热处理过程中TC17钛合金组织与取向的关联性

为了进一步研究热压缩及热处理过程对组织及取向变化的关联性, 通过对TC17进行热压缩变形及后续热处理, 利用光学显微镜和背散射电子衍射等分析方法, 结合晶粒尺寸、织构分布图、极图以及反极图, 研究变形后及热处理后的TC17的组织结构、晶粒尺寸的变化和取向的演变规律以及两者之间的关联性.结果表明: 随着变形温度升高, 初生α相含量大幅减小, 尺寸减小, 大部分α相晶粒分散分布, 且位于高温β相晶粒的三叉晶界上; 热处理后, α相和β相组织特征清晰, 界限明显, 初生α相依旧存在, 且趋于等轴化, 亚稳定β相发生转变, 形成片层状β转变组织; 热变形使α相织构极密度值减小, 且随之温度增加, α相织构极密度值也变小; 热变形后的α相已不存在明显的强织构, 热变形对α相晶粒的取向影响较大, 很明显的改善了其取向的均匀性; 热变形同样使β相织构极密度值减小, 但效果不明显.β相仍存在取向集中现象, 取向均匀性相对较差.      

Deformation of metastable betaTi-15Mo-5Zr alloy single crystals

Deformation modes have been investigated in metastable beta Ti-15Mo-5Zr alloy single crystals using both transmission electron microscopy techniques and multisurface trace analysis. {332} twinning and 〈111〉 crystallographic slip were observed to occur at an initial stage of deformation depending on deformation axis. {332} twinning occurs in a crystal whose tensile axis lies around 〈111〉, while 〈111〉 slip appears in a crystal having the tensile axis in the neighborhood of 〈001〉 to 〈011〉. The twinning system which possesses the maximum resolved shear stress is always operative in both tensile and compressive deformations. Single crystals of this alloy exhibit an asymmetry of the active slip plane and of the yield stress in a manner similar to other bcc metals and dilute alloys.     

Composition,Structure, and Properties of Sintered Silicon-Containing Titanium Alloys

The main factors limiting the application of high-temperature creep-rupture resistant titanium alloys synthesized from powder components by pressing and subsequent vacuum sintering for the manufacture of parts for gas turbine engines are analyzed. The method for synthesizing the VT1-0 alloy and an alloy whose chemical composition corresponds to the high-temperature creep-rupture resistant VT8 alloy is described. Their chemical and phase composition, strength, hardness, and distribution of doping elements are examined. Upon analysis of the composition, structure, and properties of the samples produced from the test alloys synthesized from PT5 titanium powders with different particle sizes by powder metallurgy methods, it was concluded that semi-finished products could be produced from the VT1-0 and VT8 titanium alloys. The effect of the particle size of the titanium matrix on the chemical composition of the synthesized alloys is studied. The chemical composition of the test alloy complies with the industry standard for semi-finished products of hightemperature creep-rupture resistant titanium alloys. The influence of the particle-size distribution of titanium powder on the strength, hardness, and residual porosity of the synthesized alloys is established. Regardless of the particle size of the powder mixture matrix (ranging from 40 to 400 μm), the strength, ductility, and hardness of the test VT8 alloy do not comply with the requirements of standards OST 90002–70 and OST 90006–70, which govern these properties for bars and blanks of gas turbine engine blades. It is concluded that a series of measures are required to eliminate the residual porosity and impart the blade structure to the material to improve the strength properties.     

Correlation of tensile properties, deformation modes, and

The effect of plastic deformation mode on tensile properties of quenched commercial β-phase titanium alloys has been investigated at approximately constant grain size and oxygen content. In addition, stability of β-phase has been estimated from ω-reflections or diffuse streaking in electron diffraction patterns in a manner similar to the previous works on binary β-phase titanium alloys. Dominant mode of plastic deformation is 332 〈113〉 twinning in the alloys with large instability of β-phase, such as Ti-ll.5Mo-6Zr-4.5Sn and Ti-15Mo-5Zr, and is crystallographic slip in the alloys with small instability of β-phase, such as Ti-15Mo-5Zr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-15V-3Cr-3Al-3Sn, Ti-8Mo-8V-2Fe-3Al, and Ti-13V-11Cr-3Al. Twinning leads to low yield strength and large elongation, while slip results in high yield strength and small elongation in agreement with binary and ternary β -phase titanium alloys.     

Microstructural effects on the tensile properties and deformation behavior of a Ti-48Al gamma titanium aluminide

The effects of microstructure on the tensile properties and deformation behavior of a binary Ti-48Al gamma titanium aluminide were studied. Tensile-mechanical properties of samples with microstructures ranging from near γ to duplex to fine grained, near- and fully-lamellar were determined at a range of temperatures, and the deformation structures in these characterized by transmission electron microscopy (TEM). Microstructure was observed to exert a strong influence on the tensile properties, with the grain size and lamellar volume fraction playing connected, but complex, roles. Acoustic emission response monitored during the tensile test revealed spikes whose amplitude and frequency increased with an increase in the volume fraction of lamellar grains in the microstructure. Analysis of failed samples suggested that microcracking was the main factor responsible for the spikes, with twinning providing a minor contribution in the near-lamellar materials. The most important factor that controls ductility of these alloys is grain size. The ductility, yield stress, and work-hardening rate of the binary Ti-48Al alloy exhibit maximum values between 0.50 and 0.60 volume fraction of the lamellar constituent. The high work-hardening rate, which is associated with the low mobility of dislocations, is the likely cause of low ductility of these alloys. In the near-γ and duplex structures, slip by motion of 1/2<110] unit dislocations and twinning are the prevalent deformation modes at room temperature (RT), whereas twinning is more common in the near- and fully-lamellar structures. The occurrence of twinning is largely dictated by the Schmid factor. The 1/2<110] unit dislocations are prevalent even for grain orientations for which the Schmid factor is higher for <101] superdislocations, though the latter are observed in favorably oriented grains. The activity of both of these systems is responsible for the higher ductility at ambient temperatures compared with Al-rich single-phase γ alloys. A higher twin density is observed in lamellar grains, but their propagation depends on the orientation and geometry of the individual γ lamellae. The increase in ductility at high temperatures correlates with increased activity of 1/2<110] dislocations (including their climb motion) and twin thickening. The role of microstructural variables on strength, ductility, and fracture are discussed. 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.