Deformation behavior of HCP Ti-Al alloy single crystals

Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for <a> slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temprature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and <c+a> slip at higher Al concentrations, with a mixture of <c+a> slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that, for <c+a> slip observed in compression loading. 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 and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Deformation behavior of HCP Ti-Al alloy single crystals

Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading. 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 and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Ti-5A1-2Mo-3Zr合金的低温形变

对Ti-523合金由室温到-75℃之间的拉伸性能和形变行为作了系统研究,发现淬火态在-75℃拉伸仍保持高塑性,而经回火的试样则塑性较低。两者的强度在低温下均升高,可比室温强度高10%。形变机理则由室温的滑移为主变为低温下的以孪生为主,交替温度可能在-15℃~-30℃之间。低温形变出现一种双重\     

Interfacial Phase in Ti-5Al-2Mo-3Zr Alloy

Under appropriate cooling condition, intcrfacial phase appeared between α and β phases in Ti-5Al-2M0-3Zr alloy. Its thickness could reach 100 nm. The interfacial phase was either single layer structure or double layer structure. The monolithic layer adjacent to 3 phase was indentified as fee structure with relationship （110）_β// （001）_m, [11$\\mathop 1\\limits^{{\\rm{ - - }}} $]_β /[110]_m. The striated layer adjacent to a phase was indentified as hep with twin relationship {10$\\bar 1$1}<1$\\bar 0$12> to α. Thus, the study provided a new experimental fact for the controversial topic on structure of striated layer. Chemical composition of interfacial phase varied slightly with heat treatments, but always between that of α and β phases.     

Deformation structure ofα-SiC single crystals

Conclusions The deformation of-SiC single crystals in bending with the direction of extension and compression approximately parallel to the basal face (the axis of bending parallel to (0001), T = 2000–2100°C) involves slip over the basal plane; the directions of slip are <11¯20> and <10¯10>. Accordingly, crystallographic planes rotate about parallel axes, <10¯10> and <11¯20>. Slip in the <10¯10> direction may be a consequence of migration of dislocations with Burgers vectors equal to b=2/3 <10¯10> (a full dislocation and <10¯10> (a partial dislocation). X-ray microdiffraction observations show [12] that partial dislocations with Burgers vector components parallel to [101¯1] frequently form in-SiC; travel of partial dislocations apparently plays an important part in the deformation of -SiC. The deformation of-SiC is accompanied by kink band formation [1].During the deformation process, dislocations pile up in slip planes, and their density increases by two to three orders; their distribution is very uneven, typical values of being 10⁸–10¹⁰ cm^–2 and values of _max of 10¹¹cm^–2 being recorded in zones with a large local curvature.Because of the changing specimen structure, two stages may be detected in the deformation process: 1) formation of elastically bent regions and 2) comminution (formation of microcracks within the specimen).The crystals investigated exhibited only micropolygonization, when the block size in the basal plane was 30 and the block disorientation 10. Neither postdeformation annealing for 10–30 h at 2000–2100°C nor prolonged (4–8 h) holding of specimens under load produced any macroscopic polygonization. The structure of naturally deformed specimens, too, was found to contain no polygonization macroblocks. It would appear that, because of the appreciable activation energy for diffusion and strong covalent linkage in-SiC, dislocations cannot readily climb over large (>10) distances in this compound; in this connection, the relaxation properties of-SiC would be expected to be weak.The methods of investigation employed in our work are comparatively simple to use. They do not give any information about elementary deformation acts, but throw a light on the character of lattice variation in various volumes (V ranging from 0.1t to 6.0t mm³, where t=200–500 is the crystal thickness) and are particularly useful in topographic x-ray photography when there is no contrast on individual linear defects in crystals.Translated from Poroshkovaya Metallurgiya, No. 8 (128), pp. 63–72, August, 1973.     

Deformation of beta-brass single crystals under explosive shock

Abstract

Beta-brass single crystals were explosively shock loaded at several temperatures after various thermal treatments. “Twinning” was observed when some disorder occurred. Mechanisms which are thought to explain the absence of “twinning” in ordered crystals are reviewed. It is suggested that beta-brass “twins” under strong impact but that elastic “de twinning” takes place immediately after the shock wave.

Résumé

Des monocristaux de laiton bêta ont été défonnés par chocs explosifs a plusieurs températures après différents traitements thermiques. Des “mac1es” de déformation ont été observées uniquement dans les cristaux au moins partiellement désordonnés. On discute des mécanismes qui peuvent expliquer l'absence de “maclage” dans les cristaux ordonnés. Il semble possible que le laiton bêta se “macle” sous l'influence d'un impact mais cette déformation serait suivie d'un “démaclage” élastique immédiatement après le passage de l'onde de choc.     

Deformation of semi-solid Sn-15 Pct Pb alloy

The rheological behavior of semisolid Sn-15 pct Pb alloy was studied using a parallel-plate viscometer. Small nondendritic and dendritic semisolid samples of the alloy were deformed under a constant load at initial pressures up to 232 kPa (33.6 psi) and at fractions solid from 0.15 to 0.60. Strain-time data for the nondendritic material obey the non-Newtonian, two-parameter, Ostwald-de-Waele, power-law model,i.e. μ = mγ ⁿ⁻¹, where μ is viscosity γ shear rate andm andn are constants. For fractions solid above about 0.30, the following empirical equation relates viscosity, shear rate and fraction solidμ = a exp (bf_s) γ^(cf _s ^+d) 0.3 <f _s < 0.60 wheref _s is fraction solid anda, b, c, d are constants. The nondendritic alloy deformed homogeneously without cracking to very large strains (up to 80 pct). Dendritic alloys required much higher loads and cracked easily. For the nondendritic alloys the forging pressures to obtain 50 pct compression were of the order of 7 to 70 kPa (1 to 10 psi) for fractions solid under 0.55 and 172.5 to 207 kPa (25 to 30 psi) for fraction solid of about 0.60. For the dendritic alloys, the forging pressure required to achieve 10 pct compression is about 85 kPa at a fraction solid of 0.35 and increases rapidly with increasing fraction solid.     

Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700 °C to 1400 °C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200 °C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb₂O₅ and the vaporization of MoO₃. An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900 °C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.     

Kinetics of the phase transformation in a Ti-15Mo-5Zr-3AI alloy as studied by X-ray diffraction

The alloy Ti-15Mo-5Zr-3Al is a metastatale β-alloy. X-ray diffraction data are obtained on the time-temperature-transformation behavior of this alloy as it decomposes into the hexagonal α-phase and the body centered cubic β-phase of a higher Mo concentration. Special X-ray techniques are described to overcome the problems encountered due to rolling and annealing textures as well as grain size. Lattice parameter data from the β-phase permit an approximate phase diagram to be determined.     

Deformation of NiTiCu shape memory single crystals in compression

Single-crystal orientations of NiTi10Cu alloys were studied under incremental, cyclic compression conditions to establish the pseudoelastic and shape memory response of this class of alloys. This material exhibits a two-step transformation involving cubic to orthorhombic martensite (B2 → B19) followed by orthorhombic to monoclinic martensite (B19 → B19′). The transformation parameters (shear magnitudes and directions for habit and twin planes) were determined associated with the B2 → B19 transformation. The growth of monoclinic martensite correspondent variant pairs (CVPs) emanating from the orthorhombic structure was also analyzed. The transformation strain for the B2 → B19 case was orientation dependent and lower than the B19 → B19′ transformation in compression for all orientations except those near the [001] pole. The experimental results show that the critical transformation stress is orientation dependent and is in the range 30 to 58 MPa. Orientations that exhibit lower transformation stress (or high resolved shear stress factors, [100] and [012]) produce higher recoverable strains (as high as 4 pct), while other orientations ([011], [111], and [123]) with lower resolved shear stress factors result in recoverable strains less than 3 pct. At higher strains, inelastic deformation develops, limiting recoverability. The recoverable strains are lower than the theoretical values for two main reasons: the transformation is curtailed first by austenite slip and subsequently by martensite slip, and the orthorhombic structure does not fully transform to the monoclinic martensite.     

Effects of heat treatments on microstructure and mechanical properties of Mg-15Gd-5Y-0.5Zr alloy

Microstructure and mechanical properties of Mg-15wt.%Gd-5 wt.%Y-0.5wt.% Zr alloy were investigated in a series of conditions. The eutectic was dissolved into the matrix and there was no evident grain growth after solntionized at 525 ℃ for 12 h. The evolution of the phase constituents from as-cast to cast-T4 was as follows： α-Mg solid solution＋Mg5（Gd,Y） entectic compound→α-Mg solid solution＋ spheroidized Mg5（Gd, Y） phase→α-Mg supersaturated solid solution＋cuboid-shaped compound （Mg2Y3Gd2）. And the precipitation sequences of Mg-15Gd-5Y-0.5Zr alloy were observed, according to the hardness response to isothermal ageing at 225-300 ℃ for 0-128 h.     

Deformation and fracture of NiAl single crystals tested in torsion

Binary NiAl single crystals were tested in torsion. [001] oriented samples exhibited lower shear strength and higher shear strain to failure compared to [110] oriented samples. Transmission electron microscopy revealed an increasing dislocation density from the center of the sample to the outer surface. Most dislocations that were analyzed were of the b= <100= type, however, some 〈110〉 type dislocations were observed in the [110] oriented samples. Electron backscatter patterns and tilting experiments were used to determine the crystallography of two types of facets on failed samples: those near {112} and those ∼10° away from {001}.     

Deformation of Nb-W single crystals: Athermal solid solution strengthening

The athermal component of the flow stress of single crystals of niobium and four substitutional solid solution strengthened Nb-W alloys (1, 3, 6, and 15 at. pct) has been measured. The magnitude of the athermal stress was obtained by the method utilizing the temperature dependence of the flow stress and by a stress relaxation method. The agreement between these two methods is good. Athermal strengthening is a linear function of concentration over most of the composition range. No current athermal hardening theory is completely consistent with the experimental results. Formerly with the Advanced Materials Research and Development Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.,     

Dislocation mobility in lead and Pb-In alloy single crystals

Dislocation velocities were determined from the length of slip bands produced on the surface of single crystals of lead and Pb-In alloys by the application of stress pulses of a few microseconds duration. Experiments were conducted at temperatures varying between 4.2° and 298°K in both the normal and the superconducting states of the metals. The velocities studied were in the range 10³ to 10⁵ cm per sec. At each temperature studied, the dislocation velocity in lead was found to vary linearly with applied stress in the early stages. At higher stresses the velocity tends to approach some limiting velocity equal to the shear sound velocity in the metal. The damping coefficientsB were calculated from the slopes of the linear portion of the dislocation velocity-stress curves. The value ofB at 77°K was smaller than that at 298‡K. Tests carried out in the normal lead at temperatures between 10° and 30°K showed thatB increases with decreasing temperature. The value ofB at 4.2°K in the superconducting lead was smaller than the values ofB in the normal lead at higher temperatures. Qualitative comparison of these results with existing theories indicates that the dislocation mobility in lead, within the velocity range studied, is controlled by interaction of the moving dislocation with thermal vibrations and the viscosity of the conduction electrons in the lattice. The damping coefficients in the alloy are higher than those in the pure metal. This paper is based upon a thesis submitted by V. R. PARAMESWARAN in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

Deformation induced alpha and epsilon martensites in Fe-Ni-Cr single crystals

The orientation dependence of an applied tensile stress on the formation of specific α martensite variants of the Kurdjumov-Sachs (K-S) orientation relationship in Fe-15Ni-15Cr single crystals are presented. Test temperatures were 185, 242 and 273 K,i.e., aboveM _s. Quantitative volume fraction measurements of deformation induceda ande martensite were made on crystals with the [100]γ, [1•10]γ, [•1•12]γ and [•2•13]γ tensile axes, α-martensite was only observed after 5 pct strain at 185K in all orientations, but the total volume fraction ofa martensite varied from <0.002 to 0.07 for the [100]γ and [•2•13]γ tensile axes respectively. The distribution of the K-S variants was also found to be sensitive to the direction of the applied stress, and 80 pet of the α martensite present in crystals oriented for easy glide had the same K-S variant. Epsilon martensite was found in all specimens but occurred only in the (111) planes which had slipped. Glen Stone, formerly graduate student, University of California at Berkeley     

Deformation and fracture in laser-shocked NiAl single crystals and bicrystals

Oriented single crystals and a [3 4 55]/[5 7 17] random bicrystal were used to study dynamic behavior in NiAl due to laser-driven shocks at moderate pressures (3 to 20 GPa). Disks 5 mm in diameter and 100- to 400-μm thick were tested at the TRIDENT facility at Los Alamos National Laboratory (LANL). Particle velocities were measured using laser velocimetry, which showed that shock-speed variations with orientation in monocrystals were consistent with anisotropic elasticity predictions, whereas the bicrystal showed spatial and temporal variations in the velocity field due to the grain boundary. The shocks displayed strong elastic precursors at the free surface, which agrees with transmission electron microscopy observations of a low dislocation density in 〈100〉 and 〈111〉 monocrystals and in the [5 7 17] grain of the bicrystal. The latter developed a damage zone in the [3 4 55] grain, with cracking and slip present close to the boundary. Orientation-imaging microscopy showed that the boundary produced in-plane misorientation gradients in the bicrystal and that all specimens developed through-thickness lattice rotations, which were more pronounced for the 〈111〉 and 〈110〉 loading axes. High rotations occurred within 20 μm of the shocked surface and decreased toward the bulk, indicating a fast decay of the plastic shock wave, which explains the strong elastic precursors observed.     

Creep of precipitation-hardened nickel-base alloy single crystals at high temperatures

Single crystals of a γ′ precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F. It was found that shear of the γ′ precipitate by pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep. This contrasts with 1400°F creep behavior where shear of γ′ is dominated by α/3 (112) dislocations in primary creep, but by pairs of α/2 (110) dislocations in steadystate creep. The orientation dependence of the steady-state creep rate at 1575°F is explained by the nature of dislocation junction reactions for the different orientations. Crystals along the [001]-[1•11] boundary have the greatest creep resistance because of the formation of stable dislocation networks at the matrix-particle (γ⊃ interfaces, whereas the lower creep resistance of crystals oriented along the [001]-[0•11] boundary is a consequence of the low probability for the formation of stable junction reactions. Finally, evidence, in the form of resolvable α/2 (110) dislocation pairs within the γ′ precipitate, is presented for a reduction in the local antiphase boundary energy of γ′ at high temperatures.     

Deformation structure and nucleation of dynamic recrystallization in copper single crystals

The deformation structure of [189] oriented copper single crystals was investigated at incipient dynamic recrystallization for different deformation temperatures (T > 0.5 T_m and strain rates. For this purpose the dislocation arrangement was studied, and the cell size distribution was measured. The deformation structure contained kink bands as deformation inhomogeneities at all temperatures. With increasing temperature, the dislocation structure changed its nature from a typical cell structure with a tangled dislocation arrangement in the cell boundaries to a typical subgrain structure, where the subgrain boundaries had the character of small angle grain boundaries. In the investigated range of strain rates the structure transition occurred in a narrow temperature interval around T_t = 0.75 T_m. In the kink bands the dislocation structure was more recovered than in the matrix and the structure transition proceeded at a lower temperature. The average cell diameter was found to vary approximately reciprocally with the flow stress. The cell size distribution consisted of two components, which could be associated with matrix and kink band. The distribution of the matrix shifted continuously to larger cell sizes with increasing deformation temperatures. In the kink bands a stronger increase in subgrain size as well as the occurrence of very large subgrains was recognized for T >T₀ resulting in a pronounced broadening of the distribution. It is concluded that these subgrains grow discontinuously and thus, lead to dynamic recrystallization, when exceeding a critical size. At medium high temperatures the recrystallized structure was dominated by twin chains originating from the scatter of the deformation texture. The twinning of subboundaries is proposed as the nucleation mechanism of dynamic recrystallization in this temperature range.