Solid solution strengthening in Nb-Ta and Nb-Mo alloy single crystals

The strength of Nb, Nb-Ta, and Nb-Mo alloys was investigated in tension and compression as a function of composition, temperature, and strain rate. The room-temperature properties show molybdenum to have a significant effect on the strength of niobium while the effects of tantalum are nearly zero. The room-temperature strength is shown to be well correlated with the atomic misfit parameter. Analysis of the low-temperature behavior of Nb-Ta alloys indicates that they behave similarly to the pure metal while Nb-Mo alloys show a marked deviation from this behavior. The strengthening phenomena occurring in the Nb-Mo alloys at low temperature is interpreted as being an impurity interaction. A transition between the mechanism operating in the pure metal and the impurity interaction occurs at low solute contents. Formerly Graduate Student, Michigan Technological University, Houghton, Mich.     

Low temperature strengthening in niobium-hydrogen single crystals

The mechanical behavior of high purity niobium and niobium-hydrogen single crystals at temperatures in the range 77° to 380°K was investigated. Thermally activated hardening by hydrogen in solution is appreciable at temperatures ≲150°K. However, hydride precipitation occurs at these temperatures, can be strain-induced during mechanical testing, and accounts for the largest part of the decrease in ductility in alloys with up to 720 at. ppm of hydrogen. The influence of hydrogen additions and the effects of purification on the low temperature flow stress of niobium demonstrate the large influence of interstitial solutes on the strength of bcc materials at low temperatures. Formerly Graduate Assistant, Division of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, Ohio     

Modeling solid solution strengthening in nickel alloys

The yield stress of multicomponent nickel solid solution alloys has not been modeled in the past with respect to the effects of composition and temperature. There have been investigations of the effect on the yield stress of solutes in binary systems at a fixed temperature, but the effects on the yield stress of multiple solute elements and temperature changes have not been investigated. In this article, two different forms of the trough model are considered for nickel-base alloys to determine the most applicable model for solid solution strengthening in the system. The yield stresses of three binary nickel-chromium and three ternary nickel alloys were determined at a range of temperatures. The yield stress of the alloys was then modeled using the Feltham equation. The constants determined in fitting the Feltham equation to the experimental data were then applied to other experimental solid solution alloys and also to published information on commercial solid solution nickel alloys. It was found that the yield stress of the nickel solid solution alloys could be modeled successfully using the Feltham equation.     

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 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.     

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 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.     

The temperature independent plateau stress of solid solution crystals

A calculation of the plateau stress in solid solution crystals is presented assuming an arbitrarily oriented dislocation loop of lengthL, that moves under an applied stress. At high concentrations of solute atoms the dislocation segment does not interact with an individual solute atom but instead with all the solute atoms along the dislocation segment within a certain radius. The macroscopic flow stress is assumed to be determined by the maximum force that is encountered when a dislocation is moved over a distance equal to the distance between the position at zero stress and the critical position of an activated Frank-Read source. If the dislocation segment is assumed to be large compared to atomic distances, the interaction with groups of atoms will lead to an athermal process and therefore can explain the origin of the temperature independent flow stress in solid solution crystals. From this model the flow stress can be calculated with the help of statistical methods similar to those used in calculations of the movement of Bloch walls in magnetic materials. Besides the proper temperature dependence of the plateau stress the above model yields a dependence of the plateau stress upon the square root of the solute concentration, a result that is in good agreement with the measurements on silver, gold, and copperbased alloys. A linear relation between the solid solution hardening parameter dT/d√c and the strength of the solute atoms is obtained which is confirmed by the experimental results on copper-based alloys.     

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.     

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.     

On the additivity of precipitation and solid solution hardening in under-and over-aged single crystals of (CuAu)-Co

The superposition of precipitation (τ_p) and solid solution (τ_s) hardening has been experimentally investigated for the system copper-gold-cobalt. The copper-rich copper-gold solid solution is precipitation hardened by cobalt-rich particles which have a lattice mismatch. Under- as well as over-aged single crystal specimens have been analyzed. It has been attempted to relate the total measured critical resolved shear stress τ_t to τ_p and τ_s by: τ_t^k = τ_p^k + τ_s^k. This is an ad hoc generalization of suggestions published with k = 1.0 and k = 2.0. τ_t and τ_s were measured as functions of the gold-concentration of the matrix, of the particle dispersion and of temperature (90–500 K). k is to be chosen such that τ_p derived with the aid of the above equation from τ_t and τ_s, varies with the particle dispersion as predicted theoretically. The result is: only for under-aged specimens containing no more than 3.2 at.% gold in the matrix, the above equation describes the experimental results satisfactorily, k equals 1.25. For higher gold-concentrations the above equation does not yield an acceptable representation of the data, k = 1.0 and k = 2.0 can be ruled out.     

镍元素对18K金固溶强化的影响

采用氧气乙炔火枪制备了4种镍质量分数分别为0,2.5,5,8%的18K金合金;利用显微硬度仪和X射线衍射仪研究了镍质量分数对18K金固溶强化的影响。其研究结果表明:随着镍质量分数的增加,合金的硬度不断增加;这主要是由于镍质量分数的增加提高了合金的固溶度,加强了固溶强化机制。此外,镍元素对金的固溶强化效果要强于其他补口元素,这也是合金硬度提高的原因。     

Computer simulation of solid solution strengthening in Fcc alloys: Part I. Friedel and mott limits

It has been assumed for a number of years that dilute solid solutions obey a Friedel limit;i.e., the concentration dependence of the yield stress varies as 1/2 power (c ^1/2), whereas in more concentrated solid solutions, the Mott limit is upheld with a predicted concentration dependence ofc ^2/3. These two limits were examined both analytically and numerically. As expected, the Friedel limit can be reproduced without any difficulty. However, in the Mott limit, a detailed analytical treatment of the concentrated solution case resulted in a concentration dependence of the yield stress asc ^1/2. The numerical data are in agreement with this analytical result in the Mott limit. Earlier work which predicted a,c ^2/3 dependence is analyzed, and certain key assumptions are shown to be invalid.     

Computer simulation of solid solution strengthening in Fcc alloys: Part II. At absolute zero temperature

Computer simulation techniques were used to explicate solid solution strengthening in fcc metals. With this approach, it was not necessary to make a number of the assumptions used in analytical investigations. A semicontinuum model, with a flexible dislocation line interacting with a random array of solute atoms, was employed. The dislocation line tension was assumed constant, and classical elastic size or modulus interactions were assumed between the solute atoms and the dislocation line segment. The size effect was again found to be dominant, and the yield stress was predicted as { }=0.069 με^{4/3 1/2}. Although there were no adjustable parameters in this study, agreement with experimental data was quite good.     

The growth of Zirconium single crystals

Abstract

The growth of zirconium single crystals by three different techniques has been studied. For crystals grown by passing samples through a steep temperature gradient in an electron beam furnace, it is shown that there is preferred growth when the 〈111〉 direction in the beta phase which transforms to a 〈12¯10〉 direction in the alpha phase is almost parallel to the longitudinal temperature gradient. Large crystals were also grown by cycling samples through the transformation temperature followed by long annealing just below this temperature and by strain-annealing. For these latter techniques, evidence is presented to show that crystal growth proceeds to reduce the amount of substructure present.

Résumé

On a étudié la croissance de monocristaux de zirconium par trois techniques différentes. Pour des cristaux obtenus en passant des échantillons à travers un fort gradient thermique dans un four à bombardement électronique, on a montré qu'ily a croissance préférentielle lorsque la direction 〈111〉 dans la phase β, qui se transforme en la direction 〈12¯10〉 dans la phase α, est presque paralléle au gradient longitudinal de température. De grands cristaux ont aussi été obtenus par la méthode de l'ecrouissage critique et par la méthode dans laquelle on utilise un cyclage à travers la température de transformation suivi par un long recuit légèrement en dessous de cette température. Pour ces deux derniéres techniques, on a montré que la croissance s'effectue en réduisant la sous-structure dans les grains.