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Interstitial oxygen embrittles titanium, particularly at cryogenic temperatures, which necessitates a stringent control of oxygen content in fabricating titanium and its alloys. Here, we propose a structural strategy, via grain refinement, to alleviate this problem. Compared to a coarse-grained counterpart that is extremely brittle at 77 K, the uniform elongation of an ultrafine-grained (UFG) microstructure (grain size ~ 2.0 µm) in Ti-0.3wt.%O is successfully increased by an order of magnitude, maintaining an ultrahigh yield strength inherent to the UFG microstructure. This unique strength-ductility synergy in UFG Ti-0.3wt.%O is achieved via the combined effects of diluted grain boundary segregation of oxygen that helps to improve the grain boundary cohesive energy and enhanced dislocation activities that contribute to the excellent strain hardening ability. The present strategy will not only boost the potential applications of high strength Ti-O alloys at low temperatures, but can also be applied to other alloy systems, where interstitial solution hardening results into an undesirable loss of ductility., 結晶粒超微細化により、酸素に起因したチタンの低温脆性を克服 --悪者とされてきた不純物酸素の有効利用に期待--. 京都大学プレスリリース. 2023-02-01.

Individually, increasing the concentration of either oxygen or aluminum has a deleterious effect on the ductility of titanium alloys. For example, extremely small amounts of interstitial oxygen can severely deteriorate the tensile ductility of titanium, particularly at cryogenic temperatures. Likewise, substitutional aluminum will decrease the ductility of titanium at low-oxygen concentrations. Here, we demonstrate that, counter-intuitively, significant additions of both Al and O substantially improves both strength and ductility, with a 6-fold increase in ductility for a Ti-6Al-0.3 O alloy as compared to a Ti-0.3 O alloy. The Al and O solutes act together to increase and sustain a high strain-hardening rate by modifying the planar slip that predominates into a delocalized, three-dimensional dislocation pattern. The mechanism can be attributed to decreasing stacking fault energy by Al, modification of the “shuffle” mechanism of oxygen-dislocation interaction by the repulsive Al-O interaction in Ti, and micro-segregation of Al and O by the same cause., Individual addition of O or Al is known to deteriorate ductility of Ti alloys, especially at cryogenic temperatures. Here the authors demonstrate a counter-intuitive effect where significant addition of both Al and O substantially improves both strength and ductility in a Ti-6Al-0.3 O ternary alloy.

Using ex-situ transmission electron microscopy and the recently developed nanoprobe diffraction (NPD) technique, we characterize a nanoindented solution treated gum metal. Lattice rotations are resolved at a 1.2 nm length-scale and shown to be continuous within the nanoindentation pit; further, it is shown that these can be accommodated by a reasonable number of geometrically necessary dislocations at a density of ∼1015/m2. We additionally provide direct evidence that dislocations within the nanoindent, rather than secondary phase nanoparticles, can serve as potent barriers to dislocation motion. We also demonstrate that plasticity in these alloys under nanoindentation can be accommodated solely by dislocation nucleation and propagation, with no competing deformation mechanisms present. Conventional transmission electron microscopy and “g•b” analysis reveal the presence of dislocations on 〈 1 ¯ 11 〉 {110} slip systems and highly localized plastic deformation in the form of shear bands on { 1 ¯ 1 ¯ 2 } slip systems, similar to previously observed “giant faults”.

Fe-12Mn steel has the unique and interesting property that, when quenched to α′ martensite from the austenite phase, it forms an ultrafine grained microstructure that has exceptional resistance to cleavage fracture at cryogenic temperatures. The present research was undertaken to complete the characterization of this microstructure and understand why it forms and why it has such exceptional crack resistance. A combination of EBSD and TEM analysis shows that the microstructure is a dislocated lath martensite in which the laths have the Kurdjumov-Sachs relation to the parent austenite. As in other dislocated lath martensites, the prior austenite grains are divided into packets, each of which contains the 6 (of 24) KS variants that mate with the same { 111 } γ plane. Uniquely, however, the packets are stacks of thin plates that contain all 6 KS variants. The variants within the plate are organized into 3 pair of twinned KS variants that are elongated along their { 112 } α twin planes, rotated 120° from one another, and interwoven to form the 6-variant plate. The ultrafine grains are the laths themselves; twin boundaries between KS variants are known to provide strong barriers to cleavage crack propagation. This unusual microstructure is apparently due to the transformation path; austenite transforms to the hexagonal e martensite phase before its ultimate transformation to α′, and the 6-variant plate is the preferred element to minimize elastic energy in a microstructure created by a dominant γ → e → α ′ transformation path.

Austenite grains that have undergone a martensitic transformation are typically composed of 24 variants that can be categorized by their Bain axis of transformation. There are 3 〈001〉 axes for Bain transformations, therefore the (001) pole figure of a prior austenite grain displays 3-fold symmetry. However, we observed superficially similar prior austenite grains containing 6-fold symmetry in the (001) pole figure. This paper introduces evidence of this 6-fold symmetry and explores the crystallographic origins.

Recent experimental studies of the microstructure of dislocated martensite in low-carbon steel have shown that its superficially complex microstructure is, in fact, a simple hierarchical pattern whose basic element is a composite block containing two Kurdjumov–Sachs variants of the α′ phase that share the same Bain axis. The blocks are plates with interfaces generally parallel to {0 1 1} α planes that can be stacked into four crystallographically distinguishable packets. When the prior austenite grain contains all four packets its transformation can be a simple dilatation that involves no net shear. In the present paper we use the crystallographic theory of dislocated martensite to show that the stackable composite martensite plate is the simplest transformed plate that can be constructed that preserves a close-packed plane ({0 1 1} α ||{1 1 1} γ ) and has an invariant {1 1 1} γ habit plane, as the experimental results require. The 12 distinct variants of this plate can be configured into a microstructure that duplicates the patterns and pole figures found experimentally. The results suggest that there is no single-variant martensite plate that can be stacked in this way without substantial strain, though the theory does predict the single-variant plates with {5 5 7} α habit planes and Greninger–Troiano orientation relations that are found in alloys with large fractions of retained austenite.

Many of the most useful structural steels have dislocated lath martensitic structures. The microstructures of these steels are complex since each prior austenite grain contains as many as 24 different crystallographic variants of the γ(fcc)–α′(bcc) transformation. Recent research, using electron backscatter diffraction (EBSD), has significantly clarified the “block-and-packet” structure of lath martensite in low-carbon steel. The blocks are bivariant composites of two transformation variants, with the three distinct blocks stacked so that all six of the possible variants of the packet are used. The present work was undertaken to complete the description of this structure and identify its underlying causes. We address these issues in two steps. First, we present an EBSD characterization of lath martensite in low-carbon 9Ni steel. The results show that all packets have the bivariant block structure, and, with the proper notation, the full hierarchical structure has a simple pattern that is easily described and visualized. The results are readily explained on the basis of two assumptions: (i) the bivariant block, in contrast to a single-variant block, has an α′–γ invariant plane very near {0 1 1} α″ ||{1 1 1} γ , which permits the plate-shaped blocks to stack without significant strain to form a packet; (ii) the transformation goes to completion, with the consequence that the net strain in the prior austenite grain is a simple dilatation, and polygranular bodies can transform martensitically without significant residual stress. In a companion paper we use an appropriate modification of the crystallographic theory of lath martensite to validate the first of these assumptions.

The operation of 〈c + a〉 slip is explored using in situ mechanical testing in a transmission electron microscope by compressing a single-crystal Ti alloy oriented along the [0 0 0 1] direction. In this direction 〈c + a〉 slip is the preferred slip mode. We observed the operation of a 〈c + a〉 dislocation source while simultaneously measuring the mechanical response. Our results show that 〈c + a〉 dislocations could operate as single-arm sources. A model for the source mechanism of 〈c + a〉 slip is proposed based on the experimental observations.

“Gum Metal” refers to β-Ti alloys that achieve exceptional elastic elongation and, with a specific alloy composition, appear to deform via a dislocation-free mechanism involving elastic instability at the limit of strength. This paper describes the current status of research on its strength, deformation mechanism and the possible role of stress-induced martensite. The theoretical basis for deformation at ideal strength is presented. The relevant experimental data is then discussed, including ex situ nanoindentation behavior and in situ pillar compression observed by transmission electron microscopy.

The inherent brittle mode in dislocated lath martensitic steel is cleavage on {100} planes in the microstructure. The transition to {100} cleavage fracture on cooling determines the minimum value of the ductiule-brittle transition temperature. A half-century of research on the microstructure and toughness of lath martensitic steels has produced a semi-quantitative understanding of the brittle transition to cleavage. The results identify the crystallographic “block” of lath martensite as the effective grain size that controls cleavage, and clarify why the internal structure of a block has the microstructure it adopts. The ductile-brittle transition temperature is strongly affected by the block size. Several effective metallurgical processes are now available to refine the block size without excessive strengthening, leading to martensitic structural steels that combine high strength with good low-temperature toughness.

The name “Gum Metal” has been given to a set of β-Ti alloys that achieve exceptional elastic elongation and, with appropriate preparation, appear to deform by a dislocation-free mechanism triggered by elastic instability at the limit of strength. We have studied the compressive deformation of these materials with in situ nanocompression in a quantitative stage in a transmission electron microscope. The samples studied are cylindrical nanopillars 80–250 nm in diameter. The deformation pattern is monitored in real time using bright-field microscopy, dark-field microscopy or electron diffraction. Interesting results include the following: (i) nanopillars approach, and in several examples appear to reach, ideal strength; (ii) in contrast to conventional crystalline materials, there is no substantial “size effect” in pillar strength; (iii) the deformation mode is fine-scale with respect to the sample dimension, even in pillars of 100 nm size; (iv) shear bands (“giant faults”) do form in some tests, but only after yield and plastic deformation; and (v) a martensitic transformation to the base-centered orthorhombic α′′ phase is sometimes observed, but is an incidental feature of the deformation rather than a significant cause of it.

The present paper addresses several connected issues that concern the mechanical properties of ultra-fine grained martensitic steels. Recent research, particularly including EBSD studies, has clarified the complex microstructure of dislocated martensitic steels and shown the central importance of martensite blocks, which are subvolumes of laths that share a Bain variant of the parent austenite. The block-and-packet structure of the martensite appears well-designed to minimize the elastic energy introduced during the martensitic transformation. The martensite block is, ordinarily, the effective grain size for both strength and cleavage fracture. However, the role of the block in imparting strength is sensitive to carbon contamination of the block boundaries. To optimize strength carbon should be present; to minimize the ductile-brittle transition temperature it should be eliminated. When fine grain size produces high strength, it also causes low elongation. The elongation can be improved by including mechanisms, such as TRIP, that lower the initial work hardening rate.

Intercritically tempered (5–9)Ni steels are toughened by the introduction of islands of precipitated austenite along the boundaries of dislocated martensite laths. The interlath islands of precipitated austenite transform to martensite under the stress field ahead of a cleavage crack. Since the fresh martensite is a different crystallographic variant than that of the surrounding packets, it efficiently refines the effective grain size for transgranular cleavage. A straightforward calculation shows that this behavior is dictated by the need to minimize the elastic strain energy of the fresh martensite.

Tension levelling is a process used in the steel industry to remove any shape defects (i.e. out of flatness) present in coiled material. It is generally the final process before the cold rolled product is dispatched to the customer, and is therefore a process which plays an important role in delivering the desired material properties and the product standards required by the customer. In this paper a designed factorial analysis has been employed to study the effect of tension levelling process parameters on the shape characteristics of the levelled product. These characteristics include residual flatness (longitudinal curvature, referred to as longbow), centreline elongation and residual (or internal) stress imbalances; criteria that determine whether the customer accepts the material or not. It has been identified that in a basic five roll leveller the final (adjustable) roll wrap angle has the most significant influence on these characteristics in over 70% of cases. It has also been recognised t...

The effective grain size of a martensitic steel is the coherence length of structure domains that share the relevant crystallographic features. These, in turn, depend on the property of interest. The crystallographic feature that governs transgranular fracture is the {1 0 0} cleavage plane while that governing dislocation plasticity is normally the {1 1 0} slip plane. However, two adjacent sub-volumes that have different orientation relationships do not necessarily mean two effective grains. In this paper, effective grain size is discussed based on 24 Kurdjumov–Sachs and 12 Nishiyama–Wassermann relationships which can be divided into three groups according to their Bain strains. Stereographic projection showed that only those from different Bain groups will have large-angle misorientations between {1 0 0} cleavage planes while dislocation glide planes {1 1 0} usually do not exhibit large-angle misorientation. Therefore, effective grain sizes for transgranular fracture and plastic deformation are not the same, and do not necessarily respond in the same way to certain grain refinement measures.

In situ nanoindentation within a transmission electron microscope is used to investigate the deformation mechanisms in ultrafine-grained Al films. Deformation-induced grain growth resulting from grain boundary migration, grain rotation and grain coalescence is commonly observed as the indentation proceeds. In situ studies of nanograined films suggest that the same mechanisms are operative, though the difficulty of imaging nanosized grains makes the evidence less clear. The results suggest that grain growth and coalescence are important modes of response in the deformation of ultrafine- and nanograined materials.

Aged alloys of Al-0.5Si-0.5Ge (at. pct) contain diamond-cubic-A4 precipitates in a dispersion that is much finer than is found in alloys with Si or Ge alone. To help understand this aging behavior, the present work was undertaken to determine alloy composition as a function of aging temperature. The composition was estimated theoretically using a CALPHAD approach, and measured experimentally with energy-dispersive spectroscopy (EDS) in a high-resolution electron microscope. Theory and experiment are in reasonable agreement. As the aging temperature rises, the precipitates become enriched in Si, changing from 50 at. pct in the low-temperature limit to about 80 at. pct Si as the temperature approaches 433 °C, the high-temperature limit of the precipitate field.

The effect of a superimposed hydrostatic tension or compression on the ideal tensile strength of Fe has been computed ab initio using the Projector Augmented Wave method. Interestingly, a superimposed hydrostatic tension increases the ideal tensile strength while hydrostatic compression decreases it. This result can be explained by a simple crystallographic model.

The Y modification of a two-phase (γ+α2) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method. The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge harmful oxygen. Deformation was found to be mainly provided by 1/2

The ideal strength of a material is the stress at which the lattice itself becomes unstable and, hence, sets a firm upper bound on the mechanical strength the material can have. The present paper includes an ab-initio calculation of the ideal shear strength of Fe. It is, to our knowledge, the first such computation for any ferromagnetic material. The paper also elaborates on our earlier calculation of the ideal tensile strength of Fe by studying the effects of strains which break the tetragonal symmetry. The strengths were calculated using the Projector Augmented Wave Method within the framework of density functional theory and the generalized gradient approximation. In tension the ideal strength is determined by an elastic instability of the ferromagnetic phase along the ''Bain'' strain path from bcc to fcc. An tensile strain also leads to instability with respect to transformation into a face centered orthorhombic structure, and to various magnetic instabilities. However, these are encountered at larger strains and, thus, do not affect the ideal strength. We also investigated the ideal shear strength of bcc iron in two prominent shear systems, {l_brace}112{r_brace} and {l_brace}110{r_brace}. In both shear systems the ideal strength is determined by the body centered tetragonal structure that defines a nearby saddle point on the energy surface. The ideal shear strengths are thus very similar, though they are not identical since the two shears follow slightly different strain paths from bcc to bct. We investigated the magnetic instabilities encountered during {l_brace}112{r_brace} shear. These instabilities do not appear until the strain is significantly greater than the instability strain of the ferromagnetic crystal. Hence while Fe exhibits some novel effects due to magnetism, they do not affect the ideal strength, which is determined by the same elastic instabilities that determine the strengths of most other bcc metals.

The present work addresses the response of Si-Ge precipitates in Al-0.5Si-0.5Ge (at. pct) to thermalcycling treatments of the sort known to refine the shapes of Ge precipitates in binary Al-Ge alloys. Alloys aged at 250 °C contained both small, platelet precipitates on {111} planes and larger, equiaxed precipitates that were heavily twinned. Thermal cycling between 250 °C and 360 °C led to partial or complete dissolution of the platelets. However, the equiaxed particles coarsened at an essentially constant shape; the shape refinement that led to untwinned, single-variant octahedral precipitates in binary Al-Ge did not occur. The apparent reason is the heavy twinning of the Si-Ge precipitates, which produces particles with a nearly spherical shape and rounded, incoherent interfaces and is, hence, a viable mechanism for relaxing the large misfit strain of the precipitate structure. The twinned particles undergo normal coarsening at an essentially constant shape. After thermal cycling, the precipitates contain Si and Ge in the approximate ratio of 70Si-30Ge, which is in the composition range expected for the cycling temperature.

Al−Cu−Si−Ge alloys display a unique combination of ultrarapid aging response, high peak hardness, and extended-aging microstructural stability. The purpose of this work is to explain these properties in terms of the role that the Si−Ge additions have on modifying the conventional Al−Cu aging sequence. In both AlCu and AlCuSiGe, the room-temperature microstructure consists of both Guinier-Preston (GP) zones and ϕ″ precipitates. Upon aging at 190°C, Al−Cu displays the well-known precipitation sequence: the slow dissolution of GP zones and ϕ″ and the gradual formation of ϕ′. In the quaternary alloy, Si−Ge particles quickly nucleate and grow during elevated-temperature aging (they are detected after as little as 30 minutes at 190 °C). The Si−Ge particles then act as nucleation sites for ϕ′ precipitates, resulting in a peak-aged microstructure consisting of a dense distribution of ϕ′ attached to Si−Ge.

Advanced research on structural steels has recently focused on the improvement of properties through the control of grain size. Grain refinement increases strength via the Hall-Petch relation, lowers the ductile-brittle transition by increasing resistance to transgranular cleavage, and reduces hydrogen embrittlement by minimizing interfacial fracture along grain or lath boundaries. However, given their different mechanisms, these properties require slightly different measures of the effective grain size. When the grains are smooth and random, all measures of the effective grain size are roughly equivalent. However, transformations in steel are often crystallographically coherent, producing a martensitic, bainitic or ferritic product that has either a Kurdumov-Sachs (KS) or a Nishiyama-Wasserman (NW) relation to the parent austenite. The 24 KS variants and 12 NW variants divide into three sets of eight, corresponding to the three Bain variants of the fcc→bcc transformation. Grain, packet or block boundaries that separate different Bain variants have signifi-cant misorientations of the {100} cleavage planes, but may have only slight misorientations of the {110} slip planes. It follows that grain refinement through coherent transformation is very effective in improving resistance to cleavage fracture and, if the boundary facets are small, to hydrogen embrittlement, but is often relatively ineffective in increasing strength. For this reason, grain refinement for increased strength is best done with incoherent transformations (such as the strain-induced ferrite transformation) while grain refinement for low-temperature toughness or hydrogen resistance is best done with coherent transformations that refine the effective grain size without overstrengthening to unacceptably low ductility.

Tension levelling is a process used in the steel industry in order to remove any shape defects present in temper rolled strip. It is generally the final process before the cold rolled product is despatched to the customer, and therefore plays an important role in delivering the desired material properties and the product standards required. In this paper a designed factorial analysis has been employed in order to study the effect that tension levelling process parameters have on the shape characteristics of the levelled product. These characteristics include residual flatness (longitudinal curvature or longbow), centreline elongation and residual (or internal) stress imbalances; criteria which determine whether the customer accepts the material or not. It has been identified that in a basic five-roll leveller the final, adjustable roll wrap angle, has the most significant influence on these characteristics in over 70% of cases. It has also been determined that final flatness is dictated by many second-order process interactions.

Using pseudopotential density functional theory within the local-density approximation, we calculate the ideal shear strengths of W for slip on {110}, {112} and {123} planes allowing for complete structural relaxation orthogonal to the applied shear. The strengths in the weak directions on all planes are found to be very nearly equal (about 18GPa, or 11% of the shear modulus G). Moreover, the shear instability occurs at approximately the same applied shear strain (17–18%). This unusual isotropy is explained in terms of the atomic configurations of high-energy saddle points reached during shear. Analysis of these saddle points may also offer a simple explanation for the prevalence of the pencil glide of dislocations on planes containing a (111) direction in bcc metals. Finally, we calculate the ideal cleavage strengths of W on {100} and compare our calculated ideal shear and cleavage strengths with experimental nanoindentation and whisker measurements. All these results can be rather simply unders...

improve shape removal and reduce internal stress remove the incoming defect, is dependent upon the number levels. However, residual stresses and specifically and diameter of bending rolls (workstations) employed. In the imbalance of these stresses post-levelling is an pure stretch levelling no specific bending rolls are used. issue that has failed to be addressed in its entirety. High tensions, therefore, must be generated as a result of To investigate the generation and effects of residual considerable velocity differentials at large diameter bridle stresses owing to tension levelling, a finite element drums. Consequently, this can lead to plastic instabilities (FE) model has been developed with validation

The ideal structural steel combines high strength with excellent fracture toughness. In this paper we consider the limits of strength and toughness from two perspectives. The first perspective is theoretical. It has recently become possible to compute the ideal shear and tensile strengths of defect-free crystals. While the ferromagnetism of bcc Fe makes it a particularly difficult problem, we can estimate its limiting properties from those of similar materials. The expected behavior at the limit of strength contains many familiar features, including cleavage on {100}, slip on multiple planes, "conditionally" brittle behavior at low temperature and a trend away from brittle behavior on alloying with Ni. The behavior of fcc materials at the limit of strength suggests that true cleavage will not happen in austenitic steels. The results predict an ideal cleavage stress near 10.5 GPa, and a shear strength near 6.5 GPa. The second perspective is practical: how to maximize the toughness of high-strength steel. Our discussion here is limited to the subtopic that has been the focus of research in our own group: the use of thermal treatments to inhibit transgranular brittle fracture in lath martensitic steels. The central purpose of the heat treatments described here is grain refinement, and the objective of grain refinement is to limit the crystallographic coherence length for transgranular crack propagation. There are two important sources of transgranular embrittlement: thermal (or, more properly, mechanical) embrittlement at the ductile–brittle transition, and hydrogen embrittlement from improper heat treatment or environmental attack. As we shall discuss, these embrittling mechanisms use different crack paths in lath martensitic steels and, therefore, call for somewhat different remedies.

This paper investigates the conditions of elastic stability that set the upper limits of mechanical strength. Following Gibbs, we determine the conditions that ensure stability against reconfigurations that leave the boundary of the system unchanged. The results hold independent of the nature of properties of the loading mechanisms but are identical with those derived previously for a solid in contact with a reservoir that maintains the Cauchy stress. Mechanisms that control the stress in some other way may add further conditions of stability; nonetheless, the conditions of internal stability must always be obeyed and can be consistently used to define the ultimate strength. The conditions of stability are contained in the requirement that λ ijkl δe ij δe kl ≥ 0 for all infinitesimal strains, where λ ijkl = 1/2(B ijkl + B klij ), and B is the tensor that governs the change in the Cauchy stress t during incremental strain from a stressed state τ: t ij = T ij + B ijkl δe kl . Since λ has full Voigt symmetry, it can be written as the 6 x 6 matrix λ ij with eigenvalues λ α . Stability is lost when the least of these vanishes. The conditions of stability are exhibited for cubic (hydrostatic), tetragonal (tensile) and monoclinic (shear) distortions of a cubic crystal and some of their implications are discussed. Elastic stability and the limits of strength are now being explored through first-principles calculations that increment uniaxial stretch or shear to find the maximum stress. We discuss the nature of this limiting stress and the steps that may be taken to identify orthogonal instabilities that might intrude before it is reached.

The work reported here concerns the effect of Si on the precipitation of θ′ phase (metastable Al2Cu) during the isothermal aging of Al-2Cu-1 Si (wt pct). The binary alloys Al-2Cu and Al-1 Si were studied for comparison. Only two precipitate phases were detected: essentially pure Si in Al-1 Si and Al-2Cu-1 Si, and θ′ (metastable Al2Cu) in Al-2Cu and Al-2Cu-1Si. On aging the ternary alloy at 225 °C, Si precipitates first and catalyzes the θ′ phase. The precipitates in the ternary alloy are smaller, are more densely distributed, have lower aspect ratios, and coarsen more slowly than those in the binary Al-2Cu aged at the same temperature. While the shapes of individual θ′ precipitates in binary Al-2Cu are strongly affected by the kinetic problem of nucleating growth ledges, which produces a significant scatter in the aspect ratio for samples of given thickness, the overall evolution of particle shape with size follows the predictions of the Khachaturyan-Hairapetyan (KH) thermoelastic theory, which reduces to κ=L/d ∞ √L at large sizes. The KH theory provides an estimate for the interfacial tension of the broad Al-θ′ interface of 85 to 96 mJ/m2, which is near the values for other low-energy interfaces in Al, such as the twin boundary energy (100 mJ/m2) and the antiphase boundary energy in δ′ Al3Li (70 mJ/m2). Si and θ′ precipitates in Al-2Cu-1 Si have a strong elastic interaction because of their compensating strain fields. This elastic interaction promotes the nucleation of θ′ precipitates on Si, decreases the expected aspect ratio of θ′, and inhibits coarsening. Finally, Si precipitation in ternary Al-2Cu-1 Si differs from that in binary Al-1 Si in that the Si precipitates are coarser, more equiaxed, and more extensively twinned. These changes appear to be effects of Cu, which increases the solubility of Si in Al and adsorbs on the Si-Al interface, promoting twinning by a “step-poisoning” effect at the interface.

The remanent magnetization of ferromagnetic steel with five different microstructures has been studied using a microscope based on a high-transition-temperature d.c. Superconducting QUantum Interference Device (SQUID). The samples were prepared using different heat treatments, beginning with the same material composition. Each sample was magnetized in turn in a magnetic field of 50 mT, and the remanent magnetization was studied by scanning the sample over the SQUID using a two-dimensional translation stage with a range of 50×50 mm 2 . With a sample-to-SQUID separation of 400–500 μm, this procedure yielded a two-dimensional magnetic field image produced by the local remanent magnetization of the sample. In addition, microstructural information has been obtained using optical imaging, allowing the magnetic images to be correlated with the underlying microstructure.

This work was done to clarify the metallurgical mechanisms that are used in thermal treatments to refine the effective grain size of lath martensitic steels. The alloy chosen for this work was Fe–12Ni–0.25Ti, which provides a well-defined lath martensite structure in the as-quenched condition. The alloy was given four prototypic heat treatments: (1) an anneal-and-quench treatment, (2) an intercritical anneal, (3) a single or double anneal in the austenite range, and (4) a single or double reversion to austenite at a rapid heating rate. Two effective grain sizes were identified for each heat treatment: the packet size, or dimension over which adjacent martensite laths have a common crystallographic orientation, and the prior austenite grain size. The former controls the ductile–brittle transition temperature (DBTT), the latter controls intergranular embrittlement. The different heat treatments suppress the DBTT and increase resistance to intergranular embrittlement to the extent that they decrease the relevant grain size. Rapid reversion is the most successful of the heat treatments since it destroys the alignment of laths within a packet. Rapid reversion readily yields an effective grain size near 1 μm, leading to a very low ductile–brittle transition temperature and a substantial resistance to temper or hydrogen embrittlement.

The effects of isothermal tempering on the coarsening behavior of hexagonal M2C precipitates and the secondary hardening reaction in ultrahigh-strength AerMet 100 steel were investigated. The tempering temperatures were 468 °C, 482 °C, and 510 °C, and the tempering time spanned the range from 1 to 400 hours. Experimental studies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The hardness at the secondary hardening peak was about HRc 55. The average length and diameter of M2C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, at all three tempering temperatures; hence, the aspect ratio was almost 3, an equilibrium value in this case. The size of the M2C carbides increased monotonically with time, but the growth kinetics did not exactly follow the classical coarsening behavior. The amount of precipitated austenite increased with tempering time and temperature. M2C precipitates were still relatively fine even after 200 hours of tempering. This feature seemed to be closely related to the high hardness maintained after prolonged tempering.

This article reports the results of computer simulation studies of reversible, athermal martensitic transformations in idealized, two-dimensional crystals. The transformation is accomplished by se-quentially transforming elementary cells. The model accounts for the elastic strain developed during the transformation, assuming homogeneous elastic constants, negligible interfacial tension, and no external stress. The effects of frictional resistance to the transformation and plastic relaxation of the elastic strain are included in a simple way. The model is used to study the sources of hysteresis in the temperature-transformation (TT) curve and in the microstructural transformation path when the transformation is reversed. The central result is that some hysteresis is inevitable in a transformation of the type studied here. Even in the absence of friction and plastic relaxation, the transformation follows a path in which sequential elements of martensite relax the elastic strain of those that have previously formed. This causes the martensite to form in bursts and has the consequence that the reverse transformation does not reverse the path of the forward transformation. Friction and plastic relaxation increase hysteresis.

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (Kmax). Small surface cracks initiated predominantly at (Nb,Ti)xCy, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔKth, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσth) and the threshold cyclic stress intensity (ΔKσth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.

This study was conducted in order to determine and understand the effect of substrate on the behavior of eutectic In-Sn. Samples for mechanical testing were produced with either bare Cu or Ni on Cu substrates. Both the microstructure and the mechanical behavior are strongly dependent on substrate. When eutectic In-Sn is joined to bare Cu, Cu diffusion into the joint causes the alloy to become off-eutectic, giving a nonuniform and irregular microstructure. The addition of a layer of Ni acts as a diffusion barrier, preventing Cu diffusion sufficiently such that a uniform, normal colony-based eutectic forms. Deformation is more uniform in the In-Sn on Ni, while it is concentrated along the length of the joint in the In-Sn on Cu. This distinction is reflected in the different shapes of shear stress-strain curves between In-Sn on Cu and In-Sn on Ni. The stress exponents and activation energies for creep also vary with substrate. Creep deformation is governed by the In-rich β phase for In-Sn on Cu and by the Sn-rich γ phase for In-Sn on Ni. If In-Sn on Ni samples are aged, the microstructure coarsens and the mechanical behavior changes to resemble that of the as-cast In-Sn on Cu.

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.

This article reports a computer simulation study of the microstructures produced by martensitic transformations. In the present work, the transformation strain is dyadic, and the transformation is athermal and irreversible. The transformation occurs in a two-dimensional crystal that is constrained in a matrix that has no net transformation strain and may be subject to external stress. The crystal is divided into elementary cells. The transformation is simulated by computing the elastic strain energy in the linear elastic approximation and transforming the most-favored cell in each step to generate the minimum-energy transformation path. The simulation generates the microstructure at each step of the transformation and plots a temperature-transformation (TT) curve by computing the chemical driving force required to maintain the transformation and assuming that it is proportional to the undercooling. The results show that the matrix constraint causes complex, multivariant microstructures and separatesMsandMf. Multiple variants partly relax the shear part of transformation strain but interfere so that the transformation is difficult to maintain. The dilational part of the transformation strain produces interesting microstructures, such as “butterfly martensite,” in partially transformed crystals. It also increases ΔM since it produces a hydrostatic stress that cannot be compensated by mixing variants. The applied stress can be divided into hydrostatic and deviatoric components. The hydrostatic component changesMswithout altering the microstructure or ΔM. The deviatoric stress changes the relative energies of the variants and produces a microstructure that is rich in the favored variant. It also increases ΔM, since single-variant transformations must be sustained against an accumulating, uncompensated shear. The thermal resistance (ΔM) increases with the magnitude of the deviatoric stress until a high-stress limit is reached and only one variant appears. The microstructure is most complex at intermediate stress, where both variants develop in a complex internal stress field. Cyclic stress dramatically increases the extent of transformation at given maximum load. The martensite that has already formed becomes a source of intense internal stress when the stress is reversed, promoting further transformation.

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant Kmax, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σy. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔKth (or ΔK th eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔKeff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK th eff form and grow in statistically weak regions of the microstructure.

The microstructure that results from a martensitic transformation is largely determined by the elastic strain that develops as martensite particles grow and interact. To study the development of microstructure, it is useful to have computer simulation models that mimic the process. One such model is a finite-element model in which the transforming body is divided into elementary cells that transform when it is energetically favorable to do so. Using the linear elastic theory, the elastic energy of an arbitrary distribution of transformed cells can be calculated, and the elastic strain field can be monitored as the transformation proceeds. In the present article, a model of this type is developed and evaluated by testing its ability to generate the preferred configurations of isolated martensite particles, which can be predicted analytically from the linear elastic theory. Both two- and three-dimensional versions of the model are used. The computer model is in good agreement with analytic theory when the latter predicts single-variant martensite particles. The three-dimensional model also generates twinned martensite in reason- able agreement with the analytic predictions when the fractions of the two variants in the particle are near 0.5. It is less successful in reproducing twinned martensites when one variant is dom- inant; however, in this case, it does produce unusual morphologies, such as “butterfly mar- tensite,” that have been observed experimentally. Neither the analytic theory nor the computer simulation predicts twinned martensites in the two-dimensional transformations considered here, revealing an inherent limitation of studies that are restricted to two dimensions.

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and e phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to e phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when the Sn is consumed; e phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases.

A multiphase diffusion model was constructed and used to analyze the growth of the e- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in thee andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the e phase(≈De) is well fit by an Arrhenius expression with D0 = 5.48 × 10−9 m2/s andQ = 61.9 kJ/mole, while that in the η phase (≈Dη) has D0= 1.84 × 10−9 m2/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that thee phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted e-phase thickness is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.

Ni/Au metallization layers are used with increasing frequency to protect Cu substrates in ball grid array microelectronic packaging. The external Au layer provides oxidation and corrosion resistance during storage prior to assembly, while the intermediate Ni layer acts as a diffusion barrier that inhibits the formation of a thick Cu-Sn intermetallic layer during aging. During soldering with eutectic Pb-Sn, the Au dissolves into the molten solder and forms fine, needle-shaped AuSn{sub 4} intermetallic precipitates that are retained in a dense distribution in the bulk of the solder joint after it has solidified. However, recent research by Mei et al. has revealed a new and potentially problematic phenomenon that seems to be peculiar to the Ni/au metallization. They found that after extensive aging (150 C for 2 weeks in their case), the Au-Sn intermetallic redeposited onto the solder-substrate interface. The reconstituted interface was significantly weakened and failed by brittle fracture along the surface between the redeposited Au-Sn and the Ni{sub 3}Sn{sub 4} layer that formed during reflow. While the interfacial redeposition of Au-Sn intermetallics has been observed previously, the mechanism remains unknown. The present work was undertaken to identify the mechanism of this phenomenon and explore methods for controlling it.

A two-step heat treatment was developed as a means of maximizing the basal texture of Zircaloy-4 sheet. The heat treatment, consisting of a brief beta heat treatment and subsequent controlled cooling, resulted in a Kearns number (fn) of 0.83 with a microstructure of uniformly fine grains and high hardness. The texture component, as determined by the crystallite orientation distribution function (CODF), was (0002)(10−10). The result was discussed in terms of phase transformation, either diffusional or martensitic, incurred by controlled cooling. The rate of stress corrosion cracking (SCC) of the alloy was evaluated with constant deflection tests in an iodine environment. The exponentn of the relationshipda/dt = C · (Kln was found to correlate with fn for KI, ranging from 4 to 17 MPa√m.

The cryogenic mechanical properties of a 12.7-mm (0.5-in.) Vintage III 2090-T81 (Al-3.05Cu-2.16Li-0.11Zr by weight) plate material were studied. The results indicate that the through-thickness anisotropy in the yield strength of Vintage I 2090-T81 is reduced in the Vintage III material by utilizing a partial recrystallization step during the thermomechanical processing but at some cost to the elongation and cryogenic fracture toughness. Sharp discontinuities in the microstructure resulting from this thermomechanical treatment give rise to distinct differences in the variation of the work hardening behavior with temperature between those regions which undergo recrystallization and those which do not. The lower elongations observed in Vintage III 2090-T81 result from a decrease in work hardenability that appears to be associated with increased slip heterogeneity. The reduced cryogenic toughness is due, in part, to the disruption of the monolithic unrecrystallized grain structure previously observed in Vintage I 2090-T81. The results suggest that a fully unrecrystallized structure is beneficial for cryogenic toughness in the longitudinal orientation.

A transmission electron microscopy (TEM) investigation of the electron beam (EB) and gas tungsten arc (GTA) fusion zones of 2090 indicates that in both the as-welded and aged conditions, the EB and GTA fusion zones lack the volume fraction and the homogeneity of strengthening precipitates found in the base metal. In the underaged and peak-aged conditions, the δ′ phase is the primary strengthener, the volume fraction ofT 1 present being too low to be effective. TheT 1, precipitates are found either in the vicinity of other inclusions or at the dendrite boundaries. As the strength increases with postweld aging, the elongation decreases to 1 pct. The presence of the boundary phases and Cu- and Cl-containing inclusions at the boundaries leads to poor elongation. The joint efficiencies of the peak-aged EB and GTA weldments (EBWs and GTAWs, respectively) are 75 and 55 pct at 293 K and 75 and 50 pct at 77 K, respectively. Both EBWs and GTAWs have relatively low elongations.

This research reports an investigation into the influence of mechanically induced martensitic transformation on the rate of fatigue crack growth in 304-type austenitic stainless steels. Two steels of different composition, 304L and 304LN, were used to test the influence of composition; two test temperatures, 298 and 77 K, were used to study the influence of test temperature; and various load ratios were used to determine the influence of the mean stress. It was found thadecreasing the mechanical stability of the austenite by changing composition or lowering temperature reduces the fatigue crack growth rate and increases the threshold stress intensity for crack growth. However, this beneficial effect diminishes as the load ratio increases, even though increasing the load ratio increases the extent of the martensite transformation. Several mechanisms that may influence this behavior are discussed, including the perturbation of the crack tip stress field, crack deflection, work hardening, and the relative brittleness of the transformed material. The perturbation of the stress field seems to be the most important; by modifying previous models, we develop a quantitative analysis of the crack growth rate that provides a reasonable fit to the experimental results.