Application of the double-shear theory of martensite crystallography to the β → α′ transformation in an U(Ga) alloy

The phenomenological double-shear theory of martensite crystallography has been applied to the β → α′ martensitic transformation in the U-1.6 at. pct Ga alloy. A correspondence matrix for the β → α′ transformation was derived from the experimentally determined β/α′ orientation relationship, and the double lattice invariant shear was considered as a combination of the principal slip (010) [100]_a with one of the minor slips in the α-uranium structure. The theoretical predictions of the habit plane are in good agreement with the experimental observations. Formerly Graduate Student, Ben-Gurion University of the Negev.     

Detwinning in NiTi alloys

This work focuses on the stress-induced transformation in solutionized and overaged single-crystal NiTi alloys. The potential role of detwinning on the recoverable strains was investigated both theoretically and also with temperature-cycling experiments. The detwinning is the growth of one variant within a martensite in expense of the other. It is shown that the experimental recoverable strains in tension (near 8.01 pct in the [123], 9.34 pct in the [111], and 7.8 pct in the [011] orientations) exceed the theoretical martensite (correspondent-variant pair (CVP) formation strains (6.49 pct in [123], 5.9 pct in [111], and 5.41 pct in [011]), lending further support that partial detwinning of martensite has occurred in both the solutionized and overaged specimens. In compression, the experimental recoverable strains are lower than the theoretical martensite (CVP) formation strain. In the compression cases, the detwinning strain contribution is calculated to be negligible in most orientations. The transformation strains observed in overaged NiTi are similar to the solutionalized NiTi, suggesting that incoherent precipitates do not restrict the detwinning of the martensite. For the [123] orientation, it is demonstrated that the thermal hysteresis is higher in solutionized NiTi compared to the overaged NiTi. The higher thermal hysteresis can be exploited in applications involving damping and shape stability, while the lower hysteresis is suited for actuators.     

Damping characteristics of Ti50Ni49.5Fe0.5 and Ti50Ni40Cu10 ternary shape memory alloys

The damping characteristics of Ti₅₀Ni_49.5Fe_0.5 and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys (SMAs) have been systematically studied by resonant-bar testing and internal friction (IF) measurement. The damping capacities of the B19′ martensite and the B2 parent phase for these ternary alloys are higher than those for the Ti₅₀Ni₅₀ binary alloy. The lower yield stress and shear modulus of these ternary alloys are considered to be responsible for their higher damping capacity. For the same ternary alloy, the B19/B19′ martensite and R phase also have a higher damping capacity than does the B2 parent phase. In the forward transformations of B2 → R, R → 519′, and B2 → 519′ for Ti₅₀Ni₅₀ and Ti₅₀Ni_49.5Fe_0.5 alloys, the damping capacity peaks appearing in the resonant-bar test are attributed to both stress-induced transformation and stress-induced twin accommodation. The lattice-softening phenomenon can promote the stress-induced transformation and enhance the damping capacity peaks. The Ti₅₀Ni₄₀Cu₁₀ alloy had an unusually high plateau of damping capacity in the B19 martensite, which is considered to have arisen from the easy movement of twin boundaries of B19 martensite due to its inherently very low elastic modulus. The peaks appearing in the IF test for the Ti₅₀Ni₄₀Cu₁₀ alloy are mainly attributed to the thermal-induced transformation due to T ⊋ 0 during the test.     

Orientation dependence of β1 → β1′ stress-induced martensitic transformation in a Cu-AI-Ni alloy

A systematic investigation was carried out to settle some critical issues on the orientation dependence of the stress-induced martensitic transformation. The transformation we picked up is the β_l (DO3) → β₁′ (18R) stress-induced transformation in a Cu-14.1 mass pct Al-3.9 mass pct Ni alloy. By utilizing tensile tests and two surface analyses coupled with X-ray diffraction, the following clear results were obtained. Concerning the variant selection rule under stress, it was found to be the shape strain which interacts with an applied stress. The critical resolved shear stress for the martensitic transformation was found to have orientation dependence, the shear stress increasing with the normal stress. The observed transformation strains were consistent with those calculated from the shape strain in all orientations. The strong orientation dependence of the Young’s modulus was consistent with that predicted by the elastic constants in the parent phase.     

Martensitic transformations, microstructure, and mechanical workability of TiPt

The TiPt phase with the B2 structure has been reported to undergo a reversible displacive transformation to B19 martensite at about 1000 °C. This system could, therefore, serve in principle as the basis of a high-temperature shape-memory alloy (SMA). However, very few additional details of the B2 and B19 forms of TiPt have been published. In the present work, the B19→B2 transformation temperatures previously reported are confirmed, but the B2→B19 temperatures are found to be about 40 °C lower than previously accepted. The hardness of B19 martensite shows a minimum at a stoichiometry of ∼50 at. pct Pt. Between 45 and ∼50 at. pct Pt, the B2↔B19 transformations appear to take place in a complex sequence, with up to two intermediate phases being stable over a range of about 50 °C. No evidence for this intermediate phase was found for Pt contents from 50 to 56 at. pct Pt. It is clear that a Pt content of about 50 at. pct marks a significant change in the nature of the TiPt phase and its martensite or martensites. It was also found that TiPt has a slightly wider stability range than is shown in the current phase diagram, extending from 45 to 56 at. pct Pt at 1300 °C.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

Elastic properties of β-uranium and the martensitic α′-phase in uranium-gallium alloys

Ultrasonic sound velocity measurements were used to determine the elastic constants, Young, shear, and bulk moduli of polycrystalline, β-stabilized uranium 1.6 and 2.3 at. pct gallium alloys, between 77 and 300 K. Measurements were also made, at constant heating rate from 273 to 473 K, to follow the variation of the elastic constants during the transformation from the stabilized β into the distorted orthorhombic α′-structure of uranium. The Young and shear moduli in the α′-structure are higher by about 30 pct than those in β-uranium.     

Martensitic transformations in β Ag-cd alloys

Phase transformations have been studied inβ Ag-Cd alloys lying in the composition range 42.3 to 50.9 at. pct Cd. Thermal martensite with an orthorhombic structure was obtained on cooling below room temperature. The M_s temperature was found to change from —44° to — 137°C as the cadmium content changed from 44.2 at. pct Cd to 47.0 at. pct Cd. The martensite had a habit plane close to (133)β in good agreement with that calculated from the phenomenological theory assuming a (011) [0•11]β lattice-invariant shear. An fct martensite was obtained on moderate degrees of deformation at room temperature. This had a habit plane close to (110)β, again in good agreement with the phenomenological theory assuming a (110)[1•10]β lattice-invariant shear. On severe deformation a close-packed structure was obtained, this being fcc up to 45.5 at. pct Cd, hcp above 47.7 at. pct Cd, and a mixture of both fcc and hcp at 46.0 at. pct Cd. A spontaneous martensite with an fcc structure was found to occur along the thin edges of perforated specimens used for electron microscopy. A mechanism suggesting the course of the transformation has been proposed. Formerly Graduate Student, Department of Metallurgy, University of British Columbia, Vancouver, Canada.     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

Tensile deformation behavior of mechanically stabilized Fe-Mn austenite

The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness reduction by rolling at 673 K (above the A_f temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However, the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel, the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel at low temperatures. M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley     

An investigation of transformation strengthening in precipitation-hardened Fe-Ni austenite

Ausaged Fe-33Ni-3Ti and Fe-34Ni-3Ti-0.5Al austenitic alloys were transformation strengthened through sequential martensite (α′ → γ) and reverse martensitic transfor-mations. An increase in yield strength of 20 to 30 ksi (130 to 210 MPa) was obtained, leading to alloy yield strengths near 180 ksi (1250 MPa) in the ausaged and transforma-tion strengthened condition. The presence of the γ′ precipitates formed on ausaging did not radically change the nature of the shear transformation nor the morphology and sub-structure of the transformation product. The presence of the γ′ precipitates did, how-ever, have the beneficial consequence that the α′→ γ reversion could be accomplished with slow heating rates without loss of transformation strengthening. The yield strength actually increased when a slow heating rate was used, a result attributed to the success-ful pinning of transformation induced defects by the γ′ precipitates and to the occurrence of additional precipitation during heating. The efficient pinning of transformation induced defects had the further consequence that the alloys could be annealed for long periods after reversion to austenite without significant loss of strength.     

Electron backscattered diffraction study of <Emphasis Type="Italic">ε/α′</Emphasis> martensitic variants induced by plastic deformation in 304 stainless steel

The electron backscattered diffraction (EBSD) technique has been used to assess crystallographic features of the residual γ phase and the strain-induced ε/α′ martensites in a 304 stainless steel, tensile tested to 10 pct strain at T=−60 °C. The martensitic transformation rate varies according to the γ-grain orientation against the applied stress and the γ-grain size. The α′-transformation textures as well as the γ-misorientation spreads observed in specific γ-grain orientations have been analyzed. Large misorientation spreads are observed in the less-transformed γ grains. This reveals an important crystallographic slip activity, even if less strain-induced martensite has been formed. A strong γ → α′ variant selection was detected in the cube- and Goss-oriented γ grains for which the transformation is less developed. For the {110} 〈1–11〉 and copper-oriented γ grains, the amount of α′ martensite is significantly higher and the γ → α′ variant selection is less pronounced. This variant selection is then analyzed on at a local scale and is related to the presence of {111} _γ localized deformation bands on which further ε/α′ martensites have nucleated.     

Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.     

Large-strain Bauschinger effects in fcc metals and alloys

We have performed Bauschinger experiments on a variety of fcc metals and alloys, after large amounts of prestrain, using torsion and a short thin-walled tube geometry. The materials we studied were 99.99 pct Al, OFE copper, 70:30 brass, Al-1 pct Mg, Al-2 pct Mg, Al-0.17 pct Fe-0.07 pct Si, Al-0.8 pct Mn, and two Al-Cu alloys (Al-2.6 pct Cu and Al-4 pct Cu) given different heat treatments. For the material systems other than the Al-Cu alloys, the stress reversal was after a prestrain in shear of ≈3.0. Two stress reversals were performed on the Al-Cu alloys. The first was at γ = 0.3 and the second at γ = 1.2. Thus, for the Al-Cu, the prestrain and the final increment of deformation were in the same direction. The Bauschinger yield stress in these experiments was characterized by a very large offset shear strain of 0.05. This definition of reverse yield minimizes the effects of heterogeneous deformation and long-range internal elastic stresses that arise mainly from second-phase particles. We attributed the effects we observed to “isotropic hardening” associated with the dislocation substructures that developed in the different materials. We found that the behavior of these materials could be divided into two categories: those which deform by planar slip and those that form a “cell” structure and are characterized as having wavy slip. When the deformation was wavy in nature, we attributed the observed Bauschinger effects to be a result of the untangling of the “cells” formed during the prestrain. Different morphologies of cells had different behaviors when the stress was reversed. The behavior of the planar slip alloys depended on whether or not the barriers to dislocation activity were rigid or shearable. The θ′ precipitates in the Al-Cu alloys and the twin boundaries in the 70:30 brass constituted rigid barriers to dislocation motion, and a very large Bauschinger effect was observed. The solid solution Al-Cu material and that containing Guinier-Preston (GP) zones and θ″ had almost no Bauschinger effect when the yield stress in reverse deformation was considered. After yield, these materials hardened very rapidly and the flow stress in the reverse direction exceeded that for the equivalent amount of monotonie deformation.     

Influence of phase transformations on the mechanical properties of high-strength austenitic Fe-Mn-Cr steel

The influence of Cr and N additions on the mechanical properties of a Fe-Mn-C steel was investigated. The chemical composition was found to have a pronounced effect on the strain-hardening behavior, due to the strain-induced sequence of the γ → ▓ → α′ martensitic transformations. It was found that Cr and N suppress this transformation sequence. At Cr levels higher than 7.5 mass pct, no α′ martensite was formed, which led to a pronounced improvement of the ductility. The differences in transformation behavior can be attributed to the change in the intrinsic stacking-facult energy (ISFE): in the compositional range studied, Cr and N additions cause an increase of the ISFE.     

Transformation behavior in a thermomechanically cycled TiNiCu alloy

The effect of thermomechanical cycling under 150 MPa on the transformation behavior in a TiNi₄₀Cu₁₀ (at pct) alloy annealed at different temperatures was investigated using electrical resistivity measurements and differential scanning calorimetry (DSC). It was found that thermomechanical cycling to failure could increase or decrease the transformation temperature for specimens annealed below or above the recrystallization temperature, respectively, but there was no obvious change of the transformation temperature for specimens annealed at the recrystallization temperature. The DSC and electrical-resistance experiments show that the B2 ⇋ B19 and B19 ⇋ B19′ two-stage transformations occurred in cold-worked and thermomechanically cycled specimens and that the electrical-resistance change due to the B2 → B19 transformation is larger than that of annealed specimens.     

Transformation plasticity in iron-nickel alloys

Plastic flow during the austenite ? martensite transformation under constant load has been studied in two Fe?Ni alloys (15.4 pct Ni; 32.9 pct Ni). Transformation plasticity, characterized by the typical linear relationship between the transformation strain per cycle and the externally applied stress,i.e., a quasiviscous behavior, was observed for both alloys. The plastic transformation strain on heating was larger than that on cooling for the 15.4 pct Ni alloy and equal to that on cooling for the 32.9 pct Ni alloy. Transformation plasticity results for both alloys are in quantitative agreement with the pseudo-creep theory of Greenwood and Johnson except for the martensite to austenite transformation in the 32.9 pct Ni alloy where the result is an order of magnitude too low. A dislocation model is proposed which considers the superposition of the large shear stresses generated by the martensite plate formation and the externally applied stress. The model quantitatively predicts the stress dependence of the transformation strain per cycle for transformation plasticity.     

Orientation and temperature dependence of slip in iron single crystals

The orientation and temperature dependence of slip in ZrH₂-purified iron crystals have been investigated at strains up to about 15 pct. Both {110} and {112} slip planes were observed at all temperatures investigated (295°, 250°, 195°, and 143°K) when the orientation of a crystal was such that its maximum resolved shear stress plane (MRSSP) corresponded to {110} or {112}. At 195° and 143°K {110} and {112} slip planes were also observed for orientations where the MRSSP deviated from {110} or {112} slip planes were also observed for orientations where the MRSSP deviated from {110} or {112}. At 295° and 250°K, the slip planes of crystals whose MRSSP was not {110} or {112} deviated from the MRSSP toward {110}. The variation in the intensity and waviness of the slip traces with orientation suggested that cross slip was easiest for orientations that slipped on a {112} plane in the twinning sense (near [001]), and hardest for orientations that slipped on a {112} plane in the antitwinning sense (near [011]). This appears to be in accord with observations of easier dislocation multiplication for orientations near [001] than for orientations near [011]. At 143°K the critical resolved shear stress law was not obeyed; the resolved shear stress was about 14 pct lower for slip on a {112} plane when the sense of the applied stress was favorable for twinning than when it was unfavorable. The slip line observations and the asymmetry of slip on a {112} plane appear to be qualitatively explainable in terms of dissociated dislocation models that are based on differences in the ease with which slip and cross slip can occur on {110} and {112}.