Observations of aging effects in a Cu-Sn shape memory alloy

A Cu-15.0 at. pct Sn alloy has been chosen as a model alloy for the study of aging effects in copper-based shape memory alloys. Different thermal aging treatments were carried out to determine the effects of both parent phase and martensite aging on the amount of shape recovery and the characteristic transformation temperaturesM _s ,A _s , andA _f . Aging of the martensite reduces both the amount of shape recovery and the extent of the reverse martensite → parent transformation. High martensite heating rates promote complete shape recovery and reverse transformation while the aging occurring during slow heating can inhibit or prohibit both. But irrespective of the martensite heating rate the transformation temperature hysteresis as given by (M _s -A _s ) is large for the Cu-15 pct Sn alloy compared to other shape memory alloys exhibiting thermoelastic behavior. On the other hand, some beneficial effects were noted when the Cu-15 pct Sn alloy was aged in the parent phase condition prior to subsequent transformation to martensite. TheM _s ,A _s , andA _f were lowered following prior parent phase aging, possibly because of a change in long range order, but prior parent phase aging was found to diminish the deleterious effect of martensite aging. Both shape recovery and the extent of the reverse martensite → parent transformation are enhanced by prior parent phase aging. The enhancement is greater the higher the aging temperature or the longer the aging time at a given temperature. J. D. STICE, formerly Research Assistant at the University of Illinois     

Microobservation of stress induced martensitic transformation in CuAlNi single crystals

Stress-induced martensitic phase transformation of CuAlNi single crystals is observed by use of an in situ optical observation apparatus. The results show that long before the transformation of the parent phase becomes macroscopically noticeable, interfaces arise. After the macroscopic phase transformation is completed, interfaces do not totally disappear. The macroscopic yield stress is the stress for the growth of the martensite in the transversal direction, this growth goes on only from one side of the acicular martensite. The number density of martensitic plates does not change with different temperatures.     

Determination of heat of transformation in a cold-rolled martensitic tini alloy

We studied the heat of transformation, ΔH, in martensitic transformations in a cold-rolled equiatomic TiNi alloy with differential scanning calorimetry (DSC), X-ray diffraction (XRD), and microhardness measurements. Results of our experiment indicate that the martensite stabilization and stress-induced parent (SIP)B2 phase are introduced when the TiNi martensite is cold rolled at room temperature. The SIP formation seems to be related to the lattice softening phenomenon occurring in the martensite, while the ΔH value of the first reverse martensitic transformation decreases enormously for the cold-rolled equiatomic TiNi alloy. We are proposing possible explanations for these results: (1) the occurrence of SIP, which reduces the transformable martensite volume; (2) the release of accumulated elastic energy induced by the cold rolling; and (3) the recovery of defects induced by cold rolling and release of the heat of recovery. We also found that the retained dislocations can depress the martensitic transformation temperatures and induce the R-phase transformation after the occurrence of the first reverse martensitic transformation. Formerly Graduate Student, Institute of Materials Science and Engineering, National Taiwan University     

Interfacial Modulus Mapping during Structural Transformation in Shape Memory Alloys

Through the modified phase-field model the local soft mode mechanism of nucleation during martensitic transformation was confirmed in shape memory alloys. It was discovered that the modulus loss (8 pct) depended on the martensitic nucleation exceeding the loss (1 pct) during the martensitic growth. The elastic modulus and the stress across the martensite/parent interface differed from those across the martensitic twin boundary. The modulus losses in systems with three variants, two variants, and one variant were compared.     

Effects of aging on pseudoelastic behaviour in Cu-15.0 at.%Sn alloy single crystals

The effect of aging on pseudoelasticity in a shape memory alloy was studied using single crystals of a CuSn alloy, in which the martensitic transformation behaviour is fairly susceptible to an aging treatment. The aging effect on the nucleation of martensite was investigated in the specimens aged in the parent phase. The effect on the growth of martensite was studied by strain aging during tensile testing, and also by the analysis of strain rate dependence of pseudoelasticity. The former effect is attributed to the embryo exhaustion due to the diffusion of Sn atoms, originally proposed by Kennon. On the other hand the latter is explained from the pinning of a martensite/parent interface by the diffusion of Sn atoms.     

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

     

Transformation characteristics of α1 Plates in Cu-Zn-Al Alloys

The formation of α₁ plates during isothermal aging of a Cu-26.7 wt pct Zn-4.0 wt pct Al alloy at 150 °C to 350 °C follows thermally activated incubation kinetics. Early stage α₁ plates possess an ordered 18R or 9R long-period stacking order (LPSO) crystal structure, with antiphase domain boundaries running continuously across the interface. The plates also exhibit invariant plane strain (IPS) crystallography consistent with calculations of the phenomenological theory of martensite crystallography (PTMC). The ordered LPSO structure and IPS crystallography are gradually annealed out only after extended aging as the structure changes to the equilibrium disordered face-centered cubic (fcc) one. High-resolution transmission electron microscopy (TEM) analyses reveal that early stage α₁ plates have straight coherent interfaces. Prolonged aging induces misfit dislocations at the interface and causes the interface to protrude into the parent phase. Although microanalytical analyses indicate that a composition difference exists between the α₁ plates and the parent matrix, solute depletion was observed at neighboring defects. These observations support the proposed transformation mechanism that the α₁ plates nucleate at solute depleted defects through a shear mechanism and that subsequent plate growth is then controlled by a diffusional process. Formerly Visiting Scientist, with the Department of Materials Science and Engineering, University of Illinois     

Microstructure and martensitic transformations in a dual-phase α/β Cu−Zn alloy

The martensitic transformations in a dual-phase α/β Cu−Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu−Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α_S) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α_S and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α_S were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α_S martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α_S martensite, an α_S lamellar martensite was found in the deformed specimen.     

Effects of Fine Particle Peening Conditions on the Rotational Bending Fatigue Strength of a Vacuum-Carburized Transformation-Induced Plasticity-Aided Martensitic Steel

The effects of fine particle peening conditions on the rotational bending fatigue strength of a vacuum-carburized transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.20 pct C, 1.49 pct Si, 1.50 pct Mn, 0.99 pct Cr, 0.02 pct Mo, and 0.05 pct Nb were investigated for the fabrication of automotive drivetrain parts. The maximum fatigue limit, resulting from high hardness and compressive residual stress in the surface-hardened layer caused by the severe plastic deformation and the strain-induced martensite transformation of the retained austenite during fine particle peening, was obtained by fine particle peening at an arc height of 0.21 mm (N). The high fatigue limit was also a result of the increased martensite fraction and the active plastic relaxation via the strain-induced martensite transformation during fatigue deformation, as well as preferential crack initiation on the surface or at the subsurface.     

Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy

The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 °C to 400 °C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) D0₃ or L2₁ atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium α and γ ₂ phases and destruction of the shape memory capacity.     

Evaluation of Interface Boundaries in 9Cr-1Mo Steel After Thermal and Thermomechanical Treatments

The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite.     

The effect of prior deformation on the strength and annealing of reverted austenite

The strength, annealing behavior, and microstructure of reverted austenite has been measured in an Fe-31 pct Ni-0.03 pct C alloy that was plastically deformed in the martensitic state prior to the reversion to austenite. Mechanical properties of reverted austenite (e.g., austenite formed by the reverse martensite shear transformation) were measured as a function of the amount of prior deformation, heating and cooling rates to the reversion temperature, austenitizing temperature and time, repetitive cycling from martensite to reverted austenite, and prereversion heat treatments. The results showed that 80 pet prior deformation increases the yield strength of reverted austenite about 30 pct. Along with this strengthening, the dislocation configuration changes from a plate-like fine structure with a random array of tangled dislocations in reverted samples without prior deformation to a equiaxed fine structure with a high density of tangled dislocations within the fine structure in samples with 80 pct deformation prior to reversion. Although smaller amounts of prior deformation (20 pct) have only a small effect on the strength of the reverted austenite, this amount of prior deformation significantly increases the driving force for recrystallization. The results are explained on the basis that the prior deformation and the reversion process produce separate components to the strength and annealing behavior. E. GOLD, formerly with the Aeronutronic Division, Philco-Ford Corporation, Newport Beach, Calif.     

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveA _f have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowM _f can be restored by transforming back to the original parent phase by heating toA _f (shape memory) or alternatively, can be recovered belowM _f by applying a small stress of opposite sign. Martensite deformed belowM _f with the same stress maintained while heating persists aboveA _f, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed.     

The nature of the parent-martensite interface in titanium-manganese

A transmission electron microscope study has been made of the ‘zig-zag’ parent-martensite interfaces found between the b.c.c. parent phase and the internally twinned h.c.p. martensite phase in a Ti-5 wt% Mn alloy. The interfaces were found experimentally to have an orientation consistent with their being in the zone of the invariant line of the transformation for the Class A (α−, ω+) solution of the phenomenological theory. No evidence was found for the ‘steps’ or ‘ledges’ in the interfaces which would be expected if the interfaces are to be explained on the basis of small scale dilatations which make the lattice strain in the martensitic transformation an invariant plane strain. The orientation of the interfaces have been explained by a new model based on the surface dislocation approach. The model suggests that the occurrence of interface faceting in materials such as TiMn is a consequence of the individual twin-parent interfaces lowering their net Burgers vector contents. Predictions from this approach are found to be in good agreement with the present results and also previous experimental results. A plausible argument for this approach is presented using results from isotropic elasticity theory.     

Pseudoelastic behavior of a CuAlNi single crystal under uniaxial loading

In order to study the basic properties of pseudoelasticity of a CuAlNi single crystal, an investigation was carried out to observe and analyze the orientation dependence of the stress-induced martensitic transformation. The transformation is the β ₁ to β′₁ stress-induced transformation in a Cu-13.7 pct Al-4.18 pct Ni (wt pct) alloy. From the uniaxial tension of three groups of differently oriented flat specimens, we obtained a series of stress-strain curves. In addition, the micrograph of martensitic evolution was observed by utilizing a long-focus microscope. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very quick process, and martensite “jumps” out until the specimen is completely transformed into a single variant. The experimental results are analyzed and compared to a constitutive model proposed recently. It is found that the constitutive model cannot describe transformation hardening, since the model ignores the surface-energy change. Nevertheless, the proposed constitutive model cannot only precisely predict the forward and reverse transformation, but can also characterize the stress-strain hysteresis behavior during pseudoelastic deformation under uniaxial tension loading.     

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy have been studied by internal friction and shear modulus measurements, hardness test and TEM observation. The martensite stabilization can be induced by cold rolling at room temperature. A variety of deformed martensite structures has been observed. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. The hardness test results also support this viewpoint. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. Experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy.     

A phenomenological study of the shape memory effect in polycrystalline uranium-niobium alloys

When uranium-niobium alloys containing between 13.9 and 17.9 at. pct Nb are quenched to room temperature from the BCC (γ) phase at elevated temperatures, diffusion-controlled precipitation of the equilibrium phases is prevented and martensitic transformations to transition phases occur instead. Dilatometry was used to detect transformation temperatures and with the help of X-ray diffraction analysis, a metastable phase diagram was established. At room temperature after quenching, alloys containing < 15.2 at. pct Nb were monoclinic (α″) and those with < 16.6 at. pct Nb were tetragonal (γ°). The deformation behavior and shape memory effects (SME) accompanying the reverse martensitic phase transformations in polycristalline specimens were surveyed and characterized phenomelogically. From uniaxial tensile tests at room temperature, macroscopic stress-strain parameters, associated with the reversible deformation modes in the α″ and γ° martensites, were defined and their composition and structural state dependencies delineated. A diffuse maximum manifested in the stress-strain diagrams was identified with the reversible strain limit, which varied inversely and continuously with composition. A concentration-independent value of 693 MPa was found for the plastic yield strength of the alloys. All the alloys exhibited heat-activated shape recovery but the degree depended on structural state and composition. The α″ alloys showed a much larger effect than γ° alloys. Shape recovery occurred in two stages in all alloys. The first stage of recovery accompanied martensite reversion but final reversion to the equilibrium y phase was not accomplished until much higher temperatures were reached. Rapid, low temperature aging reactions were thought to affect the finish of shape recovery and delay it to higher temperatures. Formerly with Metals and Ceramics Division, Oak Ridge National Laboratory     

Temper hardening of cu-al martensite

The temper hardening of quench-induced Cu-Al martensite was investigated via trans-mission electron microscopy, differential thermal analysis, microhardness and yield strength measurements. The results obtained are also viewed in conjunction with inde-pendent set of experimental data dealing with anneal hardening of the α-phase without prior martensitic transformation. A substantial low temperature hardening of a 10 pct Al martensite is indicated and attributed to the formation of small ordered clusters, en-hanced by stacking faults already existing in martensite. In the 11 pct Al martensite hardening is also linked with ordering though less pronounced due to the larger domain size reached. On the other hand the tempering of 11.8 pct Al martensite is associated with phase separation leading to the(α + γ₂) state via continuous and discontinuous pre-cipitation.     

Evidence of transformation bursts during thermal cycling of a Pu-Ga alloy

The thermodynamics and kinetics of the fcc (delta) to monoclinic (alpha-prime) phase transformation and its reversion in a plutonium-gallium alloy have been studied using differential scanning calorimetry, resistometry, and dilatometry. Under ambient conditions, the delta phase is metastable in a Pu-2.0 at. pct Ga alloy. Thermal cycling to below the ambient temperature results in a partial transformation to the alpha-prime phase; this transformation is composition-invariant and exhibits martensitic behavior. Because this transformation results in an unusually large 25 vol pct contraction that cannot be fully accommodated by purely elastic adjustments, the transformation mode is expected to involve burst formation of individual alpha-prime particles. However, upon cooling, these individual bursts were not resolved by the above techniques, although signals corresponding to the overall accumulation of many alpha-prime particles were observed. On the other hand, upon heating, signals from differential scanning calorimetry, resistometry, and dilatometry showed a series of discrete changes occurring in periodic increments beginning at approximately 32 °C. These features correspond to the cooperative reversion of many alpha-prime particles to the delta phase; they appear to be the result of an interplay between the autocatalytically driven reversion of a cascade of individual martensite units and self-quenching caused by small changes of temperature or stress accompanying each individual transformation burst. The heat of the delta/alpha-prime transformation is estimated to be about +4 kJ/mol.     

Enhanced Mechanical Properties of As-Forged Co-Cr-Mo-N Alloys with Ultrafine-Grained Structures

The ultrafine-grained (UFG) microstructures of Ni-free Co-29Cr-6Mo (mass pct) alloys, which are designed for biomedical applications, have been successively fabricated by the conventional hot-forging process. The grain size decreased with increasing hot-forging reduction, and the equiaxed UFG structures with a mean grain size less than 1 ??m were obtained in 83?pct (true strain of 1.8) hot-forged specimens. Significant grain refinement drastically enhanced tensile strength; dislocations residual in the grains also play a crucial role for strengthening of the UFG-structured specimen. The elongation decreased with the reduction in grain size. However, we revealed that the addition of nitrogen, which is one of the nontoxic ?? phase (face-centered cubic [fcc] structure) stabilizer, improves the ductility of the UFG alloys remarkably with maintaining high strength. It was deduced that the enhanced ductility in the UFG material by N doping was related to constituent phase and strain-induced martensitic transformation behavior: the addition of nitrogen eliminated athermal ?? martensite detrimental to tensile elongation, and strain-induced martensitic transformation effectively increased work-hardening rate to avoid the plastic instability at the early stage of deformation. The present method characterized by ultragrain refinement in conjunction with nitrogen addition to stabilize the ?? phase can provide a potent strategy to obtain superior combination of high strength and adequate ductility.