Stress-induced Martensite Formation Research Articles

Modeling the transformation stress of constrained shape memory alloy single crystals

Shape memory alloys (SMA) are a unique class of engineering materials that can be further exploited with accurate polycrystal constitutive models. Previous investigators have modeled stress-induced martensite formation in unconstrained single crystals. Understanding stress-induced martensite formation in constrained single crystals is the next step towards the development of a constitutive model for textured polycrystalline SMA. Such models have been previously developed for imposition of axisymmetric strain on a polycrystal with random crystal orientation; the present paper expands the constrained single crystal SMA model to encompass arbitrary imposed strains. To evaluate the model, axisymmetric tension and compression strains and pure shear strain are imposed on three SMA: NiTi, CuAlNi (β1→γ′1) and NiAl. Model results are then used to understand the anisotropy and asymmetry of transformation stress in the three SMA considered. Finally, the impact of the present results on polycrystal behavior is addressed.

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively.

Time dependence of internal friction and shape change in Cu-Zn-Al shape memory alloys

The time evolution of internal friction and shape change in different microstructural states of two Cu-Zn-AI shape memory alloys has been investigated. In the single-martensite phase state, the monotonic decrease of internal friction with time is attributed to the pinning of martensite/ martensite interfaces. When the parent phase is present, the change in the internal friction is associated with the formation of stress-induced martensite and the subsequent pinning of the martensite/parent and martensite/martensite interfaces.

Magnetic Anisotropy Caused by the Formation of Stress-induced Martensite in Small Iron Particles in a Copper Matrix.

Magnetic Anisotropy Caused by the Formation of Stress-induced Martensite in Small Iron Particles in a Copper Matrix.

The shape memory effect and superelasticity in two-phase polycrystalline α/β brasses

The shape memory effect (SME), superelasticity (SE), and cyclic deformation behavior of two-phase α/β brasses have been investigated at various temperatures, using tensile tests andin situ optical microscopic observations. The morphology and characteristics of the (thermoelastic) martensitic transformation and the mechanism of the SME are similar to those for single-phase β-brass, but the amount of irrecoverable strain is larger in the two-phase alloys due to plastic deformation of the α particles. After unloading and heating, the slipbands in the discrete a particles remain, whereas the martensite almost disappears; thus, the higher the volume fraction of α particles, the larger the amount of irrecoverable strain. The deformation behavior of alloy A at temperatures above the martensite start (Ms) temperature (with 26 pct α phase) is dominated by deformation of the α phase, so complete SE cannot be obtained after cyclic deformation, both at room temperature and at -40 °C. While in alloy B (containing 15 pct α phase), the deformation behavior is dominated by the formation of stress-induced martensite (SIM). The α particles are deformed before SIM formation on loading at room temperature, but on the contrary, SIM forms before the α particles are deformed on loading at -40 °C (>Ms). Complete SE can be obtained in alloy B after cyclic deformation at room temperature to a given strain but does not occur at -40 °C because the a particles are deformed along with the growth of pre-existing SIM under larger strain during cycling at this temperature.

The Gibbs free energies of thermal and stress-induced martensite formation in Cu-Zn-Al single crystal shape-memory alloys

The free energies of the martensitic transformation of both thermal and mechanical origin have been evaluated in Cu-Zn-Al single crystals. The free energy variation has been studied in relation to the applied stress for stress-induced martensite formation.

Deformation Twinning, Slip, Martensite Formation and Crack Inhibition in the B2-Type Zr50Pd35Ru15 Alloy

AbstractEnhanced room temperature toughness of the Zr50Pd35Ru15B2 phase alloy was found to be a result of the activation of an additional deformation mode besides the b=[001] dislocation slip mode - {114}-type mechanical twinning. The twinning is a true one, i.e. there is no change in the ordered crystal structure. Another additional mode of plastic deformation, expected for more Pd rich alloys, is the formation of stress-induced martensite. The martensite was found to have a CrBtype structure.

Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys

Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered fee γ-Ni, which contains precipitates of γ′-Ni3Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains. The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the high ductility.

Deformation of a NI-AL-FE Gamma/Beta Alloy

ABSTRACTRoom temperature tensile tests were performed on annealed melt-spun ribbons of a 50Ni-20Al-30Fe alloy. The ribbons were found to possess a duplex structure consisting of fcc γ and B2 β grains and exhibited tensile elongations in excess of 10% while still maintaining good strength. The tested specimens were found to contain high dislocation densities in both the γ and β grains, with no indications of stress-induced martensite formation. Dislocation analysis revealed that the vast majority of dislocations in the β have <100> Burgers vectors; however, <111> dislocations were also observed. Slip transfer was often facilitated by specific orientation relationships between the γ and β grains.

The effect of thermal and stress cycling on thermoelastic martensite formation in Cu-Al-Mn alloys

The effect of thermal cycling on the thermoelastic martensitic reaction and stress cycling on stress induced martensite formation and pseudoelasticity have been studied in alloys in the Cu-Al-Mn system. The martensite to high temperature phase start and finish transformation temperatures,As andAf, of alloys quenched from 850‡ C were considerably higher than after repeated thermal cycling. The excess vacancy concentration present after quenching is postulated to enhance low temperature ordering reactions in these alloys through defect assisted diffusional mechanisms and so stabilize the martensite leading to higherAs andAf temperatures. Stress cycling of aΒ phase alloy increased pseudoelastic recovery and decreased the stress necessary to form stress induced martensite. Ageing specimens at room temperature after stress cycling was found to increase considerably the stress required to form stress induced martensite on subsequent stress cycling. Both pseudoelastic and shape memory recovery were observed in a martensitic alloy.

Elastic constants and lattice stability of beta -Cu-Zn-Al alloys

Measurements of a set of third-order elastic constants of a BCC Cu-Zn-Al alloy as functions of temperature allow the study of the combined effect of temperature and strain on the elastic constants. The influence of a shear strain on the lattice stability is examined at different temperatures and the results are integrated in the localised soft-mode theory of martensite. The influence of a uniaxial compression on the shear constants C' and C44 and on the elastic anisotropy of the BCC matrix is calculated and it is shown that at the critical conditions for stress-induced martensite formation, the resistance against a (011)(011)-type shear decreases and the anisotropy increases with increasing temperature. This change in anisotropy of the matrix can influence the local stability around defects explaining in a qualitative way the occurrence of stress-induced martensite.

Room temperature age-hardening in a ? titanium alloy

A metastableβ titanium alloy containing 10 wt % Zr and 12 wt % V has been found to undergo a substantial age-hardening reaction at temperatures as low as 20° C. The reaction involves continued growth of “athermal”ω-phase particles produced during water quenching from theβ-phase field. The morphology of the as-quenchedω is retained, implying the absence of long-range diffusion during ageing: this is consistent with the low value of the activation energy measured (93 kJ kg mol−1). It is suggested that theω growth is caused by unpinning ofω/β interfaces as a result of the short-range motion of interstitials present in the alloy. The age-hardening produces a severe loss in tensile ductility and inhibition of stress-induced martensite formation.

THE THERMAL EFFECT IN PSEUDOELASTIC ALLOYS

Temperature pulses of up to 15°C have been observed in single crystals of pseudoelastic alloys and are associated with the formation of stress-induced martensite. Detailed shapes of the temperature pulses are presented for three types of transformation : β1-β'1 CuAlNi, β'2-β'2 CuZnSn and β1-γ'1 CuAlNi.

THE INFLUENCE OF STRAIN-RATE, AMPLITUDE AND TEMPERATURE ON THE HYSTERESIS OF A PSEUDOELASTIC Cu-Zn-Al SINGLE CRYSTAL

The hysteresis loop, described during the formation of stressinduced pseudoelastic martensite in a Cu-Zn-Al betaphase single crystal, was studied as a function of strain-rate, deformation amplitude and temperature. The σp→m, the stress at which the transformation starts at a given constant temperature, is strain-rate independent but the hysteresis described by the stress-strain curve shows a maximum at intermediate strain-rates (3.3 x 10-3 sec-1). For very low strain rates (3.3 x 10-5 sec-1) the relative energy-loss (ƊW/W) was independent of amplitude. The amplitude-dependence was the strongest at intermediate strain-rates. The absolute hysteresis, i.e. energy loss, measured at constant strain-rate seems to be independent of temperature in the region (Ms+60, Ms+110). The change in hysteresis as a function of strain-rate can be attributed to the heating and cooling of the sample due to the exothermic character of the beta-martensite transformation and the reverse endothermic transformation. At very low strain-rates the transformation occurs isothermally so that nearly no hysteresis is found. Only at very high strain-rates the sample is deformed adiabatically.

The thermal effect due to stress-induced martensite formation in Β-CuAlNi single crystals

The thermal effect associated with the formation and reversion of stress-inducedβ′ martensite has been studied in single crystals ofβ1-CuAlNi. The maximum temperature increase for theβ1 →β1′ transformation in a specimen at room temperature ranged from 1.2°C at a strain-rate of 0.025 min−1 to 14°C at a strain-rate of 25 min−1. The large temperature increase is due to lower heat losses at the high strain-rates and a closer approach to adiabatic conditions. The thermal effect for theβ1 →β1 transition was somewhat larger and believed to be due to the different directions of motion of the single transformation interface. A method has been developed which correlates the thermal effect at infinite strain-rate with the entropy change of the transformation resulting in a value of δSβ’1 Β1 = − 1.20 J/mol · K. This result is in good agreement with the entropy changes of −1.18 ± 0.17 J/mol · K calculated from the Clausius-Clapeyron equation. The variation of the thermal effect with specimen temperature (d|δT|/δT) was 0.04 ± 0.01, and could be explained by assuming a constant entropy change for the transformation at different temperatures.

The fatigue of pseudoelastic single crystals of α-CuAINi

The fatigue life of single crystal α-CuAINi has been studied on pseudoelastic cyclic loading. In general the fatigue life was quite poor with most specimens having less than 3000 cycles to failure. The fatigue life decreased significantly with increasing stress level. However, the fatigue failure was due primarily to stress-induced martensite formation, since if the stress level on cycling was not sufficient to form martensite, the specimen did not fail. The fatigue life appeared to be largely independent of percent strain, crystal orientation and environment of testing. Surface preparation, however, was very important with an electropolished specimen having a much longer life than an abraded one. Fatigue cracks grew only in the final few hundred cycles of the life of the specimen. Cracks initially grew in the direction of growth of the stress-induced martensite, approximately at 45 deg to the tensile axis. Final failure was due to brittle fracture caused by stress concentration at the tip of the fatigue crack, the α-CuAlNi being very brittle. Scanning electron micrographs of the fracture showed no fatigue striations but rather river markings spreading out from the point of nucleation of the fatigue crack, characteristic of a brittle material.

Transformation and deformation studies of some Zr (CoNi) alloys

Alloys of the type Zr 50Co 50− x Ni x ( x < 50) have been examined using transmission electron microscopy and X-ray pole figures in order to define the mechanisms responsible for the observed ductility of these alloys. It has been shown that strains as high as 70% reduction in thickness by rolling can be carried out at room temperature and that the only dislocations involved are of the type b = 〈100〉 . The formation of stress-induced martensite also occurs during deformation and this together with dislocations of b = 〈100〉 allows a general shape change. The formation of the stress-induced martensite is only partially reversible because of subsequent plastic deformation but the influence of this transformation on load cycling can be detected.

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 aboveAf 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 belowMf can be restored by transforming back to the original parent phase by heating toAf (shape memory) or alternatively, can be recovered belowMf by applying a small stress of opposite sign. Martensite deformed belowMf with the same stress maintained while heating persists aboveAf, 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.

Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveMd and testing betweenMd andMs. Mechanical working aboveMd had a negligible effect on the yield strength betweenMd andMs when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.

Stress-induced martensite formation in Cu−Al−Ni alloys

A study has been made of superelasticity and the strain-memory effect in Cu−Al−Ni alloys in the composition range 14 wt pct Al and 2 to 3 wt pct Ni. These alloys have a bcc structure on quenching and show a low temperature martensitic transformation which is responsible for both the superelastic and strain-memory effects. Tests on both single and polycrystalline specimens showed that the maximum superelasticity occurred close toA s. At higher temperatures the effect gradually decreased, whilst at lower temperatures it decreased very quickly. The magnitude of the effect was large in single crystal specimens (>5.8 pct), but small in polycrystal specimens (<1.5 pct). The superelastic effect was caused by stress-induced martensite (SIM). Two types of SIM were observed; thin plates of thermoelastic martensite which were always reversible, and wide plates of burst-type martensite. This burst-type martensite was responsible for the major portion of SIM, and whether it was reversible or not on removal of the stress controlled the amount of superelasticity observed. The strain-memory effect occurred on deformation either in the martensitic state (temperature <M f) or in the temperature range where the martensite once formed was stable (temperature close toM s). Deformation caused reorientation of the martensite plates and when the specimen was heated, the martensite disappeared and the specimen reverted back to its original shape. This effect was explained on the basis of development of martensite plates of favorable orientation on stressing.