A study of electrical resistivity,internal friction and shear modulus on an aged Ti49Ni51 alloy

The 400°C aged Ti₄₉Ni₅₁ alloy can exhibit the transformation sequence of B2 →r premartensite R-phase →r martensite. In the early aging stage, only the premartensitic transformation is observed due to the M_s point being deeply depressed by the coherent stress of Ti₁₁Ni₁₄ precipitates. In the later aging stage, internal friction peaks associated with premartensitic and martensitic transformations are all observed on both heating and cooling. The sharp peaks associated premartensitic transformation on heating is believed to be related to the “bias” effect of the coherent stress induced by the Ti₁₁Ni₁₄ precipitates. The serrations of internal friction appearing significantly in the temperature around −30 to −80°C are found to be caused by the stress induced accomodation of R-phase or martensite variants, and are not associated with the transformation. The Ti₁₁Ni₁₄ precipitates can enhance the amount of martensite formed by unit of temperature or time during the martensitic transformation.     

On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys

Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19′ when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19′ transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19′) and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes.     

Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

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     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

Effect of aging on shape memory behavior of Ti-51.3 At. pct ni thin films

Thin films of Ti-51.3 at. pct Ni were prepared by sputtering. The sputter-deposited thin films were solution treated at 973 K for 1 hour and then aged at various temperatures between 573 and 773 K for 3 different times of 1, 10, and 100 hours. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The aging effects on the shape memory characteristics, such as the critical stress for inducing slip deformation, the maximum recoverable strain, and the R phase and martensitic transformation temperatures, were discussed based on transmission electron microscopic (TEM) observation of the microstructure in the age-treated thin films. In all the age-treated thin films, the presence of Ti₃Ni₄ precipitates was confirmed. When the precipitate diameter was less than 100 nm, the shape memory characteristics were very sensitive to the microstructure. The aging effects on the shape memory characteristics of the Ni-rich Ti-Ni thin films were found to be almost consistent with those reported in bulk specimens, and, thus, the shape memory behavior of the sputter-deposited thin films of Ni-rich Ti-Ni can be controlled in the same manner as that of bulk specimens.     

A study of B2↔B19↔B19′ two-stage martensitic transformation in a Ti50Ni40Cu10 alloy

The B2↔B19↔B19′ two-stage martensitic transformation in a Ti₅₀Ni₄₀Cu₁₀ alloy has been investigated by electrical resistivity, DSC, X-ray diffraction and internal friction measurements. The shear modulus of B19 martensite has an unusually low value over a broad temperature range between the two shear modulus minima. The B2↔B19 transformation is thus proposed to proceed under the condition of deep shear modulus softening. X-ray diffraction results show that the B19↔B19′ is an incomplete transformation and that the monoclinic angle β of B19′ martensite will increasing with decreasing temperature. This indicates that the B19↔B19′ transformation has the characteristic of the continuously monoclinic distortion of B19′ martensite, which is similar to that of the continuously rhombohedral distortion of R-phase. The opposite behavior observed in electrical resistivity and DSC measurements for B2↔B19 and B19↔B19′ transformations is also discussed.     

Aging behavior of the Ti-29Nb-13Ta-4.6Zr new beta alloy for medical implants

The aging behavior of the Ti-29Nb-13Ta-4.6Zr alloy quenched from 1033 K was investigated by employing electrical resistivity (ρ) and Vickers hardness (HV) measurements, optical microscopy (OM), and X-ray diffraction (XRD). Upon aging at 573 K, the ρ value at room- and liquid-nitrogen temperatures initially increased to 120 and 30 ks, respectively, and decreased with continued aging time. The HV value started to increase from 0.06 ks and the isothermal ω phase was identified at 300 ks of aging by XRD. On aging at 773 K, an incubation period of 6 ks was observed for the ρ change at both temperatures, following which ρ decreased. An incubation period for the HV change was extended up to 12 ks, and then HV increased. Precipitated α was observed and identified by OM and XRD, respectively, whereas no reflections of isothermal ω phase were identified during the whole aging time at 773 K. The upper-limit temperature of isothermal ω precipitation is situated at around 673 K.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Morphology and aging of the martensite induced by cathodic hydrogen charging of high-carbon austenitic steels

The transformation by cathodic hydrogen charging of a 1.95 wt pct C austenite is studied by X-ray diffraction and Mössbauer spectroscopy, and the morphology of the induced martensite, analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is compared to that of the same alloy quenched at 78 K. Optimal charging current density is found to be at 2200 A/m₂, and charging time is varied. Martensite crystals have three different shapes: long thin plates, “spear-like,” and “palm frond.” The habit plane is essentially {225}_γ, and the serratures of the palm frond are obtained by (111)_γ accommodation slip. The state of aging of the hydrogen-induced martensite at room temperature is similar to that of a martensite obtained by quench at 78 K and subsequently aged at much higher temperature, e.g., 97 °C for 1 hour, and displays the “tweed-like” or “salt and pepper” morphology of Génin’s Fe₆C “carbon extended multiplets” which transform in situ into ε- or η-carbide precipitates at the same tem-perature, i.e. 117 °C, for 1 hour. The role of hydrogen in catalyzing the martensitic transfor-mation and affecting the aging kinetics is explored.     

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.     

Ni-Ga-Fe-Co铁磁形状记忆合金的磁性转变和马氏体相变

Ferromagnetic shape memory alloys (FSMAs) such as NiMnGa, FePd and FePt are attractive as a new magnetic actuator material. They show a large magnetic-field-induced strain of 3% - 9% due to the variant rearrangement. Recently, the present authors have reported that in the Ni-Ga-Fe alloy the martensitic transformationfrom the B2 and/or the L21 structures into a seven-layer or five-layer modulated structure occurs upon cooling. In this alloy system, however, it is impossible to obtain a martensite phase at RT with a Curie temperature (To) higher than 100℃. In this work, the effects of substitution of Co for Ni on the martensitic and magnetic transformations, crystal structures and phase equilibria in Ni-Ca-Fe alloys were studied. Ni-Ga-Fe-Co alloys were prepared by induction melting under an argon atmosphere. Small pieces of specimens were taken from the ingot and homogenized at 1433 K for 24 h followed by quenching in water. The obtained specimens were aged at 773 K for 24 h and then quenched. The compositions of each phase were determined by energy dispersion X-ray spectroscopy (El)X). The martensitic transformation temperatures and Tc were measured by differential scanning calorimetry (DSC) and vibrating sample magnetometer (VSM) measurement. The crystal structure of martensite phase was observed by X-ray diffractmeter (XRD) and transmission electron microscope (TEM). The Curie temperature Tc was increased with increasing Co content while the martensitic transformation temperature slightly decreased. In the Ni(54-x) Ga27 Fe19 Cox, Tc increases from 303 K to 408 K with increasing CO content from x=0 to x=6. The crystal structure of the martensite phase and the phase equiribria in the Ni-Fe-Ga-Co alloys will be also presented.     

Formation of hcp martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic implant alloy

The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and 950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation. The relationships between transformation, temperature, and time for this phase transition were determined using two different techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition, it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation. Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase. This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and leads to the slower transformation rates observed in water-quenched material.     

Reverse Shape Memory Effect Related to α → γ Transformation in a Fe-Mn-Al-Ni Shape Memory Alloy

In this study, we investigated the shape memory behavior and phase transformations of solution-treated Fe_43.61Mn_34.74Al_13.38Ni_8.27 alloy between room temperature and 1173 K (900 °C). This alloy exhibits the reverse shape memory effect resulting from the phase transformation of α (bcc) → γ (fcc) between 673 K and 1073 K (400 °C and 800 °C) in addition to the shape memory effect resulting from the martensitic reverse transformation of γ′ (fcc) → α (bcc) below 673 K (400 °C). There is a high density of hairpin-shaped dislocations in the α phase undergoing the martensitic reverse transformation of γ′ → α. The lath γ phase, which preferentially nucleates and grows in the reversed α phase, has the same crystal orientation with the reverse-transformed γ′ martensite. However, the vermiculate γ phase, which is precipitated in the α phase between lath γ phase, has different crystal orientations. The lath γ phase is beneficial to attaining better reverse shape memory effect than the vermiculate γ phase.     

On the Stability of Reversely Formed Austenite and Related Mechanism of Transformation in an Fe-Ni-Mn Martensitic Steel Aided by Electron Backscattering Diffraction and Atom Probe Tomography

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase (α + γ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e., controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.     

Lower bainite with midrib in hypereutectoid steels

The isothermal transformations in five hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Two types of lower bainite and a thin plate isothermal martensite were observed. One of the lower bainites was the conventional lower bainite (CLB) formed at the high temperature range of 623 to 473 K, and the other was the newly named “lower bainite with midrib” (LBm) formed at the lower temperature range of 473 to 423 K. The thin plate isothermal martensite (TIM) was also observed below 373 K. This paper brought LBm into focus. Arrhenius plots (transformation ratevs l/T) for each steel revealed an abrupt change in kinetics at the temperature range between 483 and 443 K. This change was considered to correspond to the transition from CLB to LBm. The following two-stage process for the LBm formation is proposed: at the first stage a TIM is formed, which constitutes a midrib of LBm, and secondly the bainitic decomposition of austenite at TIM/austenite interfaces takes place. That is, an LBm is a composite of isothermal martensite and lower bainite.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synchrotron Characterization of Transformation Sequences in TiNi-Based Shape Memory Alloys during Thermal Cycling

In-situ synchrotron radiation has been used to provide direct analysis of the transformation sequences in TiNi-based shape memory alloys during thermal cycling. The high resolution, narrow peak width Debye–Scherrer diffraction spectra enabled positive identification and quantification of the phase transformation sequences, which is not possible through normal laboratory studies. The results facilitate a clearer understanding of the development and influence of intermediate phases such as R or B19 on sequential martensitic transformations. Ti_50.2Ni_49.8 transformed predominately via a single-step B2 ↔ B19′ transformation, although evidence of the R phase was found during cooling in every cycle. The martensitic start temperature was depressed by ~0.6 °C per cycle, while the R-phase start temperature was found to be unaffected. Ti₅₀Ni₄₁Cu₉ transformed through a two-step B2 ↔ B19 ↔ B19′ sequence, with the B2 → B19 transformation reaching completion prior to the formation of any B19′. The transformation temperatures of Ti₅₀Ni₄₁Cu₉ were found to be insensitive to thermal cycling, remaining constant over the studied cycle range.     

The R-phase transition and associated shape memory mechanism in Ti-Ni single crystals

The shape memory mechanism associated with the R-phase transition was investigated using age-treated Ti-Ni single crystals. Tensile tests and in situ optical microscopic observations were carried out. Four distinct variants of the R-phase were found, and these could be correlated with each other by twinning relationships. Six twinning planes in each variant were determined using two-surface analysis and were found to consist of three {110} and three {100} planes. A specific morphology consisting of the four types R-phase variants was formed upon cooling below the T_R point. The combination of the four variants produces a zero macroscopic strain, indicating that the specific morphology is the result of self-accommodation. By stressing, the most favorable R-phase variant was formed from the remaining variants by twinning deformation. Upon heating after unloading, the specimen showed a shape recovery which is a function of temperature or rhombohedral angle of the R-phase. Most of the shape recovery was brought about by a change in the lattice parameter of the R-phase, but not by the reverse-transition. The manner of the shape recovery is quite different from that associated with the reverse martensitic transformation. On the basis of the results obtained, the mechanism of the shape memory associated with the R-phase transition in alloys of the Ti-Ni type has been clarified.