<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.     

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.     

Deformation of NiTiCu shape memory single crystals in compression

Single-crystal orientations of NiTi10Cu alloys were studied under incremental, cyclic compression conditions to establish the pseudoelastic and shape memory response of this class of alloys. This material exhibits a two-step transformation involving cubic to orthorhombic martensite (B2 → B19) followed by orthorhombic to monoclinic martensite (B19 → B19′). The transformation parameters (shear magnitudes and directions for habit and twin planes) were determined associated with the B2 → B19 transformation. The growth of monoclinic martensite correspondent variant pairs (CVPs) emanating from the orthorhombic structure was also analyzed. The transformation strain for the B2 → B19 case was orientation dependent and lower than the B19 → B19′ transformation in compression for all orientations except those near the [001] pole. The experimental results show that the critical transformation stress is orientation dependent and is in the range 30 to 58 MPa. Orientations that exhibit lower transformation stress (or high resolved shear stress factors, [100] and [012]) produce higher recoverable strains (as high as 4 pct), while other orientations ([011], [111], and [123]) with lower resolved shear stress factors result in recoverable strains less than 3 pct. At higher strains, inelastic deformation develops, limiting recoverability. The recoverable strains are lower than the theoretical values for two main reasons: the transformation is curtailed first by austenite slip and subsequently by martensite slip, and the orthorhombic structure does not fully transform to the monoclinic martensite.     

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.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

The thermodynamics of stress-induced martensites in Cu Al Ni alloys

Stress-induced martensitic transformations have been studied in Β₁ Cu Al Ni single crystals in which two martensite crystal structures can form, Β _i ^′ and γ′. By straining specimens at one temperature and releasing the strain at either the same temperature or a different temperature, stresses corresponding to the transitions Β₁ ⇌ Β _i ^′ ,Β ₁ ⇌ γ′,Β _i ^′ ⇌ γ′ could all be measured. This enabled a quantitative stress-temperature diagram to be drawn, giving the stability ranges of the Β₁,Β _i ^′ and γ′ phases. The slope of the stress-temperature lines separating the different phases enabled the value of the entropy changes for the transformations to be calculated. This was very small for theΒ _i ^′ → γ′ transformation (0.08 J/mole K) and much larger for the Β₁ →Β _i ^′ and Β₁ → γ′ transformations (-1.21 and -1.4 J/mole K, respectively). The hysteresis between the forward and reverse transformations enabled evaluation of the critical free energy for transformation. This was small for the Β₁ → Β _i ^′ transformation (-2.9 J/mole), and large for the Β₁ → γ′ and Β _i ^′ → γ′ transformations (-28 and -29 J/mole respectively). Formerly Post Doctoral Fellow, Department of Metallurgy, University of British Columbia     

On the effect of stress on nucleation and growth of precipitates in an Al-Cu-Mg-Ag alloy

A study has been made of the effect of an externally applied tensile stress on Ω and Θ′ precipitate nucleation and growth in an Al-Cu-Mg-Ag alloy and a binary Al-Cu alloy which was used as a model system. Both solutionized and solutionized and aged conditions were studied. The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). The volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutionized samples aged under stress, precipitation occurs preferentially parallel to the stress axis. A threshold stress has to be exceeded before this effect can be observed. The critical stress for influencing the precipitate habit plane is between 120 and 140 MPa for Ω and between 16 and 19 MPa for Θ′ for the aging temperature of 160 °C. The major effect of the applied stress is on the nucleation process. The results are discussed in terms of the role of the lattice misfit between the matrix and the precipitate nucleus.     

The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel angular extrusion

A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.     

Thermodynamic study of the low-temperature phase B19′ and the martensitic transformation in near-equiatomic Ti-Ni shape memory alloys

Experimental information on the transformation temperatures and the thermodynamic properties of the near-equiatomic TiNi alloys is analyzed. Special attention is paid to the estimation of T ₀ temperature from experimental M _s and A _f temperatures. The properties of the TiNi low-temperature phase (B19′) are evaluated from selected experimental data by using a two-sublattice model. The Ti-Ni phase diagram including the B19′ phase is then calculated. It reveals that the equiatomic TiNi parent phase (B2) remains stable from high temperatures until 370 K, and then the B19′ phase becomes thermodynamically stable as a linear compound under 370 K. Thermodynamic quantities such as the T ₀ temperature and transformation enthalpy are also calculated and compared with experimental data. Further, the M _s temperature is predicted and compared with data from different sources.     

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.     

Decomposition of the γ solid solution in a nickel-based cast alloy at a high uniform pressure

The formation of γ′ particles in an as-cast nickel alloy at barothermal treatment temperatures of 1235, 1260, and 1320°C and a pressure of 175 MPa for an action time of 2 h (isobar-isothermal holding (IIH)) is studied by quantitative metallography using optical and electron microscopy and image processing computer programs. An analysis of images of the dendritic structure of the alloy, which is formed by the morphology and sizes of γ′ particles, demonstrates an increase in the degree of homogeneity of the alloy with the IIH temperature. The concentration of γ′ particles is determined in the initial alloy and after barothermal treatment (BTT) at temperatures of 1235, 1260, and 1320°C, and the volume fraction of the γ′ phase is found to significantly decrease as the IIH temperature increases. An activation mechanism is suggested for the formation of the nucleation centers of γ′ particles in the γ solid solution, and the activation energy of the dissolution/coalescence of γ′ precipitates is determined. The fraction of nonequilibrium γ′ particles is determined in the material in the initial state and after BTT at three IIH temperatures. The precipitation of γ′ particles is characterized by a bimodal character with the formation of nanoparticles in the initial material and upon BTT at temperatures of 1235 and 1260°C.     

Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti-4Al-4Mo-2Sn-0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite (α′) is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (α″) on rapid cooling. Finally, below 1123 K, the β phase is stable—no martensitic transformation occurs on rapid cooling. This transition from α′ → α _primary + (α′ + β _retained) → α _primary + (α″ + β _retained) → α _primary + β _metastable + ω, with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature β phase.     

Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti−4Al−4Mo−2Sn−0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite α′ is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (α″) on rapid cooling. Finally, below 1123 K, the β phase is stable—no martensitic transformation occurs on rapid cooling. this transition from with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature β phase.     

Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel

In the previous paper, it was reported that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite is produced, results in the improvement of low temperature mechanical properties (287 to 123 K). The purpose of this paper is to study whether above new heat treatment will still be effective in commercial practice for improving low temperature mechanical properties of the ultrahigh strength steel when applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel which is economical because it lacks the expensive nickel component (AISI 4140 type). At and above 203 K this new heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved the strength, tensile ductility, and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel. At and above 203 K the new heat treatment also produced superior fracture ductility and notch toughness results at similar strength levels as compared to those obtained by usingγ α′ repetitive heat treatment for the same steel. However, the new heat treatment remarkably decreased fracture ductility and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel below 203 K, and thus no significant improvement in the mechanical properties was noticeable as compared with the properties produced by the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment. This contrasts with the fact that the new heat treatment, as compared with the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment, dramatically improved the notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel, providing a better combination of strength and ductility throughout the 287 to 123 K temperature range. The difference in the observed mechanical properties between the above two ultrahigh strength steels is discussed on the basis of the effect of nickel content, fracture profile, and so forth.     

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.     

Microstructures and mechanical properties of Ni3Al alloyed with iron additions

The alloying behavior of iron in B-doped Ni₃Al was studied using optical and transmission electron microscopy for microstructural analysis and tensile testing for mechanical property evaluation. The aluminide dissolves less than 15 at. pct iron. At iron levels of 15 pct, depending on the aluminum level, formation of both transformed B2 phase (β^′) and disordered fee phase (γ) is observed. The phase relationships in the aluminide containing 10 pct iron were studied in detail by quenching from temperatures below 1370 °C. Tensile properties of nickel-iron aluminides were determined as functions of iron content and test temperature. The tensile results are discussed in terms of solid-solution hardening, atomic size misfit, and precipitation of γ, β^′, and carbides.     

Deformation and fracture behavior of a directionally solidified β/γ′ Ni-30 At. pct Al alloy

The effect of a ductile γ′-Ni₃Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni₃Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately 850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties.     

Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents from 10 to 26 at. pct were investigated by means of X-ray diffraction analysis upon aging in the temperature range from 373 to 473 K. It was found that precipitation of α″-Fe₁₆N₂ below 443 K does not only occur upon aging of supersaturated α (ferrite) and α′ (martensite), but also upon transformation of γ′-Fe₄N_1-z and ɛ-Fe₂N_1-x (<20 at. pct N). No α″ was observed to develop upon aging of γ(austenite). Therefore, it is proposed that γ′ is a stable phase at temperatures down to (at least) 373 K. Phase formation upon annealing at low temperatures is apparently governed by the (difficult) nucleation and (slow) growth of new Fe-N phases: α″ forms as a precursor for α because of slow nitrogen diffusion, and nitrogen-enriched ɛ develops as a precursor for γ′ because of a nucleation barrier.     

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