<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.     

Effect of thermal cycling on the transformation temperatures of TiNi alloys

The effect of thermal cycling on the transformation temperatures was investigated in TiNi alloys which were subjected to various types of thermo-mechanical treatments. In solution-treated specimens with various Ni-contents, the transformation temperatures were found to change with thermal cycling. However, in aged Ni-rich specimens and those annealed at a temperature lower than the recrystallization temperature after cold working, they were found to be constant even after thermal cycling. Transmission electron microscopy revealed that dislocations were introduced by thermal cycling in the solution-treated specimens and their density increased with increasing number of thermal cycles. However, in the specimens with constant transformation temperatures, no substantial change in the microstructures was observed with thermal cycling. On the basis of these results, it is concluded that the shift of the transformation temperatures with thermal cycling is due to the introduction of dislocations, but not due to any ageing effect nor due to a change in the degree of order during thermal cycling.     

The effect of thermomechanical pretreatment on the allotropic transformation in cobalt

The effect of prior thermomechanical treatment on the allotropic transformation in cobalt was investigated utilizing continuous resistivity measurement, X-ray diffraction, and pole figure determinations. Initially, a significant amount of metastable cubic phase was always present at room temperature, the existence of which was independent of the prior thermomechanical treatment employed. For material fully recrystallized, a pronounced reduction in the amount of metastable cubic phase is observed in cycling through the transformation. A much less pronounced effect was observed on cycling material recovered prior to the recrystallization treatment; however, interpretation of data is less clear due to the presence of texture which was always evident. The anomalously low Residual Resistance Ratios always found in high-purity cobalt is interpreted to be due to the presence of this metastable cubic phase, since cycling resulted in significant improvements in the ratios observed. The results of this investigation are in strong disagreement with those of Bibring and Sebilleau^21–23 on the effects of prior thermomechanical treatment on the allotropic transformation in cobalt.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensitic R transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain = 0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensiticR transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain=0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

冷热循环对Ni─Ti形状记忆合金马氏体相变及形状记忆效应的影响

通过电阻及回复特性的测定,研究了在冷热循环条件下ＮｉＴｉ形状记忆合金记忆效应的稳定性问题。实验结果表明：对不同的ＮｉＴｉ合金固溶处理后,冷热循环对其形状记忆效应均产生影响。但时该合金在再结晶温度下时效处理或退火,冷热循环对其相变温度几乎均不产生影响。同时对产生以上规律的内在机制进行了初步探讨。     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy

The superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy was assessed in the temperature range of 573 to 773 K. The thermomechanical treatment (TMT) comprised of three steps of solution treatment, overaging, and warm rolling. There are large η-phase (MgZn₂) precipitate particles of average size of 1.30 μm in the overaged condition. The warm-rolled alloy undergoes continuous recrystallization at the test temperatures of 573 and 623 K, exhibiting a maximum tensile elongation of 450 pct at 573 K and a strain rate of 8 × 10⁻⁵ s⁻¹. The precipitate particles play a major role in the process of continuous recrystallization. For a given volume fraction of precipitate particles and constant amount of warm rolling (in the course of TMT), an optimum precipitate particle size is expected to maximize the rate of continuous recrystallization and render the finest recrystallized grain size. The warm-rolled alloy undergoes static recrystallization at temperatures above 673 K. The grain growth accompanying the deformation at these test temperatures limits the tensile ductility to a lower value. Irrespective of the test temperature and strain rate, the specimens undergo extensive cavitation when deformed at elevated temperatures.     

Shape memory properties of Ni-Ti based melt-spun ribbons

Shape-memory properties of equiatomic NiTi, Ni₄₅Ti₅₀Cu₅, and Ni₂₅Ti₅₀Cu₂₅ ribbons made by melt spinning have been studied by temperature inducing the martensitic transformation under constant tensile loads. Recoverable strains above 4 pct can be obtained under ∼100 MPa loads for the NiTi and Ni₄₅Ti₅₀Cu₅ ribbons, transforming to B19’ martensite. The B19 martensite is formed in the Ni₂₅Ti₅₀Cu₂₅ ribbon after crystallization, and according to the lowering in transformation strain as Cu content increases, the recoverable strain is close to 2.5 pct for ∼150 MPa load. The transformation temperatures exhibit a linear dependence on the applied stress, which can be quantitatively described by means of a Clausius-Clapeyron type equation. The NiTi and Ni₄₅Ti₅₀Cu₅ ribbons exhibited some degree of two-way shape-memory effect (TWSME) after thermomechanical cycling. Texture analyses performed on the different ribbons allow us to better understand the transformation strains obtained in each ribbon. The amounts of shape-memory effect (SME) and nonrecoverable strain shown by the studied ribbons are of the same order as those already observed in bulk materials, which makes melt spinning an ideal substitute to complicated manufacturing processes if really thin samples are needed. However, applicable stresses in melt-spun ribbons are limited by a relatively “premature” brittle fracture caused by irregularities in ribbon thickness.     

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.     

Superplasticity in a thermomechanically processed High-Mg,Al-Mg alloy

Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10^-3 s^-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al₈Mg₅) and MnAl₆ precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation. Formerly in Materials Group, Mechanical Engineering, Naval Postgraduate School Formerly Graduate Student in Mechanical Engineering, Naval Postgraduate School     

The Effect of Thermomechanical Processing on the Tensile, Fatigue, and Creep Behavior of Magnesium Alloy AM60

Tensile, fatigue, fracture toughness, and creep experiments were performed on a commercially available magnesium-aluminum alloy (AM60) after three processing treatments: (1) as-THIXOMOLDED (as-molded), (2) THIXOMOLDED then thermomechanically processed (TTMP), and (3) THIXOMOLDED then TTMP then annealed (annealed). The TTMP procedure resulted in a significantly reduced grain size and a tensile yield strength greater than twice that of the as-molded material without a debit in elongation to failure (ε _f ). The as-molded material exhibited the lowest strength, while the annealed material exhibited an intermediate strength but the highest ε _f (>1 pct). The TTMP and annealed materials exhibited fracture toughness values almost twice that of the as-molded material. The as-molded material exhibited the lowest fatigue threshold values and the lowest fatigue resistance. The annealed material exhibited the greatest fatigue resistance, and this was suggested to be related to its balance of tensile strength and ductility. The fatigue lives of each material were similar at both room temperature (RT) and 423 K (150 °C). The tensile-creep behavior was evaluated for applied stresses ranging between 20 and 75 MPa and temperatures between 373 and 473 K (100 and 200 °C). During both the fatigue and creep experiments, cracking preferentially occurred at grain boundaries. Overall, the results indicate that thermomechanical processing of AM60 dramatically improves the tensile, fracture toughness, and fatigue behavior, making this alloy attractive for structural applications. The reduced creep resistance after thermomechanical processing offers an opportunity for further research and development.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

Fracture and the formation of sigma phase,M23C6, and austenite from delta-ferrite in an AlSl 304L stainless steel

The decomposition of delta-ferrite and its effects on tensile properties and fracture of a hot-rolled AISI 304L stainless steel plate were studied. Magnetic response measurements of annealed specimens showed that the transformation rate of delta-ferrite was highest at 720 °C. Transformation behavior was characterized by light microscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy on thin foils. The initial transformation of delta-ferrite (δ) to austenite (γ) and a chromium-rich carbide (M₂₃C₆) occurred by a lamellar eutectoid reaction, δ⇄M₂₃C₆ +γ. The extent of the reaction was limited by the low carbon content of the 304L plate, and the numerous, fine M₂₃C₆ particles of the eutectoid structure provide microvoid nucleation sites in tensile specimens annealed at 720 °C for short times. Sigma phase(σ) formed as a result of a second eutectoid reaction,δσ +γ. Brittle fracture associated with the plate-shaped sigma phase of the second eutectoid structure resulted in a significant decrease in reduction of area (RA) in the transverse tensile specimens. The RA for longitudinal specimens was not affected by the formation of sigma phase. Tensile strengths were little affected by delta-ferrite decomposition products in either longitudinal or transverse orientations. Y. Shen, formerly with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is deceased.     

The Kinetics of and the Microstructure Induced by the Recrystallization of Copper

The kinetics of the recrystallization of pure copper was investigated by differential scanning calorimetry (DSC). The associated microstructural change was characterized by electron backscatter diffraction imaging (EBSD), by analyzing deformed specimens before recrystallization and specimens after partial recrystallization and after completed recrystallization. The experimental results acquired by the two methods were compared with each other and discussed in the context of the available body of literature results. The observed kinetics deviate from Johnson–Mehl–Avrami–Kolmogorov (JMAK)-like behavior. The observed grain-area distribution is unusually broad and skewed toward large grains. Comparison with mesoscopic, geometric simulations showed that previously proposed (simple) models fail to correctly describe the microstructure resulting from recrystallization, although they can successfully model the recrystallization kinetics. It was concluded that the experimental results on both the kinetics and the microstructure can be reconciled employing a recrystallization model incorporating ongoing (i.e., beyond time t = 0) nucleation and accounting for the inhomogeneous nature of the deformed material.     

Crystallization of amorphous Fe- P- C alloys

The transformation of amorphous Fe-P-C alloys from the amorphous state to crystalline state was studied by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Compositions included iron and phosphorus rich alloys and alloys with compositions near the eutectic composition. The crystallization of Fe-P-C alloys proceeded by the following processes: for the iron rich alloy, bcc α Fe precipitated first; for the phosphorus rich alloy, Fe₂P and iron carbide precipitated first; for the alloys near the eutectic line, the eutectic reactions dominated, although some α Fe or Fe₂P plus iron carbide crystals were observed. The composition dependence of the transformation can be explained successfully by a free energy model with the help of the phase diagram. From DSC and TEM results, the nucleation rate of the α Fe precipitate decreased with time in the Fe rich alloys according to the recrystallization theory of Avrami. Formerly Research Fellow in the Department of Metallurgy and Materials Science, University of Pennsylvania This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Microstructure-mechanical property relationship to copper alloys with shape memory during thermomechanical treatments

The phase transformations during fabrication and aging after cold deformation in three polychrystalline copper alloys of the Cu-Al-Ni system with shape memory effect (SME) were characterized. Some phase transformations were identified with clear repercussion in their mechanical properties during thermomechanical treatments. Around 430 °C, mutual effects of β-phase recrystallization and precipitation of γ₂ and NiAl phases were observed. Close to 600 °C the dissolution of phase α was observed, beginning transformation into β phase process. Brittle phases such as γ₂ and NiAl began to precipitate during a short exposure time at 380 °C, 585 °C, 600 °C, and 700 °C temperatures. The phase transformations were intensified due to the plastic deformation that acted as a driving force for the diffusion processes. The introduction of chemical elements inhibited the grain growth and increased the structural disorder generating an elevation in the hardness property.     

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.