Effect of alloying elements on martensitic transformation in the binary NiAl(β) phase alloys

The characteristics of the B2(β) to L1₀(β′) martensitic transformation in NiAl base alloys containing a small amount of third elements have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It is found that in addition to the normal Ll₀ (3R) martensite, the 7R martensite is also present in the ternary alloys containing Ti, Mo, Ag, Ta, or Zr. While the addition of third elements X (X: Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Ta, W, and Si) to the binary Ni₆₄Al₃₆ alloy stabilizes the parentβ phase, thereby lowering the M_s temperature, addition of third elements such as Co, Cu, or Ag destabilizes theβ phase, increasing the M_s temperature. The occurrence of the 7R martensite structure is attributed to solid solution hardening arising from the difference in atomic size between Ni and Al and the third elements added. The variation in M_s temperature with third element additions is primarily ascribed to the difference in lattice stabilities of the bcc and fcc phases of the alloying elements.     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

<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 enhancement of strengthening dislocated martensite

The basis of this work was the investigation of improving the tensile properties of dislocated martensites by dispersion of precipitates in the austeniteprior to the martensite transformation. Two types of precipitation-hardenable austenitic alloys were used. One is based on Fe-22 Ni-4 Mo-0.28 C where the precipitates are Mo₂C and are obtained by ausforming and aging, and the other is Fe-28 Ni-2 Ti where the precipitates are the coherent fccγ’ (Ni₃Ti) ordered phase obtained by ausaging. After the austenitic dispersion treatment both alloys were transformed to martensite by quenching to liquid nitrogen and the properties measured and compared to martensites obtained by conventional heat treatment (i.e. no precipitates in austenite). The results show that prior dispersions increase the strength of martensite and this is interpreted as being due to an increase in dislocation density resulting from dislocation multiplication at the particles during the γ →M _s transformation. In addition, the stabilities of the austenitic alloys are such that upon certain aging treatments, the alloys transform partially to martensite (due to precipitation) and “composite” materials are obtained whose strength depends on the volume fraction and yield strengths of the phases present. Formerly Graduate Student, Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

X-ray study of phase transformations in martensitic Ni-Al alloys

The formation of the Ni₅Al₃ and Ni₂Al phases in Ni-Al alloys with L1o ↔ B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni₅Al₃ can form both the L1o and B2 structures, but the kinetics of L1o → Ni₅Al₃ and B2 → Ni₅Al₃ reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni₂Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni₂Al retards the Ni₅Al₃ formation, so preaging in the B2 region affects the kinetics of the L1o → Ni₅Al₃ reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.     

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.     

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.     

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

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

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

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

The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, δ-ferrite, and the (Fe,Mn)₃Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe₅Ni₃Si₂ type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the γ-ɛ-α′ transformation sequence.     

Thermoelastic martensite and shape memory effect in ductile Cu-Al-Mn alloys

Ductile shape memory (SM) alloys of the Cu-AI-Mn system have been developed by controlling the degree of order in the β phase. Additions of Mn to the binary Cu-Al alloy stabilize the β phase and widen the single-phase region to lower temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disordered A2 structure or the ordered L2₁ structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically transforms to the disordered Al phase with a high density of twins. The martensite phase formed from the ordered L2₁ phase has the 18R structure. The SM effect accompanies both the A2 → Al and L2₁ → 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without cracking, and 80 to 90 pct recovery from bend test in the martensitic condition. Experimental results on the microstructure, crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-AI-Mn alloys are presented and discussed.     

Mechanical alloying of brittle materials

Mechanical alloying by high energy ball milling has been observed in systems with nominally brittle components. The phases formed by mechanical alloying of brittle components include solid solutions (Si + Ge → SiGe solid solution), intermetallic compounds (Mn + Bi → MnBi), and amorphous alloys (NiZr₂ + Ni₁₁Zr₉ → amorphous Ni₅₀Zr₅₀). A key feature of possible mechanisms for mechanical alloying of brittle components is the temperature of the powders during milling. Experiments and a computer model of the kinetics of mechanical alloying were carried out in order to esti-mate the temperature effect. Temperature rises in typical powder alloys during milling in a SPEX mill were estimated to be ≤350 K using the kinetic parameters determined from the computer model. The tempering response of fresh martensite in an Fe-1.2 wt pct C alloy during milling was consistent with the maximum results of the computer model, yielding temperatures in the pow-ders of ≤575 Ki.e., ΔT ≤ 300 K). Thermal activation was required for mechanical alloying of Si and Ge powder. No alloying occurred when the milling vial was cooled by liquid nitrogen. The pos-sible mechanisms responsible for material transfer during mechanical alloying of brittle components are considered.     

Influence of aging on transformation characteristics in shape memory CuZnAl alloys

Shape memory alloys exhibit superelasticity when they are deformed in a temperature range where the thermoelastic martensite forms on application of a strain. The martensite persists upon removal of the applied strain, and the alloy recovers the original shape on heating over the reverse-transformation temperature after removing the strain. The β-phase CuZnAl alloys have β-type superlattice in the parent case, and M9R or M18R martensites occur on quenching the alloys from the homogenization temperature. The basal plane of martensite is exposed to hexagonal distortion with martensitic transformation as well as the monoclinic distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences in atom sizes in lattice points. These pairs have a great importance as ordering criteria and satisfy a special relation between Miller indices. The present text reports the variation of the differences in interplane spacings (Δd) between some selected planes upon the further aging at room temperature at which alloys are fully martensitic. The decrease of Δd during the aging implies that the monoclinic distortion decreases. The mass increases are caused by the oxidation upon heating the alloys at high temperatures close to the betatizing temperature at free atmosphere.     

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.     

Enhanced strengthening of a spinodal fe-ni-cu alloy by martensitic transformation

A 61 at. Fe-20 at. Ni-19 at. Cu alloy has been designed which can be subsequently transformed to martensite following spinodal heat treatment (aging at 850°C). Morphological changes were followed by transmission electron microscopy.M _s temperatures were estimated by an electrical resistivity technique and microhardness measurements were done after each heat treatment. TheM _s temperature increased rapidly, then were unchanged as particle coarsening started and wavelength λ increased. The hardness first increased rapidly, then decreased as λ increased. Although the specimens were brittle, the considerable increases in hardness achieved by martensitic transformation compared to the hardness of purely spinodal alloys indicates that this double phase transformation method may be promising for improving the strength of certain alloys.     

Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.     

Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

Shape memory effect and related transformation behavior in FeNiC alloys

Shape memory effect in two representative FeNiC alloys, 31% Ni-0.4% C and 27% Ni-0.8% C, with low Ms temperatures has been studied in detail. The shape memory test was conducted not only on as-austenitized specimens but also on ausformed specimens. The effect of an external load during reverse transformation was also examined. The reverse martensitic transformation behavior related with the shape memory effect was observed in situ, using a high temperature optical microscope. The results are (1) about 50% shape recovery is observed for up to 5% initial tensile strain, while 75–95% shape recovery is obtained for 1–2% initial bending strain. (2) An ausformed specimen shows a better shape memory effect probably owing to the increased austenite strength. (3) The austenite-martensite interface moves backward on heating not only for a plate-type martensite but also for a curved or irregular shaped martensite in a specimen strengthened by ausforming. (4) The shape recovery decreases with increasing an applied load, but even in this case the reversible interface movement occurs.