Effect of thermal cycling on the R-phase and martensitic transformations in a Ti-rich NiTi alloy

The effect of thermal cycling on transformation temperature was studied on a Ti-rich NiTi alloy. The study was carried out by determining the electrical resistance, the internal friction, and the elastic modulus vs temperature. This study shows that the martensite microstructure is modified by the successive cycling transformation. In addition, we established that both the martensite internal friction and the transition peak are sensitive to the transient effect (the vibration frequency lies around 300 Hz). But the major results concern the behavior associated with the R phase occurrence and its evolution. We have stated that the premartensitic phase becomes stable following the diminishment of the beginning of the martensite formation (M _s ). Interrupted cooling has also shown that, contrary to the martensite, the R phase exhibits no hysteretic behavior.     

The effect of thermal cycling on the thermoelastic martensitic transformation in a Cu-Zn-Al alloy

The effect of thermal cycling between the M_fA_f on the thermal hysteresis, temperature dependence, and the morphology of the martensitic transformation in a Cu-Zn-AI alloy was studied. The hysteresis behavior of the cyclic transformation, as characterized by following the change in electrical resistivity accompanying the transformation changed in two respects: First, the M_s and A_f temperature shifted to higher temperatures while the M_f and A_s temperatures remained constant. Second, the amount of martensite undergoing cyclic transformation decreased. These changes appeared to saturate at 300 cycles and could be partly recovered during room temperature aging. The morphology and mode of transformation also changed. During the first few cycles, transformation occurred in two simultaneous ways: (1) as martensite plates which are observed to grow and thicken in a continuous manner and (2) as plates which form in a “burst” process. After 60 cycles no “burst” type transformation occurred during cooling and plate growth and thickening was stepwise. Formerly Visiting Scholar, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY     

Evidence of transformation bursts during thermal cycling of a Pu-Ga alloy

The thermodynamics and kinetics of the fcc (delta) to monoclinic (alpha-prime) phase transformation and its reversion in a plutonium-gallium alloy have been studied using differential scanning calorimetry, resistometry, and dilatometry. Under ambient conditions, the delta phase is metastable in a Pu-2.0 at. pct Ga alloy. Thermal cycling to below the ambient temperature results in a partial transformation to the alpha-prime phase; this transformation is composition-invariant and exhibits martensitic behavior. Because this transformation results in an unusually large 25 vol pct contraction that cannot be fully accommodated by purely elastic adjustments, the transformation mode is expected to involve burst formation of individual alpha-prime particles. However, upon cooling, these individual bursts were not resolved by the above techniques, although signals corresponding to the overall accumulation of many alpha-prime particles were observed. On the other hand, upon heating, signals from differential scanning calorimetry, resistometry, and dilatometry showed a series of discrete changes occurring in periodic increments beginning at approximately 32 °C. These features correspond to the cooperative reversion of many alpha-prime particles to the delta phase; they appear to be the result of an interplay between the autocatalytically driven reversion of a cascade of individual martensite units and self-quenching caused by small changes of temperature or stress accompanying each individual transformation burst. The heat of the delta/alpha-prime transformation is estimated to be about +4 kJ/mol.     

Microstructure and martensitic transformations in a dual-phase α/β Cu-Zn alloy

The martensitic transformations in a dual-phase α/β Cu-Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu-Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α _s ) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α _s and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α _s were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α _s martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α _s martensite, an SIM/α _s lamellar martensite was found in the deformed specimen.     

Manufacture of parts from a titanium alloy under superplasticity conditions

A pseudo-α PT3V titanium alloy with a grain size of about 10 μm is shown to demonstrate all evidence of superplasticity in the temperature range 880–920°C during tensile deformation at a strain rate of 10^–3–10^–2 s^–1: the relative elongation is higher than 300% and the strain-hardening exponent is higher than 0.4. It is concluded that the use of isothermal stamping of bottom-type parts from the titanium alloy in the superplastic state is profitable.     

Shape memory behaviour associated with the R and martensitic transformations in a NiTi alloy

The deformation and transformation behaviour associated with both the R and martensitic transformations in a near equiatomic NiTi alloy has been investigated using thermal cycling tests under constant applied load. Strain measurements exhibit separate stages of yielding associated with the R and martensitic transformations. The transformation sequence during cooling is found to depend on the applied stress, resulting from the differing stress dependencies of the R and martensitic transformation temperatures. Strain, resistivity, and DSC measurements indicate that the B2 → R transformation is characterised by a highly reversible single stage, first order reaction. Repeated thermal cycling resulted in irreversible changes to both the transformation temperatures and strains associated with the martensitic transformation.     

Spontaneous deformation during aging under stress in a copper-beryllium alloy

An anomalous phenomenon has been found in a Cu-Be alloy during an investigation of the aging behavior. A platelike testpiece of a cold-rolled Cu-Be alloy (Japanese Industrial Standard No. C1720) was held at 1073 K for 2 hours, followed by quenching to solutionize. The testpiece was set in a hand-made cantilever jig and aged by applying an external stress with a screw at the free end of the cantilever. The free end of the cantilever leaves the screw in 30 minutes and warps further, within 1 hour, during aging. The spontaneous warping deflection is closely related to the hardness change through aging and is dependent upon the elastic strain introduced initially by the external load. Transmission electron microscope (TEM) observation makes clear that matastable γ′ precipitates, which bring about the most effective hardening and shrinkage, are predominant at the compression side of the cantilever, while extremely fine Guinier—Preston (G—P) zones are observed at an early stage of aging and coarse γ-phase precipitates are observed at a prolonged aging time in the tension side. The spontaneous warping results from the difference of the precipitation sequence between the two sides.     

Isothermal studies of bainitic and martensitic transformations in some low alloy steels

Ohne Zusammenfassung     

Microstructural effects of martensitic transformation cycling of a Cu-Zn-Al alloy: Vestigial structures in the parent phase

Martensitic transformation kinetics and microstructural effects are correlated in a study of transformation cycling in a Cu-Zn-AI alloy. During the first few cycles of martensitic transformation and reversion, vestigial features develop in the parent phase. During these same cycles, the parent-to-martensite transformation temperature range shifts upward by several degrees and the martensite-to-parent reversion temperature range shifts down by about a degree, in effect reducing the transformation hysteresis. The vestigial ridge features are associated particularly with burst martensite phenomena, and this and other manifestations of imperfect thermoelastic martensite behavior during initial cycling lead to the kinetics changes. These changes virtually stabilize after 20 to 25 transformation cycles as the martensitic transformation achieves a completely reproducible pattern, both in terms of microstructure and kinetics.     

Effects of thermal cycling on the martensitic transformation in two-phase α/β brasses

The effects of thermal cycling between the parent and martensite phases of two-phase α/β CuZn alloys have been studied by electrical resistance-temperature measurements, optical microscopy, and transmission electron microscopy (TEM). The martensite start (M_s) temperature is dominated primarily by the composition of the β phase but increases substantially between the first and second cycles because of deformation of the α particles and a resultant change in the internal strain fields of the system. With increasing thermal cycling, the M_s temperature increases slightly and eventually becomes constant. However, the transformation hysteresis becomes smaller, and more perfect thermoelastic behavior is found. The number of vestigial deformation markings in the β phase is increased by thermal cycling and becomes more distinct; the dislocation density in the β phase is also increased and features a more crystallographic arrangement. The vestigial deformation of the β phase is instrumental in subsequent martensite nucleation and in creating a martensite microstructural memory.     

Martensite crystallography—the apparent controversy between the infinitesimal deformation approach and the phenomenological theory of martensitic transformations

The aim of this article is to demonstrate that the apparent controversy between the infinitesimal deformation (ID) approach and the phenomenological theory of martensitic transformations (PTMTs) in predicting the crystallographic characteristics of a martensitic transformation is entirely based on unjustified approximations associated with the way in which the ID calculations are performed. When applied correctly, the ID approach is shown to be absolutely identical to the PTMT. Nevertheless, there may be some advantages in using the ID approach. In particular, it is somewhat simpler than the PTMT; it is based on a physical concept that is easier to understand and, most important, it may provide a tool for investigating some of the features of martensitic transformations that have eluded explanation via the PTMT.     

Inelastic deformation and dislocation structure of a nickel alloy: Effects of deformation and thermal histories

The inelastic deformation behavior of a nickel-base alloy, B1900 + Hf, has been examined under nonisothermal conditions by performing step temperature tensile tests and thermomechanical cyclic tests. In both cases, the temperature ranges of interest are 538 °C to 760 °C, where the peak strength of γ′occurs, and 760 °C to 982 °C, where static thermal recovery is important. The dislocation structures of the deformed specimens are characterized using transmission electron microscopy and correlated with the macroscopic inelastic behavior. The present results for nonisothermal loading are compared with previous isothermal data to assess the effects of deformation and thermal histories on high-temperature, inelastic deformation behavior and dislocation structures. Relations of dislocation structures and the unified constitutive theories for representing all aspects of inelastic deformation, including plasticity under monotonic and cyclic loading, creep, and stress relaxation, are then discussed.     

Orientation dependence of β1 → β 1 1 stress-induced martensitic transformation in a Cu-Al-Ni alloystress-induced martensitic transformation in a Cu-Al-Ni alloy

The online version of the original article can be found at     

The effects of prior deformation and transformations on the microstructure of an iron-nickel alloy

The microstructure of an Fe-31.4 pet Ni-0.3 pet C alloy was examined via transmission electron microscopy as a function of thermomechanical treatment. The effects of prior deformation, rapid reversion to austenite and thermal cycling on the microstructure were investigated, and operative strengthening mechanisms under various conditions were correlated to observed structures. When midrib twinned, plate martensite of this alloy was deformed at room temperature, dislocation glide was the operating mode, and the midrib twins and plate like structure were completely dissolved after 80 pet cold rolling. The microstructure of reverted austenite without prior deformation was composed of sheared plates, but became finely equiaxed with prior deformation of the martensite. The superior strength of reverted austenite in comparison to annealed austenite was due to a grain size refinement and a higher dislocation density. However, the strengthening observed in reverted austenite with prior deformation in comparison to reverted austenite without prior deformation was due to a grain size effect alone. Repeated thermal cyclings increased the strength of reverted austenite. This was due to increases in the dislocation density since the grain structure was principally dictated by the first martensite transformationreversion cycle.     

Precipitation effects during hot deformation of a copper alloy

Hot compression tests were performed on a previously solution-treated Cu-3Ni-lSi-O.8Cr-O.1 Mg alloy below the solvus temperature. The effects of precipitation occurring during hot deformation and the accompanying flow stresses were analyzed on the basis of microstructural evolution using optical, scanning, and transmission electron microscopy, and microhardness measurements. It was found that the hardening stage was followed by strain-induced localized Ni₂Si-precipitate coarsening at the temperature related to the most effective dynamic precipi-tation. Intensive coarsening of precipitates began at grain boundaries. Very fine Ni₂Si precip-itates were transformed into elongated particles at grain boundaries, producing flow localization, softening, and finally sample fracture. Formerly Graduate Student, Academy of Mining and Metallurgy, Cracow, Poland     

Microstructure and deformation rate during long-term cyclic creep of the martensitic steel X22CrMoV12-1

The microstructure of three specimens of the martensitic steel X22CrMoV12-1 which had been subjected to long-term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = ?1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20(22)CrMoV12-1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth.     

热变形及热处理过程中TC17钛合金组织与取向的关联性

为了进一步研究热压缩及热处理过程对组织及取向变化的关联性, 通过对TC17进行热压缩变形及后续热处理, 利用光学显微镜和背散射电子衍射等分析方法, 结合晶粒尺寸、织构分布图、极图以及反极图, 研究变形后及热处理后的TC17的组织结构、晶粒尺寸的变化和取向的演变规律以及两者之间的关联性.结果表明: 随着变形温度升高, 初生α相含量大幅减小, 尺寸减小, 大部分α相晶粒分散分布, 且位于高温β相晶粒的三叉晶界上; 热处理后, α相和β相组织特征清晰, 界限明显, 初生α相依旧存在, 且趋于等轴化, 亚稳定β相发生转变, 形成片层状β转变组织; 热变形使α相织构极密度值减小, 且随之温度增加, α相织构极密度值也变小; 热变形后的α相已不存在明显的强织构, 热变形对α相晶粒的取向影响较大, 很明显的改善了其取向的均匀性; 热变形同样使β相织构极密度值减小, 但效果不明显.β相仍存在取向集中现象, 取向均匀性相对较差.