Niti and NiTi-TiC composites: Part III. shape-memory recovery

The transformation behavior of near-equiatomic NiTi containing 0, 10, and 20 vol pct TiC particulates is investigated by dilatometry. Undeformed composites exhibit a macroscopic transformation strain larger than predicted when assuming that the elastic transformation mismatch between the matrix and the particulates is unrelaxed, indicating that the mismatch is partially accommodated by matrix twinning during transformation. The thermal recovery behavior of unreinforced NiTi which was deformed primarily by twinning in the martensite phase shows that plastic deformation by slip increases with increasing prestrain, leading to (1) a decrease of the shape-memory strain on heating, (2) an increase of the two-way shape-memory strain on cooling, (3) a widening of the temperature interval over which the strain recovery occurs on heating, and (4) an increase of the transformation temperature hysteresis. For NiTi composites, the recovery behavior indicates that most of the mis-match during mechanical deformation between the TiC particulates and the NiTi matrix is relaxed by matrix twinning. However, some relaxation takes place by matrix slip, resulting in the following trends with increasing TiC content at constant prestrain: (1) decrease of the shape-memory strain on heating, (2) enhancement of the two-way shape-memory strain on cooling, and (3) broadening of the transformation interval on heating. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

NiTi and NiTi-TiC composites: Part 1. transformation and thermal cycling behavior

The transformation behavior of titanium-rich NiTi containing 0 vol pet, 10 vol pct, and 20 vol pct equiaxed TiC particles was studied by differential scanning calorimetry. The thermoelastic phase transformation of the unreinforced matrix exhibits multiple steps. Upon multiple transformation cycles, the rhombohedral phase (R phase) appears and all transformation temperatures decrease. The TiC particles inhibit the R phase and also lower some of the transformation temperatures. These effects can be explained by the internal misfit stresses resulting from both thermal expansion and transformation mismatch between matrix and reinforcement. The measured transformation enthalpy of bulk and reinforced NiTi is discussed in light of a thermodynamical model, taking into account the elastic energy stored upon cycling. The model indicates that a significant fraction of the matrix is stabilized and thus does not contribute to the transformation enthalpy. Formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology.     

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology D. MARI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Metastable phases in the Ti-V system: Part I. Neutron diffraction study and assessment of structural properties

This article presents the results of a neutron diffraction study of a series of quenched Ti-V alloys and an assessment of the composition dependence of the structural properties in the Ti-V system. Upon quenching to room temperature and atmospheric pressure, three metastable phases occur, viz., the hcp (α′) phase formed by a martensitic transformation, the omega (Ω) phase formed by a displacive transformation involving the collapse of the (111) planes of the bcc structure, and the untransformed bcc (β) phase. The lattice parameters (LPs) of the α′, β, and Ω phases are determined as functions of the V content in the composition range 3 ≤ at. pct V ≤ 70. This information is combined with a detailed analysis of the available experimental data on the α′, β, and Ω phases in pure Ti and Ti-V alloys and the β phase of V. New estimates for the LPs of β and Ω Ti and expressions describing the composition dependence of the LPs are presented. Using the assessed values, various open questions are discussed, i.e., the composition range where the hexagonal to trigonal symmetry change is observed in the Ω phase, the applicability of an approximation involved in the plane collapse model for the β → Ω transformation, and the extent to which the so-called Jamieson correlation for interatomic distances in the Ω phase holds for Ti.     

Combustion synthesis and mechanical properties of dense NiTi-TiC intermetallic-ceramic composites

Combustion synthesis (SHS) coupled with a quasi-isostatic densification step was employed to produce dense NiTi-TiC composites. The synthesis and characterization of five composites are presented, including ceramic-intermetallic (≥50 pct ceramic) composites and intermetallic-ceramic (≥50 pct intermetallic) composites. Particle size, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis was conducted to characterize the microstructure of the composites. Refractory TiC and NiTi intermetallic phases become more stoichiometric and the TiC particle size decreases with increasing intermetallic content. Micro- and nanoindentation and quasi-static compression tests were performed, to determine mechanical and material properties. The Vickers hardness decreases as the matrix shifts from ceramic to intermetallic. Modulus and compressive strength decreases with increasing amounts of Ni-Ti intermetallic. The SEM photomicrographs of fractured surfaces are included.     

Neutron diffraction study of austempered ductile iron

Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20 μm, was observed by optical micrography.     

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti₃Ni₄ precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti₃Ni₄ precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Lüders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Lüders band propagation and result in a more gradual transformation.     

Uniaxial tensile deformation of uranium 6 wt pct niobium: A neutron diffraction study of deformation twinning

The deformation of polycrystalline uranium 6 wt pct niobium (U6Nb) was studied in situ during uniaxial tensile loading by time-of-flight neutron diffraction. Diffraction patterns were recorded at incremental stresses to a maximum of 450 MPa (∼4 pct macroscopic strain). Consistent with reorientation of the martensite variants by twinning, significant changes in the diffraction peak intensities, which were proportional to the plastic contribution of the macroscopic strain, were observed. Both the lattice parameters (a, b, c, and γ) and interplanar spacings (d _hkl ) were determined as a function of applied stress. Phenomenologically, the highly anisotropic stress response of the lattice parameters as well as the individual lattice spacings can be related to deformation twinning. Preliminary transmission electron microscopy (TEM) studies identified the ( 30) and ( 72) as active deformation twinning systems of U6Nb in tension.     

Neutron diffraction study of texture development during hot working of different gamma-titanium aluminide alloys

The evolution of preferred orientations during processing appears to be of significant importance for the use of γ-titanium aluminide alloys, since the desired lamellar microstructures exhibit a strong anisotropy of mechanical properties. In this work, texture development has been investigated after hot extrusion and sheet rolling, which are considered to be technologically relevant wrought processes. As texture evolution certainly is dependent on several factors, involving deformation properties, recrystallization kinetics, and, particularly, the phase constitution at hot-working temperature, different processing conditions and alloy compositions were investigated. By comparing the results, it is indicated that the determined textures can be understood by the deformation modes of the dominating phase at hot-working temperature and the subsequent phase transformations. However, the current understanding of texture evolution is far from being complete, as no model can be presented which quantitatively accounts for the contribution of the different processes mentioned.     

Mechanical behavior of Al-Li-SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.     

The modified quasi-chemical model: Part IV. Two-sublattice quadruplet approximation

The modified quasi-chemical model is further extended, in the quadruplet approximation, to treat, simultaneously, first-nearest-neighbor (FNN) and second-nearest-neighbor (SNN) short-range ordering (SRO) in solutions with two sublattices. When one sublattice is occupied by only one species, or is empty, the model reduces to the modified quasi-chemical model for one sublattice in the pair approximation. The coordination numbers and the ratio of the number of sites on the two sublattices are permitted to vary with composition, thereby making the model ideally suited to the treatment of liquid solutions such as molten salts. The model also applies to solid solutions if the number of sites and coordination numbers are held constant and may be combined with the compound-energy formalism to treat SRO in a wide variety of types of solutions. A significant computational simplification is achieved by formally treating the quadruplets as the “components” of the solution.