K.H. Wu

Development of constitutive relationships for the hot deformation of boron microalloying TiAlCrV alloys

Abstract Isothermal compressive tests are performed on TiAlCrV and TiAlCrV+B alloys to study hot deformation behavior. A hyperbolic-sine Arrhenius-type equation is used to derive the constitutive equation to correlate the flow stress to the deformation temperature, and the strain rate to strain level. The results show that all experimental data of TiAlCrV and TiAlCrV+B alloys can be fully described using this hyperbolic-sine Arrhenius-type relationship. The activation energy, Q , the material constants, n and ln nA, of both alloys decrease as the deformation strain increases. The strain dependent term is successfully incorporated in the constitutive equation through the parameters, Q , n , and ln A . The preliminary results also suggest that the constitutive relationship between the flow stress and strain and between the strain rate and temperature can be used to predict the hot-workability map for the hot deformation of intermetallic alloys.

Fatigue and fracture behavior of nickel–titanium shape-memory alloy reinforced aluminum composites

Abstract A shape-memory alloy, nickel–titanium (NiTi), has been distributed throughout an aluminum matrix, using powder-metallurgy processing, in the hope of using the shape-memory effect to achieve strengthening and improve the fatigue resistance, as compared with the aluminum matrix. The shape-memory effect was activated by cold rolling the samples at −30°C. Upon reheating to the austenite phase, the NiTi was expected to return to its original shape, while embedded in the aluminum matrix. This action created residual, internal stresses around each particle, which strengthened the material. The yield and ultimate strengths, and fatigue lives of the NiTi reinforced aluminum composites, have been improved considerably, as compared with the unreinforced material. The cross-sectional microstructures of the composites, as well as, the modes of crack growth, have been examined with scanning electron microscopy (SEM) to identify fatigue and fracture mechanisms.

Composite Materials with Adjustable Thermal Expansion for Electronic Applications

In this paper, we discuss a newly characterized compound, ZrW 2 O g , that has been introduced into the composite materials with an adjustable and low thermal expansion for electronic applications. Offering a negative coefficient of thermal expansion (CTE) of approximate ‐9xlO –6 /°C in a large temperature range, ZrW 2 O g was used as a particle filler in polymer‐matrix composites. The paper presents two kinds of composites, that is, polyester and epoxy with various volume fractions of ZrW 2 0 g . The CTEs of the polyester/ZrW 2 O g and epoxy/ZrW 2 O g composites have been proven adjustable in the ranges of 94 to 56x10 –6 / °C and 54 to 18х 10 –6 /°C, respectively, with ZrW 2 O g filler from 0 to 30 vol%. In addition, the analysis about the interfaces between the matrices and filler indicated that the interfaces may be beneficial to reduce the overall thermal expansion of the composites. The methods to further decrease composite CTEs are also discussed.

Kinetic Model of Thermoelastic Martensite Transformation in Niti and NiMn Based Shape Memory Alloys

A new, quantitative model was developed to describe the martensite transformation kinetics of thermoelastic shape memory alloys (SMAs). In addition, a series of experiments were conducted to study the kinetics of thermoelastic martensite transformation in four SMA systems: NiTi, NiTi-15at%Hf, NiTi-20at%Zr and NiMn-7.5at%Ti alloys. Comparisons between data of the kinetic of martensite transformation with the present theoretic models show that the proposed model is in good agreement and concurs with the experimental data. Also, a comparison of data from the proposed model with data from existing kinetic models, such as Liang`s and Magee`s indicates that the proposed model can better describe the experimental data, including the relationship between d{xi}(T)/dT and {xi}, and d{xi}(T)/dT and T.

Influence of aging treatment on Ni46Ti44Ta10 shape memory alloy

The effect of aging treatments on the microstructure, martensite transformation temperature and shape memory effects (SME) of the Ni46Ti44Ta10 alloy was carefully studied in this work. The result show that the microstructure of Ni46Ti44Ta10 alloy contains an NiTi phase, a eutectic structure, which formed by NiTi, and (beta) -Ta phase, and some dispersed NiTa2 compound. The aging treatment can produce Ti11Ni14 precipitates in the alloy. Meanwhile, precipitation hardening was noticed during the aging process. The phase transformation temperatures and hardness of the sample alloy vary with the changing of the composition and redistribution of each phase during the aging treatments. After the aging treatment, the samples demonstrated an enhanced shape memory effect compared with untreated samples. It is also evident that there is a strong correspondence between the enhanced shape memory effect and the increase of the hardness value.

NiTi-Ta shape memory alloys

This paper studies the effect of Ta addition on the martensitic transformation characteristics and the X-ray visibility of NiTi shape memory alloys. It was found that: (1) The transformation temperatures of the Ti49Ni51 binary alloy increase drastically by an addition of 0 -- 4at%Ta, but only slightly when the concentration exceeds 4at%. (2) The addition of Ta greatly decreases the sensitivity of the martensitic transformations to the variation in the Ni-Ti ratio. (3) The addition of Ta to a TiNi binary alloy can improve its X-ray visibility.

Crystallization of Amorphous Ribbon in NiTi-Cu Shape Memory Alloy

The objective of this work is to study the crystallization process of the amorphous ribbon of NiTi-Cu based shape memory alloys. An amorphous material with a composition of Ti{sub 50}Ni{sub 25}Cu{sub 25} was used to conduct this study. First, a study was conducted to understand the crystallization kinetics of this amorphous material at the isothermal model and continuous heating mode, respectively. The characteristic parameters associated with the crystallization process, such as the start and finish time for isothermal crystallization, the peak temperature for continuous heating crystallization, and activation energy, are obtained. Based on the study of experimental data of crystallization kinetics, a series of isothermal annealing experiments was conducted to study the crystallization process and microstructure of fully crystallized materials. The TEM micrograph shows the crystal phase growths in the amorphous matrix as a perfect geometric sphere. A system analysis was conducted to explain the crystallization micro-mechanism.

Influence of the grain size of fully lamellar structure on the mechanical properties of a TiAl based alloy

A preliminary investigation was conducted for developing a thermomechanical procedure to refine the lamellar colony size of a fully lamellar structure and improve the balance of the mechanical properties of TiAl based alloys. The main experimental results are as follows: (1) The lamellar colony size in a fully lamellar structure can be reduced to 50 {mu}m using a proper thermomechanical procedure. (2) The influence of colony size on the compressive yield stress of the fully lamellar structure is complex. In the large colony size region, the yield stress can be described by the Hall-Petch relation. In the small colony size region, the yield stress decreases as the colony size decreases. (3) The TiAl alloy, with a fine fully lamellar structure, has the best balance of tensile properties and fracture toughness.