Materials Science and Engineering

TiAl has potential of being a strong, light-weight structural material. Nanocrystalline TiAl samples were synthesized by sputtering, particle condensation in inert gas and high-pressure compaction of the powder. Structure and chemical composition of individual grains were investigated by X-ray diffraction, conventional transmission electron microscopy, high resolution electron microscopy and micro-chemical analysis. The grains ranged from 3 to 35 nm in

ABSTRACT Zr-rich Zr–Ti–Ni(Cu) alloy system with deep eutectics was investigated for bulk metallic glass (BMG) compositions. The BMGs were investigated by locating deep eutectic compositions and by evaluation of alloys near it. When Cu was added to the Zr45Ti21Ni34 eutectic alloy as an additional melting point depressant, even lower melting eutectics were obtained. A new quaternary eutectic alloy was found with a composition of Zr49Ti17Ni20Cu14. The microstructure consists of three glass forming compounds of Zr2Ni, Zr2Cu, and (Zr,Ti)2Ni. By examining alloys around the new quaternary eutectic alloy, a BMG with a glass forming ability of 4 mm diameter was found at the off-eutectic Zr49Ti14Ni20Cu17 composition. The study confirmed the composition of the best glass former near the eutectic point. The searching method for BMGs starting from a deep eutectic alloy as a reference point is described.

ABSTRACT Composites composed of closed solid wall cells enclosing a fluid are examined in this paper. The analysis of their stiffness in compression includes the effects of fluids in the cell interiors. A specialized finite element program is developed to account for the internal pressure. Both two-dimensional and three-dimensional geometries are considered. The finite element calculations are then used to predict the compressive stiffness of fluid filled metal cell composites.

ABSTRACT Fatigue crack propagation (FCP) experiments were conducted on beta Ti-15-3 alloy under various loading conditions to examine the constancy of the specific enthalpy for fracture, advanced by the Crack Layer (CL) theory as a material parameter characteristic of its intrinsic toughness. The energy release rate and the irreversible work were determined from load-displacement curves during crack propagation. Microscopic and diffraction analyses were conducted to identify the damage mechanisms ahead of the crack tip. A damage zone whose geometry exhibited plane strain character at the initial stage of crack propagation was observed optically. The damage zone transformed into plane stress configuration when the crack reached half its critical length. Damage mechanisms involved slip lines and microcracking which is believed to ensue from intense accumulation of slip processes. The magnitude of microcracking became more weighty as the crack moved deeper into plane stress dominance. The damage preceding crack advance was quantitatively assessed as the ‘crack resistance moment’ which is the volume of transformed material per unit crack extension. Application of the CL theory to the data generated under a wide range of applied stress levels gave rise to a constant value of the specific enthalpy of fracture, 20 MJ/m3. This value is in close agreement with the specific energy of slip lines computed from microstructural considerations.

ABSTRACT The mechanism of crack tip deformation in metastable beta Ti-15-3 alloy under fatigue loading has been examined. In spite of the small thickness of the test specimens (1 mm), the plastic zone revealed plane strain conditions which was transformed to a plane stress zone when its size became 0.25 of the crack length. Slip processes whose density increased with crack length were the dominant microscopic feature of crack tip plasticity. Microcracks emanating from the main crack appeared as a result of extensive slip damage. Transmission electron microscopy (TEM) and X-ray evidence indicate the absence of twinning or phase transformation and that dislocation processes constitute the microstructural origin of crack propagation resistance in the alloy. Energy calculations show that the specific energy of slip, 20 MJ m−3, exceeds that of microcracking by three orders of magnitude.

ABSTRACT Fatigue crack propagation (FCP) in Ti-15V-3Cr-3Al-3Sn alloy (where the composition is in approximate weight per cent) was studied using thin single-edge-notch specimens. The crack tip was preceded by a zone of transformation, the size of which increased considerably with crack advance. Microscope examination revealed a mixture of surface traces (due to shear deformation by slip and/or twinning) and microcracking. The extent of microcracking increased markedly with the crack depth. The crack layer theory is employed to relate the rate of FCP to the observed damage mechanisms. The analysis revealed two stages of FCP. Slip and/or twininng processes apparently dominated the first stage with a specific enthalpy of damage of 7 MJ m−3. The second stage, approaching critical crack propagation, is dominated by microcracking and is characterized by a specific enthalpy of damage of about 20 MJ m−3.

ABSTRACT The accumulation of lattice defects during fatigue cycling of nickel was investigated by electrical resistivity measurements, positron annihilation lifetime spectroscopy and transmission electron microscopy. Dislocations and vacancy clusters were found to be the main defect types. During cycling of axial and flexural samples at constant load amplitude, the dislocations form a saturated structure early in the fatigue life. This saturated structure consists of a cellular dislocation matrix, in which persistent slip bands (PSBs) begin to operate after the saturation has been established. Vacancies and vacancy clusters are formed during fatigue as a consequence of repetitive dislocation glide in the PSB structure. When PSBs operate, the matrix is assumed to be dormant, allowing vacancies to accumulate preferentially in the PSBs. The increase in vacancy concentration then accounts for the monotonic accumulation of fatigue damage, which points to the importance of vacancy accumulation as a precursor to crack nucleation.

Elastic moduli and tensile and physical properties were measured at room temperature on texture-free powder metallurgical (PM) Ti-6Al-4V alloy as a function of heat treatment and quenching (HTQ) from temperatures between 600 °C and 1200 °C. The elastic moduli and other physical properties, which are often regarded to be constant, were found to depend strongly on heat treatment. The property

ABSTRACT This paper is concerned with ‘a probing pulse’ propagating in an inhomogeneous condensed state medium. The pulse expands as it travels. This expansion may be correlated to a material-characteristic dimension – such as the size of a grain or of a free path between scattering sites or of a domain – which is a random variable with a mean denoted as gs . Moreover, we may ascribe to the propagation a loss constant which is proportional to gs . The process derives from the fact that, for a relatively large number of scattering sites, it is possible to model the propagation media in terms of a repetitive electrical network. The model supports a Gaussian (bell shape) impulse response and defines gs . Both theoretical and experimental (ultrasonic) results are provided. The experimental method is relatively fast to perform and the results are highly reproducible.

The “average acoustic pulse dispersion length in condensed matter channels” is expanded to show that propagating acoustic pulses may behave as phonons, which can be characterized and used to evaluate the channel through which they have travelled. Employing transducers, pulser/receivers and condensed matter channels, it is possible to generate and detect a ‘harmonic Gaussian’ pulse (i.e. a Gaussian pulse modulated