Polypropylene nucleation on a glass fibre after melt shearing

Thermoplastic composites may exhibit a wide range of crystalline morphologies in the neighbourhood of fibres. It was found that glass fibre shearing of a molten polypropylene at high temperature modifies the subsequent isothermal crystallization (T _c=122°C) under static conditions. The crystallization results have been analysed as a function of the previous high temperature, shear, , shear rate, , and shear stress, . The mechanical parameters at high temperature, , , , have been calculated for two shearing temperatures (T=170, 210°C) and two fibre displacements. An -phase nucleation process took place at the fibre surface after shearing at the higher temperature (T=210°C). The nucleation increased with shear but did not appear for static conditions. A strong nucleation process in phase appeared on the fibre surface after shearing of the polymer at the lower temperature (T=170°C). These strong morphological modifications with shearing temperature have been analysed as a function of mechanical and thermodynamical parameters.     

Crack growth in lamellar titanium aluminides containing beta phase precipitates

Mode-I fracture behavior of fully-lamellar polycrystalline -TiAl + ₂-Ti₃Al intermetallic alloys and the role of Ti-V base -phase precipitates of different thermodynamic stability have been studied using a finite element method. A rate-dependent, finite-strain, crystal-plasticity based materials constitutive model is used to represent the deformation behavior of both the -TiAl + ₂-Ti₃Al lamellar matrix and the -phase precipitates. Within the matrix colonies, fracture is assumed to take place throughout the ₂-Ti₃Al lamellae. In addition, fracture along colony boundaries and matrix/precipitate interfaces is considered. The constitutive behavior of all fracture interfaces is modeled using a cohesive-zone formulation. The analysis is carried out using the commercial finite element program Abaqus/Standard within which the material state is integrated using an Euler-backward implicit formulation. The results obtained show that the main mechanism of crack growth is nucleation of secondary cracks along ₂-Ti₃Al lamellae ahead of the main crack and their subsequent link-up with the tip of the main crack. The resulting fracture resistance curve acquires the characteristic step-wise shape. Both stable and metastable -phase precipitates are found to have a beneficial effect on the fracture resistance of the material. However, the effect is not very significant and metastable -phase precipitates appear to be a little bit more beneficial. All these findings are consistent with their experimental counterparts.     

Effects of the variation in α-phase volume fraction on the thermal stability of TiAl alloys with a lamellar microstructure

The thermal stability of two-phase ( + ) lamellar microstructure in Ti-Al-Mo PST (polysynthetically twinned that has single colony) crystals, containing C or Si, was investigated. In addition, the variation of -phase volume fraction in Ti-Al-Mo-(C,Si) systems was investigated at several temperatures. Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys did not recrystallized (stable in this paper) during heat treatments at various heating rates and temperatures. Moreover, the -phase volume fractions of Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys which were stable compositions, changed less than those of Ti-47Al and Ti-46Al-1.5Mo alloys which were unstable compositions. The instability of the latter alloys was caused by their relatively higher variation of -phase volume fraction during heating. Therefore, it is suggested that the variation of -phase volume fraction is an important factor in controlling the thermal stability of lamellar microstructure.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

The formation of α-Al2O3 from θ-Al2O3: The relevance of a “critical size” and: Diffusional nucleation or “synchro-shear”?

The coarsening of -Al₂O₃ crystals to a 'critical size' is often interpreted as the first step in the shear nucleation of -Al₂O₃. The existence of this so-called critical size has also been used to explain the observation that -Al₂O₃ nuclei are generally twice as large as the crystals in the -Al₂O₃ matrix. This paper discusses the important issues in the nucleation of -Al₂O₃ from -Al₂O₃. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical -Al₂O₃ crystal size is not a prerequisite for -Al₂O₃ nucleation, but is primarily a result of the incubation time required to produce -Al₂O₃ nuclei by diffusional nucleation. It is proposed that the large observed -Al₂O₃ crystal size also does not result from a shear nucleation event in a 'critical size' -Al₂O₃ crystal, but is due to the intrinsically low -Al₂O₃ nucleation density, together with rapid growth of -Al₂O₃ after nucleation.w     

The effect of explosive loading on the microstructure of some metals and alloys

The effects of explosive pulses on the microstructure of Al-4.5% Cu duralumin, tough pitch copper, pure magnetic iron and 18 chromium 8 nickel stainless steel have been studied using transmission electron microscopy. An explosive loading assembly was designed to investigate the effect of both the flanks and the length of shock pulses on the material microstructure. In copper and duralumin it was found that the microhardness increased with the duration of the rarefaction flank. The size of the shock transformed martensite particles in stainless steel also increased with rarefaction time. Three wave interactions could be related to the microstructure of baratol-loaded iron and were attributed to the double shock front caused by the a to phase transformation. Twinning was found to occur when the reflected elastic wave interacted with the first part of the double shock front. The abrupt hardness increase caused by the interaction of the first reflected wave and the second transformation shock front was observed. In addition, a second more gradual hardness increase is attributed to the interaction between the low pressure and the high pressure rarefaction waves which caused a relatively slowly increasing rarefaction shock. The microstructure of shocked stainless steel showed that hexagonal plates and twins could occur in the same grain and that martensite particles were formed at the intersection of two twins, or of plates and twins. Both Kurdjumov-Sachs and Nishyama-Wassermann orientations were observed and sometimes occurred on the same fault band. The short rise and fall times in the pulse flanks made it possible to discuss formation mechanisms for twins and martensite particles in terms of the available formation times.     

Grain-boundary sliding and intergranular cavitation during superplastic deformation of α/β brass

Intergranular and interphase cavitation in binary alpha/beta brass has been investigated in tension at 600° C under conditions of superplastic deformation. The sites for nucleation of cavities has been studied by quantitative metallography and the cavities are observed to nucleate preferentially at- interfaces. The process of cavitation is associated with grain boundary sliding and cavity nucleation occurs at points of stress concentrations in the sliding interfaces. Measurements of grain and phase boundary sliding at various interfaces demonstrate that sliding occurred on- boundaries more readily than on- and-gb interfaces. The predominance of- interface cavitation is believed to be as a result of greater sliding at the- boundary and of an unbalanced accommodation of sliding adjacent to this type of boundary.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Change in the structure of alloy VT22 with the combined action of ultrasonic radiation and thermal cycling treatment

A study is made of the structure of alloy VT22 with thermal cycling treatment (TCT) and also with TCT and prior ultrasonic irradiation (USI) by the methods of metallography, electron microscopy, and hardness measurements. It is shown that TCT for annealed specimens promotes recrystallization, fragmentation, and spheroidization of and phases.It is established that TCT of specimens quenched to phase previously treated with ultrasonic radiation leads to formation of -phase platelets of considerably smaller dimensions and more strongly fragmented compared with -phase platelets after TCT for unirradiated specimens.Translated from Problemy Prochnosti, No. 10, pp. 47–49, October, 1990.     

A crystal plasticity materials constitutive model for polysynthetically-twinned γ-TiAl + α2-Ti3Al single crystals

Deformation behavior of polysynthetically-twinned lamellar -TiAl + ₂-Ti₃Al single crystals has been analyzed using a three-dimensional, isothermal, rate-dependent, large-strain, crystal-plasticity based materials constitutive model. Within the model it is assumed that plastic deformation parallel to the -TiAl/₂-Ti₃Al lamellar boundaries is controlled by the softer -TiAl phase while deformation which contains a component normal to these boundaries is dominated by the harder ₂-Ti₃Al phase. The parameters appearing in the crystal-plasticity materials constitutive relations are assessed using the available experimental information pertaining to the active slip systems, their deformation resistances and hardening and rate behavior of the two constitutive phases both in their single-crystalline and in polysynthetically-twinned lamellar forms. The constitutive relations are implemented in a Vectorized User Material Subroutine (VUMAT) of the commercial finite element program Abaqus/Explicit within which the material state is integrated during loading using an explicit Euler-forward formulation. The results obtained suggest that the adopted crystal-plasticity model and the parameters assessed in the present work account quite well for the observed room-temperature deformation behavior of polysynthetically-twinned lamellar -TiAl + ₂-Ti₃Al single crystals.     

Kinetics of α- and β-Si3N4 formation from oxide-free high-purity Si powder

Available kinetic data for the nitridation of high-purity oxide-free Si powder are analysed. The analysis suggests that the - and -phases of Si₃N₄ are formed by separate and parallel reaction paths, and kinetic expressions for their formation are reported. The formation of the -phase follows first-order kinetics, while the -phase is formed by a phase-boundary-controlled rate law. These conclusions are consistent with other kinetic and micrographic analyses reported in the literature.     

Preferentially orientated precipitation in MnTiO3 single crystals

Preferentially orientated precipitation of -TiO₂ (rutile) needles in single crystals of MnTiO₃ grown by a floating-zone method under a controlled atmosphere of oxygen fugacity was studied. Optical microscopy and X-ray analysis revealed that the orientation relation between the -TiO₂ precipitates and the MnTiO₂ matrix is: {000 1}MnTiO₃// {111}-TiO₂ and 11¯20MnTiO₃// 110-TiO₂. The precipitation phenomenon was explained by introducing a nucleation and growth mechanism which was experimentally supported by heat treating experiments.     

Structural features of certain Ni-Al-Ta and Ni-Al-Hf alloys containing the γ′- and β -phases

An investigation has been made of the structure of certain alloys of the Ni-Al-Ta and Ni-Al-Hf systems. The compositions chosen for investigation were: Ni-28.75 at% Al-2.5 at% Ta; Ni-26.75 at% Al-2.5 at% Ta; Ni-20.0 at% Al-7.5 at% Ta; Ni-20.0 at% Al-7.5 at% Hf; Ni-22.5 at% Al-5.0 at% Hf. The Ni-Al-Ta alloys consisted of +-phases (or-phase transformed during rapid cooling to a martensitic product). Two types of morphology were observed in the transformed-phase, one consisting of parallel sided plates and the other of acicular shaped plates; the plates contained fine twins on {1 0 1} b c t planes. The as-cast Ni-Al-Nf alloys consisted of the -phase, together with eutectic regions of-phase and (Ni, Al)₇Hf₂; both lamellar and discontinuous eutectic morphologies were observed. Extensive solid solution of tantalum and hafnium in the -phase occurred.     

Self-similar heating regime upon destruction of the surface of materials

The applicability of the dependence ^* Ka in mass transfer from the surface of heat insulating materials is experimentally demonstrated. A formula for calculating the temperature coefficient K is suggested.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 50, No. 2, pp. 236–240, February, 1986.     

Effect of high-temperature deformation on the texture of a two-phase titanium alloy

Changes in the texture of both the - and -phases of the two-phase alloy Ti-6AI-4V have been determined in order to clarify the mechanisms of high-temperature deformation. A strain of 1.5 was applied to the alloy at 928 °C, at strain rates representative of superplastic and nonsuperplastic conditions. The -phase texture changed very little with strain rate whereas that for the -phase was much more sensitive. The -phase texture was weakened at superplastic strain rates but developed a fibre texture at non-superplastic rates. It is postulated that under superplastic conditions the alloy deforms predominantly by grain boundary sliding of the soft -phase grains, with the hard primary -phase grains remaining in their original orientations and the measured loss in texture intensity being attributed to the loss in texture of the secondary -phase only. Under non-superplastic conditions there is a greater contribution from plastic deformation in the -phase which, in turn, can enhance the secondary -phase texture.     

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable phase was identified in the first stage of dissolution nucleating at the - boundaries. Interface dislocations at the - boundaries forming a perpendicular net in the 110 directions were identified as of screw character and of Burgers vector b=1/2a 110. During dissolution a change of shape of precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 110 directions.     

X-ray diffraction and X-ray photoelectron spectra of Fe-Cr-N films deposited by DC reactive sputtering

The effects of nitrogen flow ratio, target area ratio of Cr, and substrate temperature on the structure of DC reactive sputtered Fe-Cr-N ternary films have been studied. X-ray diffraction measurements show that Fe-Cr-N films consist of -Fe(Cr) and -(Fe,Cr)₄N_x (x<1) phases. The crystal grain of the -Fe(Cr) phase becomes finer and a (200) texture of the -(Fe,Cr)₄N_x phase becomes more marked with increasing the nitrogen flow ratio. X-ray photoelectron spectra of the films show that oxidation resistance of Fe-Cr-N films is superior to that of Fe-N films, and oxides are formed only in the film surface due to contacting with the ambient atmosphere and oxygen contamination is very small in the inner parts of these films.     

Eutectic solidification characteristics of Bridgman grown AI-3Fe-0.1V alloy

Eutectic solidification characteristics of Al-2.85 wt%Fe-0.12 wt% Valloy have been investigated by steady-state growth over the range of solidification front velocity from 51 to 1030m/s and temperature gradient 8 to 15 K/mm. Increasing growth velocity displaced the Al-Al3Fe eutectic by Al-AlxFe eutectic rather than by the Al-Al6Fe eutectic obtained for the binary Al-3 wt%Fe alloy. A fully Al-AlxFe eutectic structure has been obtained for the first time in the vanadium-containing alloy over the growth velocity range from 71 to1030m/s except at 100 and 510m/s where some Al dendrites were present in the eutectic matrix. TheAl-AlxFe eutectic was observed to undergo a morphological transition from lamellar to rod-like with increasing growth velocity concurrently with formation of a cellular eutectic structure. It was found that the relationship = Av–1/2, between eutectic spacing and growth velocity v, was applicable with A = 22.4 ± 1.8 and 13.8 ± 2.1 m3/2s–1/2 for lamellar and rod-like Al-AlxFe eutectics, respectively.     

Kinetics of alpha precipitation in Ti-6 wt % Cr and Ti-11 wt % Mo

The morphology and kinetics of the precipitation of the alpha phase produced by two different heat treatment routes, namely, (a) direct isothermal decomposition and (b)-quenching and subsequent ageing, were studied. In isothermally decomposed samples the (supersaturated) + transformation was seen to occur mainly through the discontinuous growth of the transformed zone consisting of groups of parallel side plates from the grain boundary regions towards the interior of the grain. Unlike for the case of a regular discontinuous precipitation, here the transformed regions are not separated from the untransformed by an incoherent interface and the growing-plates do obey a fixed orientation relationship with the grain from which they are evolved. The theory of cellular reaction has been applied to explain the growth rate of the duplex ( + ) region. The overall reaction kinetics were analysed on the basis of the Johnson-Mehl formulation and were found to be consistent with that of a discontinuous precipitation reaction, where grain boundary nucleation sites were saturated at an early stage of the transformation. The structure of the-quenched samples showed a uniform distribution of athermal omega particles which acted as precursors to the-precipitates. As a consequence, the reaction rate was greatly enhanced and-precipitation in the quenched and aged samples was seen to occur continuously in the entire body of the grain.     

Microstructural evolution in rapidly solidified Al-Cu-Si ternary alloys

Several Al-Cu-Si alloys were melt spun to produce stable, fine scale microstructures suitable for superplastic deformation and consolidation. Scanning electron microscopy of the ribbon cross-sections reveal two distinct alternating microstructural morphologies, suggesting transitions in solidification behavior. One structure consists of intimately interlocked -Al and (Al₂Cu) phases with dispersed spheroids of (Si). The other structure consists of equiaxed or cellular-dendritic -Al with interdendritic and (Si). The latter was found in the middle portion of the ribbon cross-section when cast at a low speed, and throughout the ribbon cross-section when cast at high speed. The dendritic structure appears to result from independent nucleation events in the undercooled liquid ahead of the solid-liquid interface. The solidification mechanism for the interlocked structure appears to involve multiple nucleation of the phase followed by its cooperative growth with the -Al phase. This cooperative growth is unlike that which forms a lamellar structure, as it results in a branched, randomly oriented network. We postulate that the (Si) phase is the first phase to form from the undercooled liquid, and it is uniformly dispersed throughout the undercooled melt. The (Si) spheroids provide nucleation sites for the phase because of its observed association with the phase. The -Al grain size varies from 1 m near the wheel side surface of the ribbon to 8 m with sub-grains near the free surface. The size of the and (Si) phases is on the order of a m and less. The microstructural size scale appears to be small enough for this material to exhibit superplastic behavior when deformed.