H.-J. Kestenbach

On coherent carbonitride precipitation in commercial microalloyed steels

Fine vanadium carbonitride particles nucleated during interphase precipitation have been investigated in the transmission electron microscope under controlled conditions of diffraction contrast. On dark-field micrographs taken with a carbonitride reflection, particle sizes ranged from 5 nm down to below 2 nm in diameter. No matrix strain around these particles could be detected when matrix reflections were used to produce diffraction contrast under two-beam conditions. It was concluded that the vanadium carbonitrides had precipitated in the form of incoherent particles.

Correlation between texture and substructure of conventionally and shock-wave-deformed aluminum

Abstract The changes in texture and substructure induced in commercial purity aluminum by conventional (cross-rolling) and shock wave deformation (peak pressure, 5.8 GPa; pulse duration, 2.8 μs) were studied by X-ray diffraction and transmission electron microscopy. Three different grain sizes were investigated (26, 70 and 440 μm) and shock wave deformation did not produce any noticeable change in texture, while cold rolling induced significant changes. The deformation substructure consisted of tangled dislocations after shock loading and of cellular arrays after rolling. Misorientations within the grains were investigated using Kikuchi lines and were found to be small after both shock loading and rolling (to an equivalent hardness).

The effects of temperature and pulse duration on the shock-loading response of nickel

Abstract The effects of driver plate thickness and shock-loading temperature on the shock wave response of nickel were investigated; driver plate thicknesses of 3.18, 6.36 and 25.44 mm and temperatures of 77 and 300 K were used at a constant pressure of 15.7 GPa. Calculations conducted for the three driver plate thicknesses showed that the acceleration histories of the three plates are different; consequently, different stand-off distances were used (5, 10 and 20 mm for the 3.18, 6.36 and 25.44 mm plates respectively). Post-explosion observation by transmission electron microscopy showed that the low temperature events exhibited cell structures that were less well developed than the ambient temperature events. All six events exhibited work softening; the ambient temperature events showed a very slight amount of recovery, as evidenced by the tensile curves. The high pulse duration events (for the 25.44 mm driver plate) exhibited larger cell sizes, at both 77 K and ambient temperature, than did the lower pulse duration events. This is attributed to a substantial deviation from uniaxial strain conditions for the experimental system. It is therefore recommended that further experiments in the high pulse duration range use experimental set-ups where the lateral system dimensions are appropriately scaled up to propitiate a uniaxial strain configuration.

Effect of Fe2Nb precipitation on the creep properties of niobium-bearing austenitic stainless steels

Abstract The creep behaviour of niobium-rich AISI type 347 austenitic stainless steels has been re-examined under conditions which changed the grain boundary precipitation from niobium carbonitride to intermetallic Fe 2 Nb particles while maintaining the level of intragranular precipitation strengthening by the carbonitride phase. Fe 2 Nb precipitation reduced the creep rupture ductility.

Determination of the pearlite nodule size in eutectoid steels

Abstract A simple metallographic technique is described that permits the preferential etching of pearlite nodule boundaries in a fully pearlitic microstructure. Practical results are shown to be good enough for a quantitative assessment of the pearlite nodule size by the American Society for Testing Materials (ASTM) comparative grain size method. Arguments are presented that suggest that the pearlite nodule size should play an important role in determining the fracture toughness of eutectoid steels.

Transcrystallization at the interface of polyethylene single-polymer composites

The phenomenon of transcrystallization was studied at the interface of UHMWPE fibers embedded in an HDPE matrix. It was hoped that epitaxial crystallization in such model composites could eventually be used to improve adhesion between these high-strength fibers and the thermoplastic matrix material. Matrix crystallization was induced and accompanied on a specially designed hot stage which made the crystallization front advance slowly along a thermal gradient. Transcrystalline interfacial layers were observed without regard to temperature conditions, but with widely varying dimensions. Lamellar resolution within these layers was achieved by low voltage scanning electron microscopy, and the very beginning of transcrystallization was observed in sample areas where UHMWPE fiber segments were only partially embedded into the HDPE matrix. Lamellar alignment on the fiber surface indicated that transcrystallization in this system was associated with epitaxial nucleation.

Micellar morphology and its plastic deformation behaviour in ultra-high molecular weight polyethylene

Highly oriented samples from ultra-high molecular weight and normal high-density polyethylene (UHMWPE, HDPE) were prepared under the same experimental conditions. The morphology of the UHMWPE, investigated by transmission electron microscopy, was found to be oriented micellar, while the HDPE samples had a shishkebab morphology. Tensile test experiments under cyclic loading conditions exhibit a strain-hardening effect (increase in stress for the onset of plastic deformation) for HDPE, while the UHMWPE shows a decrease in Youngs modulus, which is attributed to similar molecular mechanisms as for the Mullins effect in particle-filled elastomers.

Resolução lamelar num novo microscópio eletrônico de varredura

ABSTRACT: Working with low energy electrons (in the range of 1keV), the new scanning electron microscope permits the lamellar (supermolecular) structure of semicrystalline polymers to be observed directly without the need of specimen coating or of any other sample preparation technique. Microscope performance is demonstrated by several examples of high resolution micrographs which show spherulitic, lamellar and fibrilar morphologies developed by the a, b and g phases of PVDF as a function of processing conditions and crystallization temperature. Another example reveals the early stages of transcrystalline layer formation in HDPE reinforced by UHMWPE fibers.

Molecular orientation of individual LCP particles in injection-moulded PPS/LCP blends

Polarized light microscopy was used to investigate the presence of preferred molecular orientation in the LCP phase of PPS/LCP blends after injection moulding. Normal birefringence effects appeared to be complicated by artifacts due to sample preparation and by the complex nature of polarized light transmission through a multicomponent sample. It was found, however, that, during low-temperature cutting of optically transparent thin sections on a standard microtome, individual LCP particles could be separated from the PPS matrix, and their birefringence analyzed separately. Preferred orientation was detected only in LCP fibrils which dominated in skin regions, but not in droplet-shaped particles which had formed in core regions. Quantitative measurements indicated that the molecular orientation of the fibrils increased linearly with their length-to-diameter aspect ratios which ranged from 15 to 50. Even for the highest aspect ratios, however, the degree of orientation was always less than that which could easily be introduced into pure LCP thin-film samples by manual shearing.

Secondary Carbide Precipitation in Low Silicon Hot Work Tool Steels

A reduction from 1.0 to 0.3%Si has recently been shown to improve mechanical properties of H11-type hot work tool steels. The present paper shows that an important improvement in toughness can be explained by the effect of Si content on the precipitation sequence of secondary carbides during tempering after quenching. Carbide particle distributions were observed and identified by electron microscopy, allowing to relate the effect of Si on mechanical properties directly to its effect on cementite and subsequent alloy carbide formation during high temperature tempering.