Materials Science Forum Vols. 638-642

Abstract: The phase transformation behavior of 9Cr–3W–3Co–VNb steels with boron contents of 47 and 130 ppm has been investigated using differential thermal analysis (DTA). In our DTA experiments, disk-shaped samples were normalized and then tempered at temperatures between 10 and −50 °C from the ferrite-to-austenite transformation temperature (Ac1) at a rate of 30 °C/min. The measured Ac1 temperatures for 47 ppm B steel and 130 ppm B steel were 878 and 884 °C, respectively. The general features of the phase transformation behavior and the changes in hardness with tempering temperature for these boron containing steels were almost identical, irrespective of the boron content. In the DTA cooling curves during tempering at temperatures between −30 and 10 °C from the Ac1 temperatures, an exothermic peak due to the formation of fresh martensite was observed and the peak area increased with increasing tempering temperature. The Vickers hardness values measured after the DTA experiments decreased gradually with increasing tempering temperature, reaching a minimum value of about 180 HV around Ac1−40 °C, and then increased markedly, reaching a value of about 350 HV at Ac1+10 °C.

Abstract: Nb-20Mo-15Si-5B-20(Cr,Ti) alloys have been subjected to annealing treatment for 2 hours in a range of temperatures from 700 to 1400oC and quenched in water to perform the microstructural characterization for these two new alloys. As cast structure consists of a solid solution phase, , and silicides (Nb5Si3) in both alloys but Cr alloy also contains NbCr2 and Nb3Si phases. Heating to higher temperature introduces Ti silicides in the microstructure for Ti alloy. The oxidation in air has been conducted on these alloys in the same temperature range for 24 hours. Weight gain per unit area as a function of temperature provides the oxidation curves while characterization techniques using SEM, EDS on SEM, x-ray mapping, and XRD has yielded the analyses of the oxide scale. The scale consists of various oxides of Nb, Mo, Cr, Si, and Ti. Cr alloy appears to offer higher oxidation resistance in the selected range of oxidation temperatures.

Abstract: Compression test for GH4586 superalloy was carried out at 1273 K and 1373 K with stain from 20% to 60%. The characteristics of dynamic recrystallization were investigated by confocal laser scanning microscope, TEM and EBSD. It was found that at 1273 K dynamic recrystallization mainly took place along preexisting grain boundaries creating a necklace structure. It was confirmed by TEM observations that the first layer of the necklace structure was formed by the mechanism of bulging of preexisting grain boundaries and the following layers were nucleated at the triple junctions of former dynamic recrystallization grains. When specimens were deformed with different strains at 1373 K, dynamic recrystallization was almost completed. Our observation showed that twinning played an important role during dynamic recrystallization by promoting dynamic recrystallization grains growing. Substructures of grains were constructed by the reconfiguration of dislocations hindered at large ' particles in interior of initial grains during dynamic recovery.

Abstract: The second phase precipitation strengthening effect of , +, + phases in INCONEL718 type and newly developed INCONEL740® Ni-base superalloys and their structure stability have been studied. Experimental results show that careful control of Nb, Ti and Al for alloy modification can not only raise their strengths but also improve structure stability at high temperatures. These modified newly developed alloys are suggested to be used at higher temperatures.

Abstract: In order to further improve machining quality of superhard materials, it was presented that adds a squeeze film damper on the wheel spindle of ultrahigh speed grinder as a assistant elastic sustain to attenuate the vibration of the wheel spindle. Work principle of squeeze film damper was analyzed; the squeeze film pressure distribution was researched through simulation and damper parameters effect on damping coefficient was studied. Base on the theory research the damper was designed and experiments was done. Experimental result shows the amplitude of the grinding wheel spindle can be reduced 20% and machining quality of superhard materials can be improved 10%～20%. Research works provides a new method for superhard materials machining.

Abstract: A new method for the oxidation protection of Ni-base superalloys with relatively low Al-content is proposed. By using the halogen effect the Al activity on the surface can be increased. Thus, the formation of a pure protective alumina scale becomes possible. The alloys IN738 and IN939 are considered in the present paper. Thermodynamic calculations for fluorine and chlorine predict the existence of the halogen effect for both alloys at temperatures between 900°C and 1200°C. The results also predict a change of the oxidation mechanism from internal alumina formation to external oxidation.

Abstract: Premature collapse in terms of cohesive damage of PVD-coated carbide cutting tools often results in a time and cost consuming immediate interrupt of the cutting process. It is assumed that the residual stress state of the composite coating – substrate in combination with external loads during tool use is responsible for cohesive damage. The X-ray diffraction methods sin2 and scattering vector are applied for determination of the residual stress depth distribution in the coating and the substrate’s subsurface. Investigations of the residual stress state of commercial PVD-coated carbide cutting tools are presented. It is determined to what extent the single process steps during tool manufacturing are responsible for the final residual stress state of the PVD-coated tool. Furthermore the meaning of the PVD-coating process for the substrate’s residual stress state is investigated. Moreover, possibilities of controlling the residual stress state of the substrate by changing process variables of selected process steps are analyzed.

Abstract: The fine-grained rolling steels NFG600 and the conventional usual rolling steels SM490 were processed by sand paper polishing and mechanical grinding to compare the residual stress generated after processing. The average grain size of NFG600 and SM490 is 3 μm and 15μm respectively. Therefore improvement of mechanical properties for such fine-grained steels is expected, it is important to understand the residual stress state of new fine-grained materials with processing. In this study, multi axial stresses of two kinds of specimens after polishing and grinding were measured by three kinds of analysis methods including cos-ψ method. As a result, as for σ33, the stress of NFG was compression, though that of SM490 was tension.

Abstract: The behavior of thermal stresses and residual stresses in aluminum and copper thin films were investigated by the diffraction of X-rays and synchrotron radiations. Aluminum films were exposed in an elevated temperature in order to measure the thermal stress behavior in the films. Thermal stresses in the films in thin aluminum and copper films of 10 nm to 1000 nm thickness with and without passivation payer were measured in a sequence of thermal cycles. Stresses in thick films without passivation traced along with a hysteresis loop during thermal cycles, whereas the thin films of nano-meter size with AlN passivation layer behaved in a linear way with the temperature change. Thin copper films with and without AlN passivation layer were used to investigate strengthening mechanism of thin films. Specimens with different thickness of the films were subjected to a cyclic plane bending fatigue test. Residual stresses developed in the films were measured in a sequence of bending cycles to understand the effect of film thickness and passivation layer on mechanical properties of thin films. The film of 500 nm thickness developed less residual stresses in the film than thicker films. A passivation layer was effective to prevent residual stress development in the film.

Abstract: The determination of strain from neutron diffraction data is normally based upon the fit of a Gaussian function to a Bragg reflection. The error in the fit is assumed to be that based on ‘counting statistics’ and this error propagates through the analyses until the final stress evaluation. This relies on there being a big enough number of diffracting grains/crystallites within the gauge volume to ‘approximate’ to counting statistics. The number of grains however depends on the gauge volume size chosen and the average size of the grains (and hence diffracting grains) within the gauge volume and this should be taken into account. The aim of this work is to give an estimate of the uncertainty due to these ‘grain-size statistics’ due to grain size, gauge volume, FWHM of the Bragg reflection (for angular dispersive diffractometers), scattering angle (2), size of detector (and hence number of diffracting grains ‘seen’ on the detector), hkl multiplicity (m) and eventually texture.