We present a joint experimental and theoretical study on metal dusting phenomena in Ni binary alloys containing group-14 and 15 elements in the periodic series. Laboratory metal dusting test of several Ni binary alloys was conducted in a simulated syngas atmosphere consisting of CO, H2, CO2, and H2O at 923 K for 100 h. The Ni alloys containing the group-14 and 15 elements exhibited excellent metal dusting resistance. This behavior is reasonably interpreted by the Blyholder mechanism and the electron filling concept in the p-d hybridization states for the associated alloys. Furthermore, surface segregation of the group-14 and 15 elements was analyzed by ARXPS on the Ni alloys after exposing at 923 K in a vacuum. The behavior of those segregation was quantitatively associated with the non-dissociative adsorption.
The electro-deposited pure iron has a quite sharp and isotropic 〈111〉//ND fiber texture and needle shaped grain elongated in ND. This pure iron shows the r-value over 7 and it is difficult to explain such a high r-value only from the texture. In this study the deformation behavior of the electro-deposited pure iron was investigated to reveal the mechanism of the extremely high r-value. The slip lines on the surface after the deformation indicated that the particular {110} plane slips which are parallel to ND exclusively act in the specimen. The tensile deformation by this slip system does not need the thickness decrease. Thus the limitation of the active slip system is the main cause of the extraordinary high r-value. One possibility of the limitation is the ease of the continuity of the slip plane of the adjacent grains. Because both {110} slip plane and grain boundary are perpendicular to the surface, the slip planes of adjacent grains can connect easily.
Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and explain the creep properties of the Mg-Al-Ca AX52 die-cast alloy at 473 K. TEM observations indicate that dislocations are introduced within the primary α-Mg grain interior in the die-casting process, which consist of both basal and non-basal segments. The non-basal segments of the dislocations, having smoother curvature in the as die-cast state, partially exhibit steps parallel to the basal plane during high temperature exposure. The basal segments of the dislocations bow out and glide on the basal planes under stress, and the jogs follow the basal segments with the help of climb during creep. The easy glide of the basal segments of the dislocations controls the creep rates immediately after the stress application of the creep tests, while the creep mechanism for the alloy has been identified as the dislocation climb. By comparing the dislocation movements for the Mg-Al-Ca AX52 die-cast alloy with those for the Mg-Al AM50 die-cast alloy, it is inferred that the eutectic intermetallic phase covering the primary α-Mg grains decreases the climb velocity of the jogs during creep.
Tatsuro Ochi, Kei Miyanishi, Masahiro Toda, Osamu Kada
The purpose of this study is to clarify the optimum approach to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working of medium carbon steel. Increase in warm deformation resistance during warm working by addition of Cr is small. On the other hand, Cr has the effect of suppressing the annihilation of dislocations in the air cooling process after warm working. Thus, by adding Cr, the tensile strength after warm working is effectively improved, without increasing warm deformation resistance during warm working. Increasing of Si is effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working, too. Si has an effect of the solid solution hardening. This effect by increasing Si is due to the large temperature dependence of solid solution hardening effect. The control of microstructural factors, that are pearlite fraction and cementite spacing, is not effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working.
Unprecedented phenomena were discovered by tempering the Ti-4Fe-7Al alloy quenched from the β (bcc) field. The alloy became very hard when it was tempered at 450℃ for several minutes, and severely rugged surface was generated. The inverse shape recovery phenomenon was also discovered when a quenched specimen that had been bent at room temperature was heated. The tempered microstructure showed almost β grains and some of the usual martensitic acicular structure areas. However, electron back scattering pattern (EBSP) measurements showed that the β-like grain was not the bcc structure but was the hcp or orthorhombic structure. X-ray diffraction (XRD) measurements clarified that an orthorhombic α″ structure (a=0.2995 nm, b=0.4913 nm, c=0.4659 nm) was formed from the β phase by tempering. Moreover, this α″ structure was confirmed to be a type of martensitic transformation because no concentration distribution was detected in scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis of the microstructure. It was suggested that the essential Ms point of the alloy should be higher than room temperature; however, the martensite transformation could not operate by fast quenching. The newly-discovered α″ martensite is formed without atomic diffusion by heating. When the β grain transforms into the single α″-variant, a very huge lattice strain is generated, resulting in the severely rugged surface or the inverse shape recovery phenomenon.
Indentation creep tests and finite element simulations were performed on a model material to show that a constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate maintain constant values of ps and εin(s), respectively, the contours of the equivalent stress and the equivalent plastic strain rate in the region beneath the conical indenter expand according to the increase in the indenter displacement while maintaining the geometrical self-similarity. These findings indicate that a pseudo-steady state deformation takes place around the indenter tip. The representative point exhibiting the creep behavior within the limited region, which actually determines the indenter velocity, is defined as the location where the equivalent stress σr is equal to ps/3. The equivalent plastic strain rate εr at this point is found to be εin(s)/3.6 in the case that the creep stress exponent is 3. The stress exponent and the activation energy for creep extracted from the results of Al-5.3 mol%Mg solid-solution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for σr and εr agree well with the values for the applied stress and the corresponding creep rate in tensile creep tests at the same temperature. The above results show not only that the creep characteristics of advanced materials, which are often available in minute quantities or as small-volume specimens, can be obtained from carefully designed indentation creep tests, but also that the constitutive equation for tensile creep can be predicted with sufficient precision through indentation creep test results.