Showing papers on "Grain boundary strengthening published in 1980"

The Development of Single Crystal Superalloy Turbine Blades

Abstract: Single crystal superalloy turbine blades have recently entered production for JTSD commercial engine applications. This significant technical advance was made possible by the development of an alloy with improved properties and the development of a production casting process. The absence of grain boundary strengthening elements provided considerable alloying and heat treatment flexibility that resulted in single crystal Alloy 454 with an outstanding balance of properties. Major improvements in temperature gradients, the use of helical grain selectors, and the incorporation of these advances into existing vacuum furnaces have led to the rapid development of a production casting capability.

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Abstract: Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.

On the mechanisms of grain boundary migration

Abstract: Grain boundary dislocations and steps may have a major role in migration. The perfect grain boundary dislocations discussed in the present work can have steps at their cores and the step height depends on the dislocation and the boundary concerned. A model for migration is developed and it is correspondingly sensitive to the grain boundary crystallography. The problems of shear and long-range diffusion inherent in this model govern the details of the postulated dislocation processes and diffusion paths. There is direct evidence for migration by both dislocation and step mechanisms from in situ TEM heating experiments. Deformation associated with migrating boundaries in the absence of an external applied stress is interpreted in terms of the dislocation model. Movement of grain boundary dislocations, which are analogous to twinning dislocations, gives deformation and boundary migration. In the dislocation model, diffusion creep and boundary migration are related phenomena at a mechanistic level.

Deformation of fine-grained aluminium alloys

Abstract: The deformation of an ultrafine-grained aluminium alloy has been examined in tension and torsion. At grain sizes below about 3 μm the alloy exhibits inhomogeneous yielding but this is absent at larger grain sizes. If the lower yield strength values are plotted versus grain size the strengths are inversely dependent on grain size whereas the usual plot versus d −½ is non-linear and shows an enhanced strength at the finer grain sizes. However, if the inhomogeneous yielding region is avoided by extrapolating the work-hardening portion of the curve back to the elastic line, all the data can be well represented by the Hall–Petch relationship. Torsion tests, which allow the investigation of a large strain range, show that. the gredient of the grain size plots decreases slowly wIth strain but that the grain boundaries remain effective barriers to flow at strains up to at least 1.0. The deformation behaviour does not appear to agree with the current models for the influence of grain size on the flow stre...

On the mechanisms of point-defect absorption by grain and twin boundaries

Abstract: Low- and high-angle grain boundaries, and twin boundaries in aluminium and copper have been observed during in-situ electron irradiation using a high-voltage electron microscope. Evidence is presented for dislocation mechanisms of point-defect absorption in all three types of interface. In grain boundaries, climb of extrinsic dislocation structures leading to equilibration and final inclusion in the intrinsic structure is observed. Any movement of the intrinsic dislocations in a fully equilibrated array would usually be too small to observe using this technique. In coherent twin boundaries, dislocation loops are formed : these grow, coalesce and annihilate, and then renucleate, unlike matrix loops for which nucleation ends at an early stage of irradiation.

Compositional effects on the creep ductility of a low alloy steel

Abstract: The role which P plays in determining the creep ductility of 2.25Cr-1Mo steel is examined by notched bar creep rupture tests on high purity material selectively doped with combinations of Mn, Si and P. The impurity concentrations, hardness and grain size were carefully controlled. The ductility of as-tempered samples containing dopants was found to be higher than those without dopants; however the ductility of step cooled samples containing Mn and P was found to be lower than as-tempered samples. It is suggested that P, when segregated to the prior austenite grain boundaries, enhances the nucleation of grain boundary cavities while retarding their growth. Mechanisms for each process are proposed.

Hillock formation in lead films by grain boundary sliding

Abstract: Lead films varying in thickness and grain size were deposited from the vapour onto substrates varying in their coefficient of thermal expansion. This way both the yield stress of the films and the thermal stress arising from temperature change could be varied. Hillock formation brought about by various numbers of thermal cycles between R.T. and 4.2 ? was observed in these films by SEM and the quantitative dependence of hillock volume on stress and grain size determined. These dependences together with detailed observations suggested that, under thermal stresses larger than the yield stress of the film, hillocks form by grain boundary sliding in the superplastic mode. A model is proposed in which the sliding rate is given as a product of slip band density and the rate of climb of absorbed slip dislocations in the grain boundary. Since both quantities depend linearly on stress—with the yield stress as the threshold stress—the experimentally observed second‐power dependence of sliding rate on stress...

The study of the structure of grain boundaries using diffraction techniques

Abstract: The development and use of electron and X-ray diffraction techniques to study the structure of grain boundaries in crystalline materials is reviewed. The fundamental questions that need to be answered about the structure of boundaries are discussed. The scattering expected from a small-angle twist boundary is calculated and shown to be in good agreement with electron and X-ray diffraction observations. The recent results from X-ray studies on large-angle twist and tilt boundaries in specially prepared Au bicrystals are examined and used to illustrate the type of information that diffraction techniques can provide on the detailed atomic structure of grain boundaries. It is also shown that electron diffraction can be used to examine the structure of grain boundaries of polycrystalline specimens.

Pore formation during oxidative annealing of Al2O3−∶Fe and slowing of grain growth by precipitates and pores

Abstract: High-density samples of polycrystalline Al2O3 doped with iron retain their high density when annealed at high temperature in a reducing atmosphere, but pores are formed, both inside grains and at grain boundaries, and the density decreases upon annealing in oxidizing atmospheres. An explanation for these effects is proposed. The presence of pores and second-phase particles of FeAl2O4 is found to affect the physical properties and to slow grain growth. Grain size is proportional to the number of second-phase particles to the power −1/3 indicating that the number of second-phase particles per grain remains constant during grain growth.

Strengthening mechanisms in alloys

Abstract: Metals can be strengthened by methods appropriately combined to meet the operating conditions. In this ‘alloy design’, effort, we are guided by semi-quantitative relations that have been developed over the years and which relate the efficacy of the strengthening methods to the deformation modes. In this paper, the basic concepts are expanded with specific reference to dislocation glide, diffusional creep, grain boundary sliding and high strain rate deformation.

Theory of inclusion controlled grain growth

Abstract: Different inclusion/grain boundary interactions have been analysed with emphasis on the grain boundary structure and the geometry of the inclusion/boundary profiles. Crystalline inclusions can inhibit grain growth, provided one or more inclusions intersect each boundary. An equation for the limiting grain size, in terms of the volume fraction and the particle size distribution of the inclusions, is derived and is shown to explain the inclusion controlled limiting grain sizes in calcium fluoride and in alumina, as well as grain coarsening in steel. Conditions for the drag of amorphous particles or pores, and for pore isolation are discussed.

Voids in thin silver films

Abstract: A high density (1010−1012 cm−2) of small voids (< 15 A) has been observed in as-deposited thin silver films prepared by cathodic sputtering using high-resolution electron microscopy. The annealing behaviour of these voids has been studied as a function of annealing time and thickness of the films. It has been observed that the size of the voids and the crystallites grow as a result of annealing and, after a certain stage of annealing, a large number of voids are accumulated at the grain boundaries thus inhibiting the grain coarsening by high-angle grain boundary migration. Vacancies trapped in the films and their migration offer a suitable mechanism of void growth observed in the present case.

Development of exothermically cast single-crystal Mar-M 247 and derivative alloys

Abstract: A low-cost, exothermic directional-solidification (DS) process was developed to produce single-crystal (SC) Mar-M 247 high-pressure turbine blades. Stress-rupture data indicated that SC Mar-M 247 provides only marginal improvements in longitudinal strength relative to the columnar grained DS material. Removal of grain boundary strengthening elements (B, C, Zr, Hf) from the Mar-M 247 composition (which are also melting point depressants) permitted the alloy to be solutioned at significantly higher temperatures. An order of magnitude improvement in rupture life relative to SC Mar-M 247 was observed for several derivative alloys at 103.5 MPa (15 KSI) and 1093 C. Rupture lives of the modified SC alloys were significantly affected by both alloy purity and heat treatment. Critical aspects of vacuum induction refining, exothermic casting technology, alloy development and heat treatment, which contributed to this new class of turbine blades, are reviewed

Stress-induced absorption and emission of point defects by grain boundaries

Abstract: It is postulated that the emission and absorption of point defects take place by dislocation climb in grain boundaries which contain structural arrays of dislocations accommodating misorientations either from single crystal or perfect coincidence orientations. In the absence of any extrinsic dislocation structure, the climb required will result in perturbations to an equilibrated array and the forces required to stabilize these perturbations are estimated by considering elastic interactions between the dislocations. This leads to a threshold driving force for point defect emission or absorption which may be manifested as a threshold stress for diffusional creep. Effects of the grain boundary dislocation structure on the threshold stress are investigated and the energy associated with an array perturbation is estimated.