The successful development of deformation-processed metal-metal composites (DMMC) offers the potential for ductile, high-strength structural materials with hightemperature stability. An infiltration casting process, developed as an alternative method to combine matrix and fiber materials with dissimilar melting temperatures, was used to permeate steel wool preforms with molten magnesium-lithium (Mg-Li) alloys. The selected matrix alloys were hexagonal close packed (HCP) Mg-4wt%Li or body centered cubic (BCC) CHAPTER 2: PROCESSING AND MECHANICAL PROPERTIES OF MAGNESIUM-LrrfflUM COMPOSITES CONTAINING STEEL FIBERS

Silicon‐nitride ceramic valves can improve the performance of both light‐ and heavy‐duty automotive engines because of the superior material properties of silicon nitrides over current metal alloys. However, ceramics are brittle materials that may introduce uncertainties in the reliability and durability of ceramic valves. As a result, the lifetime of ceramic valves are difficult to predict theoretically due to wide variations in the type and distribution of microstructural flaws in the material. Nondestructive evaluation (NDE) methods are therefore required to assess the quality and reliability of these valves. Because ceramic materials are optically translucent and the strength‐limiting flaws are normally located near the valve surface, a laser‐scatter method can be used for NDE evaluation of ceramic valves. This paper reviews the progress in the development of this NDE method and its application to inspect silicon‐nitride ceramic valves at various stages of manufacturing and bench and engine tests.

Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400 °C, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material. Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400 ЊC, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material. Comments This article is from Metallurgical and Materials Transactions

Two gamma titanium aluminides (Daido RNT650 and HOWMET 45XD) with fully lamellar structure but with different colony sizes were studied using a single-grit pendulum (rotational) scratch tester in order to assess their abrasive wear resistances. The maximum depth of groove was ∼ 0.07 mm and the scratch velocity used was ∼ 1 m/s. Normal and tangential forces were monitored during each scratch. The material removal mechanisms were examined using a scanning electron microscope (SEM) and also measured using a laser profilometer. Extensive thermal softening was observed. Sizable fractures were revealed in the transverse direction; however, the role of these fractures in the chip formation depends on the microstructure of materials and the size of groove. The tribological properties were characterized by instantaneous specific energy and scratch hardness as related to the depth of the groove. The overall response of materials can be effectively characterized by the HEM (Hwang-Evans-Malkin) model and the PSR (proportional specimen resistance) model, even though the underlining material removal might be subjected to the different mechanisms. The TiAl with the larger colony or grain size exhibits a strong resistance to material loss or material removal (higher depth-independent specific energy) while exhibiting lower scratch hardness. The obtained depth-independent specific energy and scratch hardness can be used to screen the candidate materials depending on whether the application is sliding or impact dominant.

The microstructure of deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers was characterized to correlate the fiber size to the deformation strain and mechanical properties of the composite material. Micrographs taken using scanning and transmission electron microscopy techniques revealed fiber sizes larger than predicted from the deformation applied to the bulk composite. Deformation strain in the fibers, therefore, was less than in the bulk material. Measurements from SEM and TEM micrographs were used to calculate the actual deformation strain present in the fibers. This strain was then used to adjust rule-of-mixture (ROM) predictions of the strength of the composite material. However, the experimental strengths of these materials were still less than the adjusted ROM values, potentially due to the presence of fibers considerably larger than the average size measured stereologically. Of the many models used to describe the strengthening observed in DMMC materials, the Hall-Petch relationship best describes the experimental data. Details of the strengthening models are discussed in relation to these composite materials. Keywords Ames Laboratory Disciplines Metallurgy CommentsThis article is from Journal of Materials Engineering and Performance 7 (1998): 375-384, doi: 10.1361/ 105994998770347828. Posted with permission. RightsCopyright 1998 ASM International. This paper was published in Journal of Materials Engineering and Performance, Vol. 7, Issue 3, pp. 375-384 and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited. The microstructure of deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers was characterized to correlate the fiber size to the deformation strain and mechanical properties of the composite material. Micrographs taken using scanning and transmission electron microscopy techniques revealed fiber sizes larger than predicted from the deformation applied to the bulk composite. Deformation strain in the fibers, therefore, was less than in the bulk material. Measurements from SEM and TEM micrographs were used to calculate the actual deformation strain present in the fibers. This strain was then used to adjust rule-of-mixture (ROM) predictions of the strength of the composite material. However, the experimental strengths of these materials were still less than the adjusted ROM values, potentially due to the presence of fibers considerably larger than the average size measured stereologically. Of the many models used to describe the strengthening observed in DMMC materials, the Hall-Petch relationship best describes the experimental data. Details of the strength...