Changes In Matrix Microstructure Research Articles

Microstructural evolution and mechanical properties of near α-Ti matrix composites reinforced by hybrid (TiB+Y2O3) with bimodal size

In this study, bimodal size (TiB + Y2O3)/near α-Ti composites were fabricated by in situ casting route. The uniformly distributed micron TiB and micro-or nano size Y2O3 lead to significant changes in matrix microstructure and compressive properties. With the decreasing of TiB/Y2O3 volume ratios, significant refinement of α colony and gradual improvement of compressive strength can be obtained. As TiB/Y2O3 volume ratio reaches 4:1, the composite displays the optimal ultimate compressive strength and ductility. And the relative strengthening mechanism along with the evolution of the compressive ductility is discussed.

Coordinate Regulation of IL-1β and MMP-13 in Rat Tendons Following Subrupture Fatigue Damage

Mechanical overloading is a major causative factor of tendinopathy; however, its underlying mechanisms are unclear. We hypothesized mechanical overloading would damage tendons and alter genes associated with tendinopathy in a load-dependent manner. To test this hypothesis, we fatigue loaded rat patellar tendons in vivo and measured expression of the matrix-degrading enzyme MMP-13 and the inflammatory cytokine IL-1beta. We also examined these responses in cultured tenocytes exposed to intermittent hydrostatic pressure in vitro. Additionally, we hypothesized load-induced changes in tenocyte MMP-13 expression would be dependent on expression of IL-1beta. In vivo fatigue loading at 1.7% strain caused overt microstructural damage and upregulated expression of MMP-13 and IL-1beta, while 0.6% strain produced only minor changes in matrix microstructure and downregulated expression of both MMP-13 and IL-1beta. Loading of cultured tenocytes at 2.5 and 7.5 MPa produced comparable changes in expression to those of in vivo tendon loading. Blocking IL-1beta expression with siRNA suppressed load-induced both MMP-13 mRNA expression and activity. The data suggest fatigue loading alters expression of MMP-13 and IL-1beta in tendons in vivo and tenocytes in vitro in a load-dependent manner. The data also suggest MMP-13 is regulated by both IL-1beta-dependent and IL-1beta-independent pathways.

Creep processes in magnesium alloys and their composites

A comparison is made between the creep characteristics of two squeeze-cast magnesium alloys (AZ 91 and QE 22) reinforced with 20 vol pct Al2O3 short fibers and the unreinforced AZ 91 and QE 22 matrix alloys. The results show the creep resistance of the reinforced materials is considerably improved by comparison with the unreinforced matrix alloys. It is suggested that creep strengthening in these short-fiber composites arises primarily from the existence of a threshold stress and the effect of load transfer. By testing samples to failure, it is demonstrated that the unreinforced and reinforced materials exhibit similar times to failure at the higher stress levels. A detailed microstructural investigation by transmission electron microscopy (TEM) reveals no substantial changes in matrix microstructure due to the presence of the reinforcement. This suggests that direct composite strengthening dominates over indirect effects.

Controlled graphitization as a potential option for improving wear resistance of unalloyed white irons

Effect of heat treatment on the microstructure and resistance to abrasive wear has been studied in an unalloyed white iron used for manufacturing cylindrical pebbles used as grinding media by the cement and other industries. Heat treatment comprised holding at 800 °C, 850 °C, 900 °C, and 950 °C for 30, 60, 90, 120, and 180 minutes followed by oil quenching. Heat treatment in general improved the wear resistance over that in the as-cast (as-received) state. The extent of maximum improvement differed with temperature and in the decreasing order occurred at (1) 180 minutes, 800 °C, OQ; (2) 30 minutes, 950 °C, OQ; (3) 90 minutes, 900 °C, OQ; and (4) 180 minutes, 850 °C, OQ. From the point of view of commercial application, the heat treatment at (2) is most favored. Microstructural changes occurring during heat treating comprised (1) changes in matrix microstructure; (2) a reduction in volume fraction of massive carbides due to its part graphitization/destabilization; and (3) changes in graphite morphology, size, and distribution. Amongst the aforesaid changes, graphitization has emerged as the key parameter in improving wear resistance. Graphite morphology in a near-nodular form of optimum size and distribution was found to be most effective. Upon increasing the heat-treating temperature, the tendency of nodules to develop spikes increased. Similarly, interlinking of graphite flakes was also observed. These features and the possible formation of free ferrite adversely affected wear resistance. The role of other beneficial changes in the microstructure, e.g., globularization of carbides, possible retention of austenite, and formation of optimum volume fraction of martensite, have been duly considered while optimizing microstructure(s). The key feature of the present study is that, despite its fundamental significance, it has a well-focused application potential.

Investigation of the effects of matrix microstructure and interfacial properties on the fatigue and fracture behavior of a Ti [sbnd]15V [sbnd]3Cr [sbnd]3Al [sbnd]3Sn/SCS9 composite

The results of a systematic study of the evolution of microstructure and damage in a metastable β titanium matrix composite are reported. The effects of matrix microstructure are studied by heat treatment below the β transus which promotes a transformation from β toα + β. Similarly, the effects of the fiber/matrix interface are investigated by annealing above the β transus which promotes interfacial coarsening without significant changes in matrix microstructure. The effects of thermal exposure above and below the β transus are rationalized by considering the diffusion of α and β stabilizers. The initiation and evolution of damage in smooth speciments is also investigated under monotonic and cyclic loading using acoustic emission techniques. Tensile strength degradation and low-cycle/high-cycle fatigue lives are associated with the coarsening of interfacial dimensions during thermal exposure. Trends in the evolution of damage are also elucidated using filtered acoustic emission data and longitudinal sections of deformed specimens.

Effect of reinforcement size and matrix microstructure on the fracture properties of an aluminum metal matrix composite

The effects of systematic changes in reinforcement size and matrix microstructure on the crack initiation and growth toughness of a 7091 aluminum alloy reinforced with SiC particulates were studied. It is shown that changes in matrix microstructure have a significant effect on both initiation and growth toughness. The effect of reinforcement size on these properties is far less marked. These observations have been related to local microstructural parameters and the nature of the distribution of the reinforcement.

Crack initiation and growth toughness of an aluminum metal-matrix composite

The effects of systematic changes in matrix microstructure on crack initiation and growth toughnesses were determined on an AlZnMgCu alloy containing 0, 15, 20% by volume of SiC particulates. Materials were heat treated to underaged (UA) and overaged (OA) conditions of equivalent matrix microhardness and flow stress. Although both the fracture initiation and growth toughnesses, as measured by J Ic and tearing modulus, were similar for the unreinforced materials in the UA and OA conditions, significant effects of microstructure on both J Ic and tearing modulus were observed in the composites. SEM and TEM observations of fracture paths in the two conditions are utilized to rationalize these observations in light of existing theories of ductile fracture propagation.

An experimental and numerical study of deformation in metal-ceramic composites

The deformation characteristics of ceramic whisker- and particulate-reinforced metal-matrix composites were studied experimentally and numerically with the objective of investigating the dependence of tensile properties on the matrix microstructure and on the size, shape, and distribution of the reinforcement phase. The model systems chosen for comparison with the numerical simulations included SiC whisker-reinforced 2124 aluminum alloys with well-characterized microstructures and 1100-o aluminum reinforced with different amounts of SiC particulates. The overall constitutive response of the composite and the evolution of stress and strain field quantities in the matrix of the composite were computed using finite element models within the context of axisymmetric and plane strain unit cell formulations. The results indicated that the development of significant triaxial stresses within the composite matrix, due to the constraint imposed by the reinforcements, provides an important contribution to strengthening. Systematic calculations of the alterations in matrix field quantities in response to controlled changes in reinforcement distribution give valuable insights into the effects of particle clustering on the tensile properties. The numerical results also deliver a mechanistic rationale for experimentally observed trends on: (i) the effects of reinforcement morphology and volume fraction on yield and strain hardening behavior of the composite, (ii) the pronounced influence of reinforcement clustering on the overall constitutive response, (iii) ductile failure by void growth within the composite matrix, (iv) the insensitivity of the yield strength of the composite to changes in matrix microstructure, and (v) the dependence of ductility on the microstructure of the matrix and on the morphology and distribution of the reinforcement. The predictions of the present analyses are compared and contrasted with current theories of elastic and plastic response in multi-phase materials in an attempt to develop an overall perspective on the mechanisms of composite strengthening and of matrix and interfacial failure.