Time-dependent deformation of metals

The basic characteristics of timedependent deformation of metals are described in terms of dislocation properties. At high temperatures, diffusion controlled climb of edge dislocations is the rate limiting process, whereas at low temperatures, other forms of recovery involving cross-slip of screw dislocations operate. A composite model of plastic flow is used to describe the coupling between these recovery processes. The model is patterned after the persistent slip band structures observed in cyclically deformed fcc single crystals. Screw dislocations are allowed to move in the cell interiors and to deposit edge dislocations into the adjoining walls. Cross-slip and climb lead to dislocation rearrangement and annihilation in the two regions. These processes are coupled not only through the dislocation microstructure, but also through the mechanics of the composite structure. The model is used to describe various deformation properties of metals, including stage II, stage III, and stage IV strain hardening and saturation of the flow stress. The coupling of cross-slip and climb controlled recovery processes leads to gradual transitions in strain hardening and gives a natural account of the transition from low temperature deformation to high temperature creep. The model also leads to polarized dislocation structures, internal stresses, and anelastic creep properties. This paper is based on a presentation made at the symposium “50th Anniversary of the Introduction of Dislocations” held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMS-AIME Mechanical Metallurgy and Physical Metallurgy Committees.     

Thermal shock behavior of a three-dimensional SiC/SiC composite

Thermal shock behavior of a three-dimensional (3-D) SiC/SiC composite was studied using the water-quenched method. Thermal shock damage of the composite was assessed by scanning electron microscopy characterization and residual three-point-bending strength. In the thermal shock process, the composite displayed the same bending mechanical behaviors as those of the original composite and retained 80 pct of the original strength in the longitudinal direction after being quenched from 1200°C to 25°C in water for 100 cycles. However, the composite displayed anisotropy in resistance to thermal shock damage. The observed microdamage processes were as follows: (1) formation of micropores and long crack, (2) transfer and growth of pores, (3) saturation of the dimension and the density of pores, and (4) accelerated growth of the long crack along the longitudinal direction. The critical thermal shock number for the composite was about 50. When thermal shock was less than 50 cycles, the residual flexural strength of the composite decreased with thermal shock cycles increasing. When the number was greater than 50, the strength of the composite did not decrease further.     

The bauschinger effect in a SiC/Al composite

An interesting mechanical property of SiC/Al composites is that the tensile yield stress is less than the compressive yield stress, even though the apparent modulus in tension is greater than that in compression. An investigation was undertaken to determine if the Bauschinger effect (BE) in a SiC/Al composite is asymmetrical. It was found that the BE is indeed asymmetrical in the case of the composite and the magnitude of the BE increases with total forward strain. These results can be explained in terms of the changes in “back stress” caused by the changes in the residual stress and the work hardening during forward strain.     

A study of the reaction zone in an SiC fiber-reinforced titanium alloy composite

Results on several aspects of a SiC fiber-reinforced IMI-829 (α-titanium alloy) metal matrix composite (MMC) are presented. Scanning Auger microscopy (SAM) of SiC fibers reveals a high concentration of oxygen which varies across the diameter of the fibers. It also shows that composition of SiC changes across the diameter and, for the most part, is carbon-rich nonstoichiometric SiC. Transmission electron microscopy (TEM) of thin MMC foils shows the presence of TiC and TiSi₂ in the reaction zone. Postfabrication thermal exposures of MMC s at 975 °C lead to void formation in the reaction zone. Concentration profiles of various elements across the reaction zone reveal a buildup of Zr, Nb, and Si and a decrease of Ti, Al, and Sn in the matrix around the reaction zone. Void formation in the reaction zone has been explained by the relatively high flow of Si atoms to the matrix leading to an accumulation of vacancies in the reaction zone which condense to form voids. In addition, an enhancement of hardness in the matrix around the reaction zone has been attributed to a strengthening of the matrix by solid solution and precipitation hardening, together with a contribution from residual stresses. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

包覆法制备Gp/SiC 复合材料的显微结构和性能

以不同粒径的石墨颗粒和SiC粉体为原料,采用SiC粉体包覆石墨颗粒的方法,于2000℃热压制备了石墨/碳化硅(Gp/SiC)复合材料.利用扫描电子显微镜(SEM,EDS)分析了材料的金相和断口显微结构.研究表明,石墨粒径较小且质量分数较少的复合材料比石墨粒径较大且质量分数较多的复合材料在热压工艺中更致密.石墨颗粒呈岛状紧密地镶嵌在SiC基体中,石墨与SiC界面处C和Si的扩散不明显.复合材料的相对密度、抗折强度,断裂韧性和硬度随石墨粒径和质量分数的减少而增加.断口形貌表明SiC陶瓷基体为脆性,石墨为韧性断裂.当石墨粒径为125μm、SiC与石墨的质量比为3.5时,复合材料的综合性能最佳,开口气孔率为0.3%,相对密度为97.9%,抗折强度为75±15 MPa,断裂韧性为5.4±0.5 MPa.m1/2,硬度为26.8±3GPa.     

TiB2/SiC陶瓷复合材料制备工艺的研究

以TiO2、B4C和C为原料,基于原位合成法在SiC基体中生成TiB2颗粒,并采用无压烧结法制备出TiB2/SiC复合陶瓷.通过对复合材料制备工艺的研究,发现:高于1 300℃的预烧结能形成TiB2/SiC复合陶瓷坯体.C含量、烧结温度和保温时间对复合材料的相对密度均有影响.当C含量(质量分数)为4%时、在1 400℃...     

Reaction zone microstructure in a Ti3Al + Nb/SiC composite

A composite of Ti-25Al-13Nb (atomic percent) matrix with a continuous SiC fiber (SCS-6) reinforcement was fabricated by hot pressing powder cloths and mats of fiber. The fiber/matrix reaction zone was studied using scanning electron microscopy (SEM) and transmission electron microscopy/analytical electron microscopy (TEM/AEM) techniques. The extent of reaction was determined, phases were identified, and solute partitioning among the phases was determined. It was found that the matrix had reacted only with a portion of the carbon-rich outer layer of the SCS-6 fiber. The reaction zone contained two concentric zones which are distinguished by the presence of different carbide phases. Both zones contained a hexagonal Si-bearing phase, and one of the zones also contained some fine scattered porosity. The results are discussed with reference to available phase equilibria data. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Role of reinforcement in sintering of SiC/316L stainless steel composite

Abstract

Austenitic stainless steels with improved corrosion resistance are gaining wide popularity. However, their applications are limited because of their poor tribological properties. The present work was undertaken to improve the overall performance of 316L stainless steel by reinforcing it with SiC. During the processing of the 316L SS composite, the 316L SS matrix was found to interact strongly with the SiC at 1100°C resulting in the formation of low melting Fe–SiC phase. An attempt to process SiC/316L SS composite above this temperature resulted in complete melting of the composite compact.     

Time-dependent,environmentally assisted crack growth in nicalon-fiber-reinforced SiC composites at elevated temperatures

Subcritical crack growth measurements were conducted on ceramic matrix composites of β-SiC matrix reinforced with NICALON fibers (SiC/SiC_f); fiber-matrix interphases were of carbon andboron nitride. Velocities of effective elastic cracks were determined as a function of effective applied stress intensity in pure Ar and in Ar plus 2000, 5000, and 20,000 ppm O₂ atmospheres at 1100 °C. Over a wide range of applied stress intensities, theV-K _eff diagrams revealed a stage II pattern in which the crack velocity depends only weakly on the applied stress intensity, followed by a stage III, or power-law, pattern at higher stress intensity. Oxygen increased the crack velocity in stage II and shifted the stage II to III transition to the left. A two-dimensional (2-D) micromechanics approach, developed to model the time dependence of observed crack-bridging events, rationalized the measured effective crack velocities, their time dependence, the stage II to III transition, and the effect of oxygen in terms of the load relaxation of crack-bridging fibers. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RLO 1830 This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

30% SiC_p/2024Al复合材料的热变形行为

在Gleeble-1 500D热模拟机上采用等温压缩实验研究30%SiC_p/Al复合材料的高温压缩变形行为,获得该材料在温度为623~773 K,应变速率为0.01-10 s~(-1)的条件下的真应力-应变曲线,并在考虑摩擦和变形热效应的基础上对真应力-应变曲线进行修正。对修正后的峰值应力进行线性回归,建立该材料的本构方程。根据材料动态模型,计算并建立30%SiC_p/Al复合材料的热加工图,据此确定热变形流变失稳区。在应变速率为0.01 s~(-1)时,随热变形温度升高,该复合材料发生动态再结晶的体积分数增加。     

Mechanical behavior of Al-Li/SiC composites: Part II. Cyclic deformation

The deformation and failure mechanisms under cyclic deformation in an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes in low-cycle fatigue life is discussed.     

Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder

The compressive creep and stress-strain behavior of the near-eutectic 60Sn-40Pb solder alloy has been investigated over the temperature range of −55 °C to 125 °C. The total primary creep strain is a strong function of stress and temperature: at lower temperatures and high applied stresses (i.e., near the power-law breakdown regime), it is quite large, while it is much smaller at higher temperatures and lower applied stresses. The compressive minimum creep rate as a function of stress and temperature is fit well by the Garofalo sinh equation. A discussion of the effective stress exponent, n _eff, in the context of the Garofalo sinh equation is presented to understand trends in the creep data. The values of n _eff, for the applied stress levels studied, are found to range from 3.09 to 5.00 at 125 °C, while they have a range of 10.75 to 15.79 at −55 °C. These trends are consistent with the interpretation of climb-dominated creep at higher temperatures and plasticity-dominated power law breakdown behavior at the lower temperatures. The microstructural observations suggest that, at elevated temperatures, deformation occurs by relative displacement of eutectic colonies in the solder microstructure accompanied by extensive grain coarsening in the colony boundaries. At lower temperatures (<0 °C), deformation occurs by cell displacement with very limited coarsening and, at high stresses, is dominated by plastic deformation. The application of the Garofalo sinh equation to other data sets for creep of eutectic Sn-Pb solder is also discussed.     

Diffusion bonding reactions between a SiC/SiC composite and two superalloys during joining by hot isostatic pressing

Diffusion reactions during solid state joining of a ceramic SiC/SiC composite to two superalloys, Hastelloy X and Incology 909, by hot isostatic pressing (HIP) have been investigated. The HIP pressure was 200 MPa in all joining cycles, and the temperature/dwell time were either 800°C/15 min, 900°C/1 h or 1000°C/ 1 h. The reaction zones formed consisted of a thin layer of carbides surrounded by several layers containing silicides and free carbon. The thickness of the reaction layers increased with increasing temperature, but were more affected by the composition of the alloy. With more carbide formers in the alloy, the thickness of the reaction layer decreased. The SiC composite was found to be considerably more prone to reactions with these superallys during HIP as compared to Si₃N₄ under similar conditions.     

Effect of thermal residual stresses on fatigue crack opening and propagation behavior in an Al/SiC p metal matrix composite

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle-reinforced aluminum alloy have been measured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements using neutron diffraction have shown that this introduces an approximately parabolic stress field into the plates, varying from compressive at the surfaces to tensile in the center. Long fatigue cracks were grown in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. Crack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. By monitoring the compliance of the specimens during fatigue cycling, the degree to which the residual stresses close the crack has been evaluated. formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

High temperature deformation of an alumina composite reinforced with silicon carbide whiskers

Four-point bending creep tests were carried out in air on an alumina matrix composite reinforced with 9.3 vol.% of silicon carbide whiskers. Typical three-stage creep was observed. In the temperature range of 1673–1823 K, the composite exhibited an average stress exponent of 3.8. The activation energy for creep was estimated as ∼820–830 kJ mol⁻¹. Microstructure of the composite was characterized before and after deformation. Dislocation networks and other configurations were observed in samples deformed to large strains. It is concluded that the deformation mechanism consists of intragranular dislocation movement controlled by the lattice diffusion of oxygen ions.