Dislocation dynamics in strain aging alloys

The problem of dynamic strain aging is treated using a new approach to the aging kinetics. The moving dislocations are considered as an ensemble of double loops which overcome the forest dislocations by thermal activation. This implies an exponential distribution of waiting times in front of the obstacles during which additional pinning agents arrive at the dislocation. The temporal development of the mean number of pinning points per loop can be calculated from an appropriate master equation which is based on the dynamic transition rates for pinning and depinning of the dislocation segments. It is shown that the two pinning mechanisms, lineal pinning and forest strengthening discussed in the literature, can be treated in a unified way. This leads to the definition of a single parameter, the mobility factor, which completely describes the effect of dynamic aging on dislocation velocity and plastic strain rate. The role of local instabilities and the problem of strain rate sensitivity are discussed.     

Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue

An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760 °C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the present study.     

Dislocation structures in fatigued polycrystalline copper

The dislocation structures in fatigued polycrystalline copper with small average grain size were investigated over a plastic strain range from 1.5 x 10⁻⁵ to 10⁻². It was found that the dislocation structures are arranged into three types of configurations, which correspond to the three regions in the cyclic stress-strain curve. Cylidirical loop patch structure are present in region A for low strain amplitudes, similar to those observed previously in coarse grained polycrystals. Moreover, irregular loop patches are also formed in this region for small grains polycrystals rather thanin region B at intermediate strain amplitudes for coarse grained polycrystals. In region B, persistent slip band (PSB) structures are formed but with a low volume content compared with the coarse grained polycrystals. In region C, at high plastic strains, the dislocation structures are dominated by dipolar walls. In addition, labyrinth structures are developed in region C instead of region B for coarse grained polycrystals. All the dislocation structures observed are viewed as forms of dipolized structures. A dipolized dislocation arrangement model is proposed to describe the formation process of dislocation structures. It is shown that all the dislocation configurations formed in cycled polycrystalline copper are low energy structures.     

Shape distortion in liquid-phase-sintered tungsten heavy alloys

Shape retention during liquid phase sintering is a major concern at high liquid contents, or large density differences between the solid and the liquid phases. This study demonstrates the role of microstructural parameters in controlling the bulk dimensional changes that occur during liquid phase sintering of tungsten heavy alloys (WHAs). Tungsten-nickel-copper alloys containing 80 wt pct tungsten, the balance containing Ni and Cu in the ratio 6:4, 7:3, or 8:2, were sintered at temperatures between 1400 °C and 1500 °C. Compact distortion was quantified using a coordinate measuring machine and related to the microstructural parameters, such as solid volume fraction, grain size, dihedral angle, grain continguity, and connectivity. Supplementary experiments were performed on W-Ni, W-Cu, and Mo-Cu alloys to compare the role of microstructural parameters in controlling distortion. A low solid solubility and a small grain size coupled with a high dihedral angle and connectivity restrict distortion. Based on the experimental observations and stereological relations, the critical solid content required to maintain structural rigidity is related to specific combinations of dihedral angle and grain connectivity.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

The breakdown of single-crystal solidification in high refractory nickel-base alloys

The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.     

Dislocation substructure in Ta-Re-N alloys deformed at 77 K

High purity Ta, Ta-Re, and Ta-Re-N alloy single crystals were deformed in tension at 77 K, and the resulting dislocation arrangements were studied by transmission electron microscopy. Re and N have similar effects on the dislocation substructure. Alloying increases the fraction of primary screw dislocations at the expense of debris, tangling, and secondary dislocations. For a given increment in yield stress, Re causes much larger changes in the substructure than N. The substructures observed in Ta-Re-N alloys are similar to those in Ta-Re alloys, even though ternary alloys exhibit alloy softening and binary alloys exhibit alloy hardening. These observations can be explained in terms of the different intrinsic mobilities of edge and screw dislocations, the different interactions which substitutionals and interstitials have with edge and screw dislocations, and the large differences in concentration of substitutional and interstitial solutes.     

Interfacial embrittlement in liquid-phase sintered tungsten heavy alloys

The variations in toughness of the liquid-phase sintered heavy alloys W-Ni-Fe and W-Ni-Cu have been examined. Toughness was found to be controlled primarily by the strength of the tungsten particle-matrix interfaces, which is sensitive to the rate of cooling from the sintering temperature. Furnace-cooling led to the embrittlement of these interfaces; in the case of W-Ni-Fe transmission electron microscopy identified interfacial precipitation of a W(NiFe) intermetallic compound, and in W-Ni-Cu Auger electron spectroscopy indicated interfacial segregation of the trace elements phosphorus and sulfur. This embrittlement was effectively reduced by solution treating beneath the sintering temperature and quenching     

Dislocation structures in disordered and ordered Ni3Fe

Transmission electron microscopy (TEM) methods have been used to examine the dislocation structures in thin foils of Ni₃Fe in four different states, corresponding to disordered and deformed; fully ordered and deformed; deformed when disordered and afterwards fully ordered; and deformed when disordered, afterwards fully ordered and additionally deformed. The study has been carried out on single crystals deformed at room temperature. In the disordered alloy slip is coarse and group motion of dislocations is prevailing, as is confirmed by the abundance of planar dislocation arrays. The dislocation structure of such a disordered deformed crystal remains unchanged after additional ordering by annealing 1000 h at 460°C. No rearrangement of the unit dislocations into superlattice dislocations is observed. The dislocations are preserved and since they are unit dislocations they are sessile. The additional deformation of this disordered deformed and afterwards fully ordered crystal proceeds by the glide of superlattice dislocations. They originate in the high internal stresses of the preserved unit dislocations. Cross slip from (111) onto (11̄1) planes is very frequent, whereas cross slip from (111) onto (010) planes is rather rare. The structure of the superlattice dislocations in fully ordered and deformed specimens consists of dipoles and bundles of dipoles of near edge orientation. Superlattice dislocations of near screw orientation are rarely observed, since they cross slip from (111) onto (11̄1) planes and annihilate in most cases. The experimental results on ordered Ni₃Fe samples differ characteristically from those reported in the literature on other alloys having the L1₂ long range ordered structure (e.g. Ni₃Al, Cu₃Au, Ni₃Ga).     

Dislocation production in alpha Fe-C alloys by rapid heating

It is demonstrated that metastable carbide particles in α-Fe act as sources of dislocations when heated rapidly to a temperature where the particles are unstable. The type and arrangement of such dislocations are dependent on the rate of heating. At moderate heating rates, the particles produce well-defined groups of equally spaced interstitial dislocation loops lying on {100} planes, with Burgers vectors ofa<100>. Cementite (Fe₃C) particles of comparable size dissolve without producing dislocations. The original metastable particle sites retain carbon in the form of small globular particles which may be graphite. At faster heating rates, the dislocation arrangement becomes more disorderly with both b =a<100> and b =a/2<111> dislocations emanating from the same particle.     

Dislocation substructure in Ta-Re-N alloys deformed at 77 K

High purity Ta, Ta-Re, and Ta-Re-N alloy single crystals were deformed in tension at 77 K, and the resulting dislocation arrangements were studied by transmission electron microscopy. Re and N have similar effects on the dislocation substructure. Alloying increases the fraction of primary screw dislocations at the expense of debris, tangling, and secondary dislocations. For a given increment in yield stress, Re causes much larger changes in the substructure than N. The substructures observed in Ta-Re-N alloys are similar to those in Ta-Re alloys, even though ternary alloys exhibit alloy softening and binary alloys exhibit alloy hardening. These observations can be explained in terms of the different intrinsic mobilities of edge and screw dislocations, the different interactions which substitutionals and interstitials have with edge and screw dislocations, and the large differences in concentration of substitutional and interstitial solutes.     

Dislocation structures ahead of advancing cracks

Previous transmission electron microscope (TEM) observations of the dislocation structure in the vicinity of a crack suggested that the region immediately ahead of a crack is devoid of dislocations. In the present paper, the results ofin situ TEM deformation experiments in numerous systems are described. The dislocation configurations are generally complex, with dislocations extending from the crack tip(i.e., no dislocation-free zone (DFZ)) and forming complex arrangements in the plastic region in front of the crack tip. Crack advance was accompanied by the emission of dislocations from both the crack tip and nearby sources. These observations are summarized, and the theory of dislocation configurations in front of a crack is reconsidered. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

钼钨合金烧结致密化行为

采用原位测量法研究了放电等离子烧结与真空热压烧结Mo–30W合金收缩和致密化行为。研究结果表明:采用放电等离子烧结Mo–30W合金时,1200 ℃以下Mo–30W合金以膨胀为主,1200 ℃以上合金开始剧烈收缩,1600 ℃以上合金收缩趋于停止,在降温阶段合金有较大收缩,温度接近室温时,收缩基本停止。经过1600 ℃放电等离子烧结后合金的相对密度可达93%以上,优于相同温度下真空热压烧结合金的相对密度89.98%。