Degassing Behavior of Nanostructured Al and Its Composites

The synthesis of bulk ultrafine-grained (UFG) and nanostructured Al via cryomilling can frequently require a degassing step prior to consolidation, partly due to the large surface area of the as-milled powders. The objective of this study is to investigate the effects associated with cryomilling with stearic acid additions (as a process-control agent) on the degassing behavior of Al powders. This objective was accomplished by completing select experiments with Al-7.5Mg, Al-6.4 wt pct Al85Ni10La5, and Al-14.3 wt pct B4C. The interaction between Al and stearic acid was determined using thermal analysis combined with Fourier transform infrared spectroscopy (FTIR). The degassing experiments were carried out under high vacuum (10−4 to ~10−6 torr) in a range from room temperature to 400 °C, with the pressure of the released gases monitored using a digital vacuum gage. The results showed that the liberation of chemisorbed water was suppressed in cryomilled Al powders and both the chemisorbed water and stearic acid were primarily released in the form of hydrogen. It was also demonstrated that under certain conditions, a nanostructure (grain size ~100 nm) can be retained following the hot vacuum degassing of cryomilled Al

Strengthening Mechanisms in a High-Strength Bulk Nanostructured Cu-Zn-Al Alloy Processed Via Cryomilling and Spark Plasma Sintering

A bulk nanostructured alloy with the nominal composition Cu-30Zn-0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (~200-400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)-O-N, Al-O-N and Al-N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength

Quasi-Static Deformation and Final Fracture Behaviour of Aluminium Alloy 5083: Influence of Cryomilling

The commercial aluminium alloy 5083 was processed via cryomilling to produce nanocrystalline (NC) powders with an average grain size of ∼25–50 nm. The powders were subsequently degassed at 723 K (450°C), pre-heated and immediately quasi-isostatic (QI)-forged to produce a thermally stable bulk ultrafine grain (UFG) material having average grain size values ranging from 190 to 350 nm, depending on the processing conditions used. In this paper, the tensile properties and fracture behaviour of the bulk UFG material are presented and compared with the tensile properties of its conventionally processed counterpart. The specific influence of preheat temperature on strength and ductility of the alloy is briefly discussed. Three different pre-heat temperatures of 523, 623 and 723 K (250, 350 and 450°C) were chosen and used with the primary objective of controlling grain growth during forging. The influence of preheat temperature on tensile deformation and final fracture behaviour is highlighted. The macroscopic fracture modes of the bulk nanostructured material (BNM) prepared following three pre-heat temperatures are investigated. The microscopic mechanisms controlling tensile deformation and final fracture behaviour are discussed with regards to the intrinsic microstructural effects in the UFG alloy, nature of loading, and the kinetics and mechanisms of deformation

The Influence of Processing on Microstructural Development, Tensile Response and Fracture Behavior of Aluminum Alloy 5083

In this paper, the specific influence of quasi-isostatic forging and rolling of cryomilled powder on microstructural development and resultant tensile deformation and fracture behavior of aluminum alloy 5083 is highlighted and comparison made with the coarse grained counterpart. The specific influence and contribution of strain hardening to enhancing strength of the ultra-fine grain microstructure of the aluminum alloy is presented and discussed. It is shown that the capability of the ultra fine grain microstructure to recover strength through the mechanism of work hardening is quite similar to the conventionally processed counterpart. The influence and role of intrinsic microstructural features in governing tensile deformation and fracture behavior is elaborated upon. The viable microscopic mechanisms governing final fracture behavior is discussed in light of the competing and mutually interactive influences of nature of loading, intrinsic microstructural effects, and deformation kinetics. Key Words: aluminum alloy 5083, processing, microstructure, tensile properties, fractur

Influence of Length-Scales on Spatial Distribution and Interfacial Characteristics of B₄c in a Nanostructured Al Matrix

To provide fundamental insight into the influence of length scales on the spatial distribution and characteristics of ceramic/metal interfaces in nanostructured metal matrix composites, we studied an Al alloy reinforced with a broad size distribution of B4C particles, ranging from several nanometers to submicrometers. The B4C was incorporated into an ultrafine grained Al matrix using cryomilling and thermomechanical consolidation. The characteristics of the B4C/Al interface, namely the local chemistry and interfacial structure, were studied in detail using transmission electron microscopy (TEM) and atom-probe tomography. Results reveal significant differences in these characteristics as a function of particle length scale. A significant proportion (~40%) of B4C nanoparticles was located intragranularly, i.e.; within ultrafine grain interiors, whereas submicron B4C particles were surrounded by multiple Al grains, creating intergranular interfaces. Mg-O-N layers, Al2O3 nanoparticles and amorphous regions were observed at the intergranular interfaces, whereas most of the intragranular interfaces were semicoherent and free of other phases or segregation. By combining crystal structure modeling and high-resolution TEM, a specific orientation relationship was identified for the intragranular interfaces: (1 -1 1)Al//(024)B4C, 3.4° angle between (0 0 2)Al and (003)B4C, and 7.8° angle between (2 -2 0)Al and (021)B4C. Mechanisms related to length scale effects on the formation of intragranular versus intergranular interfaces and corresponding structures and chemistries are discussed, as well as the implications of these interface characteristics on strength and ductility

Strengthening Mechanisms and Deformation Behavior of Cryomilled Al-Cu-Mg-Ag Alloy

In the last decade, the commercially available heat-treatable aluminum alloy (AA) 2139 (Al-Cu-Mg-Ag) has generated interest within the aerospace and defense communities because of its high strength and damage tolerance as compared to those of other AA 2XXX alloys. In this work we investigate the possibility of enhancing the performance of AA 2139 via a nanostructuring approach involving the consolidation of cryomilled powders. For comparison purposes, two types of feedstock powders (cryomilled and unmilled, gas-atomized powder), were consolidated via dual mode dynamic forging. Our results show that, following heat treatment (HT), the strength of the cryomilled material increases in the range of ~25% to ~200% relative to that of the unmilled counterparts, depending on specific processing parameters. We present microstructural data, including grain size and precipitate chemistry, to provide insight into the underlying strengthening mechanisms. Vickers microhardess tests are used to evaluate peak heat treatments, and tensile testing is performed to characterize mechanical behavior. The kinetics of precipitation, strengthening mechanisms and deformation behavior are discussed. It is proposed that the combination of elemental segregation with the presence of oxides along grain boundaries, both facilitated by enhanced diffusion paths, are responsible for the observed change in HT kinetics in the cryomilled material

Influence of pressing temperature on microstructure evolution and mechanical behaviour of ultrafine-grained Cu processing by equal-channel angular pressing

Pure Cu was processed by ECAP at five different temperatures from room temperature (RT) to 523?K. The influence of pressing temperature on microstructure evolution and tensile behavior was investigated in detail. The results show that as the ECAP temperature is increased the grain size and ductility both increase whereas the dislocation density and yield strength decrease. In the case of ECAP processing in the range of RT to 473?K the mechanism governing microstructural refinement is continuous dynamic recrystallization (CDRX), whereas at 523?K the mechanism changes to discontinuous dynamic recrystallization (DDRX). At higher ECAP temperatures, the kinetics of CDRX are retarded leading to a lower fraction of equiaxed grains/high-angle grain boundaries and a higher fraction of dislocation cell structures. At 523?K, DDRX induces a high fraction of equiaxed grains with a very low dislocation density which appears responsible for the observed high tensile ductility. The sample processed at 523?K possessed a good combination of strength and ductility, suggesting that processing by ECAP at elevated temperatures may be a suitable alternative to RT ECAP processing followed by subsequent annealing