Modeling of mechanical alloying: Part II. Development of computational modeling programs

Computational modeling programs incorporating the physics of powder deformation, fragmentation, and coalescence occurring during mechanical alloying (MA) are developed. The programs utilize the equations developed in part I of this series; equations predicting the extent of powder deformation during an effective impact in MA and those specifying criteria for powder particle fragmentation and coalescence. Two programs have been developed for these purposes. One, MAPI, considers the behavior of a single species with the option of adding dispersoids. The other, MAP2, considers two ductile species being welded to form a third, composite species. Applications of the programs to previous experimental data, and for the purpose of identifying the effect of material and process variables on alloying behavior, are provided in the article following this one. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903 Formerly Professor, Department of Materials Science and Engineering, University of Virginia     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190°C and 260°C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases. T. FOGLESONG formerly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190 °C and 260 °C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases.     

The mechanism of mechanical alloying

The mechanical alloying process is a new method for producing composite metal powders with controlled microstructures. It is unique in that it is an entirely solid state process, permitting dispersion of insoluble phases such as refractory oxides and addition of reactive alloying elements such as aluminum and titanium. Interdispersion of the ingredients occurs by repeated cold welding and fracture of free powder particles. Refinement of structure is approximately a logarithmic function of time, and depends on the mechanical energy input to the process and work hardening of the materials being processed. A condition of steady state processing is eventually achieved marked by saturation (constant) hardness and constant particle size distribution, although structural refinement continues. Evidence of this is presented, and the nature of the cold welding and characteristics of the processed powder are described.     

Modeling of heat flow in sand castings: Part II. Applications of the boundary curvature method

In Part I of this paper, a new method was presented for modeling heat flow in sand castings. In this paper, the method is used to simulate the solidification of parts of varying complexities. Comparisons are made between conventional calculations (with the mold enmeshed), calculations using the boundary curvature method, and experimental results. The results are shown to agree well with each other. Reductions in cpu time up to 96 pct are found by adopting the new method.     

The physics of mechanical alloying: A first report

In this paper, we present a first attempt to define the basic geometry, mechanics, and physics of the process of mechanical alloying. The geometry of the collision events which lead to particle fragmentation and coalescence is modeled on the basis of Hertzian contacts between the grinding media which entrap a certain amount of material volume between the impacting surfaces. This geometry essentially defines the volume of material affected per collision, and from this information and characteristics of the specific mill and the material being processed, impact times, powder strain rates and strains, powder temperature increase, powder cooling times, and milling times can be approximated.     

The chemical kinetics of mechanical alloying

The chemical kinetics of a mechanically induced double displacement reaction exhibiting a self-sustaining combustion event have been investigated using differential thermal analysis. In particular, the effect of milling time on the activation energy for the reduction of cupric oxide by iron has been evaluated. It has been shown that the activation energy decreases from 575 kJ/mol at the start of milling to 199 kJ/mol at combustion. This is an indication that the rate-controlling step changes during milling from intrinsic ionic diffusion initially to diffusion down short-circuit pathways prior to combustion. It is suggested that this is dependent on the rate at which strain accumulates in milled powders and is hence a consequence of the mechanical alloying action.     

On the kinetics of mechanical alloying

The kinetics of solid-state displacement reactions during mechanical alloying have been investigated. The effects of charge ratio and ball size on the progress of the reaction between CuO and Fe have been evaluated from measurements of ignition temperature, combustion time, and crystallite size. The reaction kinetics are shown to increase with charge ratio. This is rationalized in terms of the effect of charge ratio on the number of ball/particle collisions. Ball size influences reaction kinetics through both the particle collision frequency and collision energy.     

Fe-Si机械合金化过程的研究

采用高能行星球磨的方法研究了原子配比3：1的Fe、Si混合粉末的机械合金化过程。用XRD、TEM、SEM及EPMA对球磨不同时间粉末的结构、组织、形貌、截面进行了分析。结果表明：Fe75Si25混合粉末在球磨的过程中出现两种形态变化，一种是Fe与Si形成层状形态，另一种为Si及Fe—Si合金包覆Fe形成包覆形态；球磨至30h，合金化基本完成；球磨产物为α—Fe（Si）固溶体，颗粒粒径约为1～20μm。利用一个简单的模型来对Fe75Si25混合粉末合金化过程进行了描述。     

Properties of materials produced by mechanical alloying

Properties of the materials produced by plastic deformation (by the shear method) under pressure from powder mixtures made of elements not interacting at room temperature are studied. Powder mixtures of three systems, viz., aluminum-graphite, copper-lead, and aluminum-lead are studied. Some compositions are studied in each system. It is estabished that dispersion and strength properties of the materials produced are higher than in those made by multipass rolling.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 87–90, October, 1992.     

The evolution of solutions: A thermodynamic analysis of mechanical alloying

Normal thermodynamic theory for solutions begins with the mixing of component atoms. Many solutions are, however, prepared by mixing together lumps of the components, each of which might contain millions of identical atoms. We examine here the way in which a solution evolves from these large clusters of components, from a purely thermodynamic point of view. There are some interesting results, including the prediction that solution formation by the mechanical alloying of solid components cannot occur unless there is a gain in coherency as the particles become small. The nature of the barrier to mechanical alloying is discovered. There is also the possibility of a metastable state prior to the achievement of full solution, when the component atoms prefer like-neighbors.     

Applications of distraction osteogenesis. Part I

Distraction osteogenesis (DO) of facial bones is a recent development in the treatment of pediatric patients. The use of DO in current clinical practice of pediatric reconstructive surgery is primarily limited to severe deformities of the lower jaw, most of which are congenital in nature. Clinical experience with DO for early facial deformities remains limited, and no authoritative works are currently available to guide clinicians in the techniques or indications for DO of the facial skeleton.     

Modern computational chemistry and drug discovery: structure generating programs

OBJECTIVE: To evaluate the relevance of IgA-containing immune complexes (IC) as a predictor of lymphocytic infiltration of the minor salivary glands, and thus to determine the necessity of the minor salivary gland biopsy as a diagnostic test for primary Sj?gren's syndrome (SS) in patients complaining of dryness of the mouth. METHODS: IgA-containing IC, as well as anti-SSA and anti-SSB antibodies, were measured using enzyme-linked immunosorbent assays in 116 consecutive patients presenting with dry mouth but no connective tissue disease. The specificity, sensitivity, and positive (PPV) and negative predictive values (NPV) of these tests were calculated in relation to the results of the minor salivary gland biopsy and to the criteria for primary SS. RESULTS: Sixty-five patients had a focus score > or = 1.IgA-containing IC were detected in 45 of them, compared with five of the remainder (specificity 89%, sensitivity 69%, PPV = 88% and NPV = 69%). When the IgA-containing IC, and the anti-SSA and anti-SSB tests were associated, the sensitivity and NPV were improved (81 and 79%, respectively), while specificity and PPV were maintained (88 and 90%, respectively). CONCLUSION: Given the reliability of this combination of tests for the diagnosis of primary SS, the minor salivary gland biopsy might be indicated only in those patients without any serological abnormality.     

Amorphization reaction of Ni-Ta powders during mechanical alloying

This study examined the amorphization behavior of Ni _x Ta_100−x alloy powders synthesized by mechanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni₁₀Ta₉₀ and Ni₈₀Ta₂₀. A supersaturated nickel solid solution formed for Ni₉₀Ta₁₀, as well. X-ray diffraction analysis reveals two different types of amorphization reactions. Through an intermediate solid solution and by direct formation of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition.     

Niti and NiTi-TiC composites: Part III. shape-memory recovery

The transformation behavior of near-equiatomic NiTi containing 0, 10, and 20 vol pct TiC particulates is investigated by dilatometry. Undeformed composites exhibit a macroscopic transformation strain larger than predicted when assuming that the elastic transformation mismatch between the matrix and the particulates is unrelaxed, indicating that the mismatch is partially accommodated by matrix twinning during transformation. The thermal recovery behavior of unreinforced NiTi which was deformed primarily by twinning in the martensite phase shows that plastic deformation by slip increases with increasing prestrain, leading to (1) a decrease of the shape-memory strain on heating, (2) an increase of the two-way shape-memory strain on cooling, (3) a widening of the temperature interval over which the strain recovery occurs on heating, and (4) an increase of the transformation temperature hysteresis. For NiTi composites, the recovery behavior indicates that most of the mis-match during mechanical deformation between the TiC particulates and the NiTi matrix is relaxed by matrix twinning. However, some relaxation takes place by matrix slip, resulting in the following trends with increasing TiC content at constant prestrain: (1) decrease of the shape-memory strain on heating, (2) enhancement of the two-way shape-memory strain on cooling, and (3) broadening of the transformation interval on heating. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology