METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 26A, No. 10, October 1995 This Month Featuring: Symposium on Dynamic Behavior of Materials: Part I; Alloy Phases; Transformations; Mechanical Behavior; Electronic, Magnetic & Optical Material; Materials Processing; and Composite Materials. View October 1995 Table of Contents.

SYMPOSIUM ON DYNAMIC BEHAVIOR OF MATERIALS: PART I

Densification Behavior of Dynamically Shock Compacted Al₂O₃/ZrO₂ Powders Synthesized Through Rapid Solidification
JOHN FREIM, J. MCKITTRICK, and W.J. NELLIS
The rapidly solidified alumina-zirconia eutectic contains high volume fractions of nanocrystalline t-ZrO₂, which makes the material a promising precursor for the manufacturer of fracture-resistant ceramic specimens. Unfortunately, conventional powder processing and sintering techniques are inadequate for the fabrication of dense specimens using this material. We have used dynamic shock compaction to facilitate the achievement of high density specimens which retain the unique microstructure of the precursor material. In an attempt to quantify the dynamics of the microstructural evolution which occurs during the compaction process, we have investigated the effect of various particle size distributions on the densification behavior of the material during the shock compaction and postcompaction sintering cycles. The shock compaction process produced high densities (~73 to 78 pct of single-crystal theoretical) by inducing a highly efficient packing of the particles. A bimodal powder distribution was also compacted and this specimen exhibited a relative density of 86.2 pct, approximately 10 pct higher than those of the unimodal compositions. In this compact, the small particles efficiently filled the interstices between the larger particles. The high density of the bimodal compact did not translate to a high sintered density, however.

The Structure of Controlled Shear Bands in Dynamically Deformed Reactive Mixtures
V.F. NESTERENKO, M.A. MEYERS, H.C. CHEN, and J.C. LaSALVIA
The structure of controlled high-strain-rate shear bands generated in heterogeneous reactive porous materials (Nb + Si, Mo + Si + MoSi₂) has been investigated using axially symmetric experimental configurations in which the source of energy is the detonation of low velocity explosives. The deformation was highly localized, with profuse formation of shear bands, which have thicknesses of 5 to 20 µm. The experimental method generated overall strains up to 100 and strain rates of approximately 10⁷ S^-1. Changes in particle morphology, melting, and regions of partial reaction on three different length scales were observed. The shear band thickness is smaller than the initial characteristic particle size of the porous mixture ( 44 µm), ensuring a cooling time of the deformed material on the same order of magnitude as the deformation time (10^-5 s). In the shear localization regions, two characteristic phenomena were observed: (a) a shear fracture subdividing the Nb particles into thin parallel layers and (b) the formation of vortices. A mechanism for the reaction inside the shear bands is proposed, and an expression for the largest size of chemical products as a function of shear deformation is obtained.

Strength and Ductility Under Dynamic Loading in Fine-Grained IN905XL Aluminum Alloy
TOSHIJI MUKAI, KOICHI ISHIKAWA, and KENJI HIGASHI
Three IN9OSXL aluminum alloys with fine grain (1 µm), intermediate grain (3 µm), and coarse grain (5 µm) have been developed by a combination of mechanical alloying (MA) and conventional extrusion in order to investigate their mechanical properties at dynamic strain rates of 1 x 10³ and 2 x 10³ s^-1 and a quasi-static strain rate of 10^-3 s^-1. Flow stresses are found to increase with decreasing grain size for all the strain rates tested. Negative strain-rate sensitivity of flow stress is observed up to 1 x 10³ s^-1 in both intermediate and coarse-grained IN9OSXL. At the highest strain rate of 2 x 10³ s^-1. however, all samples showed a positive strain rate sensitivity of strength. Total elongation at high strain rates is generally larger than that at low strain rates. Total elongation also decreases with grain size for all the strain rates. This decrease in elongation results from an initiation of microcracks at interfaces between the matrix and particles finely dispersed near grain boundary regions, introduced during MA processing; then, this initiation leads elongation of alloys to small limited values.

Dynamic Fracture Toughness of 4340 VAR Steel Under Conditions of Plane Strain
YOUNGSEOG LEE and VIKAS PRAKASH
Plate impact experiments are conducted to study the dynamic fracture processes in 4340 VAR steel which occur on submicrosecond timescales. These experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 µs. The loading is achieved by impacting a precracked disk-shaped specimen by a thin flyer plate. Motion of the rear surface of the specimen caused by waves diffracted from the stationary crack and by waves emitted from the running crack is monitored at four points ahead of the crack tip using a laser interferometric system. The measured rear surface motion is compared with the calculated motion using the finite element method to gain understanding of the dynamic fields that occur near the crack tip during crack initiation and propagation. For low temperature experiments the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velocity crack propagation model. For the room temperature experiments the experimental free surface particle velocity vs time profiles show a sharp spike with a duration of less than 100 ns at the moment of crack initiation. The spike which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics is understood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation times determined both from the velocity time profiles and the elastodynamic modeling of crack advance. The toughness values obtained increase with increasing impact velocity and are as large as 170 MPa at the highest impact velocity. Such relatively high values appear to be consistent with the ductile mode of crack initiation observed at all impact velocities used in the present study.

Shock Loading Response of 6061-T6 Aluminum Metal-Matrix Composites
KENNETH S. VECCHIO and GEORGE T. GRAY III
The purpose of this research was to systematically study the influence of peak-shock pressure and second-phase reinforcement on the structure/property response of shock-loaded 6061-T6 Al-alumina composites. The reload stress-strain response of monolithic 6061-T6 Al, used as a baseline for comparison, showed no increased shock hardening compared to the unshocked material deformed to an equivalent strain. The reload stress-strain response of the shock-loaded 6061-T6 Al-alumina composites exhibited a lower reload yield strength than the flow stress of the starting composites. The degree of strength loss was found to increase with increasing shock pressure. Wavespeed measurements of shock-prestrained specimens showed no degradation compared to unshocked specimens. indicating that particle cracking had not occurred under shock. This result was supported by optical metallography, which did not reveal cracked particles or particle decohesion in the shock-prestrained samples. The reload stress-strain response of the shock-prestrained composites, after resolutionizing and T6 reaging, showed that the composites recovered their full as received preshock stress-strain responses. This result supports the finding that degradation in reload strength was attributable to matrix microstructural changes resulting from the shock. Transmission electron microscopy (TEM) examination of the shock-loaded microstructures revealed that the matrix regions adjacent to the particle/matrix interface had undergone significant recovery and partial recrystallization resulting from the shock. This type of near-interface substructure is in stark contrast to the heavily dislocated near-interface dislocation substructure of the as-received composites. The loss of dislocation density (i.e., strain hardening) in the near-interface matrix region, resulting from the shock, highlights the importance of the thermally introduced dislocation substructure changes in establishing the strength of metal-matrix composites (MMCs).

Influence of Peak Pressure and Temperature on the Structure/Property Response of Shock-Loaded Ta and Ta-10W
GEORGE T. GRAY III and KENNETH S. VECCHIO
The deformation behavior and substructure evolution of unalloyed-Ta and Ta-1OW under quasistatic conditions have been compared to their respective responses when shock prestrained to 20 GPa at 25°C as well as to unalloyed-Ta shocked to 7 GPa at 25°C, 200°C, and 400°C. The reload yield behavior of shock-prestrained Ta and Ta-1OW did not exhibit enhanced shock hardening when compared to their respective quasistatic stress-strain response at an equivalent strain level. In addition, the reload yield behavior of Ta shock prestrained to 7 GPa at 200°C or 400°C was found to exhibit increased hardening compared to the shock prestraining at 25°C. The quasistatic substructure evolution and shock-hardening responses of Ta and Ta-1OW were investigated via transmission electron microscopy (TEM). The dislocation substructures in both materials and at each strain rate condition and temperature were similar and consisted primarily of long, straight, (a/2) < 111> type screw dislocations. The propensity for long, straight screw dislocations, irrespective of the loading condition, supports the theory of strong Peierls stress control on defect generation and defect storage. The substructure evolution and mechanical behavior of Ta and Ta-1OW are discussed in terms of defect storage mechanisms and compared to the mechanisms operative in face-centered cubic (fcc) metals.

Shock Consolidation of Mechanically Alloyed Amorphous Ti-Si Powders
S.C. GLADE and N.N. THADHANI
Mechanical alloying was used to synthesize amorphous 5Ti-3Si atomic ratio powders in a SPEX mill under Ar atmosphere. X-ray diffraction analysis revealed formation of a single-phase amorphous compound after about 24 hours of milling. High-resolution transmission electron microscopy (TEM) showed that the milled powder still contained nanocrystallites of Ti and Si among regions of generally amorphous compound. The mechanically alloyed amorphous powder was shock consolidated, using a plate impact assembly, to produce bulk compacts. The compaction resulted in a significant amount of crystallization, forming 30- to 40-nm crystals of TiSi₂ and Ti₅Si₃ intermetallic compounds. The compacts were subsequently annealed above the crystallization temperature, measured to be ~640°C using differential thermal analysis. The compacts annealed at 800°C for 1 hour showed only limited grain growth to ~50-nm crystallite size. Microhardness of the shocked amorphous alloy compacts was ~1100 KHN, which increased to ~1250 KHN upon subsequent annealing, with the formation of a more homogeneous nanocrystalline microstructure.

Dynamic Consolidation of Metastable Nanocrystalline Powders
G.E. KORTH and R.L. WILLIAMSON
Nanocrystalline metal powders synthesized by mechanical alloying in a ball mill resulted in micron-sized powder particles with a nanosized (5 to 25 nm) substructure. Conventional consolidation methods resulted in considerable coarsening of the metastable nanometer crystallites, but dynamic consolidation of these powders using explosive techniques produced fully dense monoliths while retaining the 5- to 25-nm substructure Numerical modeling, used to guide the experimental phase, revealed that the compression wave necessary for suitable consolidation was of the order of 10 GPa for a few tenths of a microsecond The consolidation process is described, and the retention of the metastable nanostructure is illustrated.

Mechanical Properties of Isothermally Aged High-Nitrogen Stainless Steel
JOHN SIMMONS
The effects of nitride (Cr2N) precipitation on the tensile, impact, and hardness properties of a typical high-nitrogen, low-carbon austenitic stainless steel (SS), nominally Fe-19Cr-5Mn-5Ni-3Mo-0.024C 0.69N, were determined. Annealed and cold-rolled (20 pct reduction in thickness) specimens were isothermally aged at 700°C and 900°C for times ranging from 0.1 to 10 hours. Only grain boundary Cr2N precipitation occurred in annealed materials aged at 700°C. Precipitation at 900°C occurred sequentially at grain boundaries, by cellular precipitation, and, finally, by transgranular precipitation within the matrix. Nitride precipitation had little effect on yield and ultimate strengths but reduced tensile ductility and impact toughness. Embrittlement occurred due to grain boundary separation (700°C and 900°C) and fracture through cellular precipitate regions, initiated at nitrides (900°C). Prior deformation increased precipitation kinetics and had a controlling influence on nitride morphology, enhancing grain boundary and transgranular Cr₂N and retarding cellular precipitation. Nitride struc tures produced in cold-rolled materials were just as detrimental to material plasticity as those pro duced in annealed materials, but prior deformation increased the kinetics of embrittlement. Due to strain recovery, the yield and ultimate strengths of cold-rolled materials decreased with aging time and temperature.

ALLOY PHASES

Effects of Tensile Stress on Microstructural Change of Eutectoid Zn-Ai Alloy
YAO HUA ZHU and ELIGIO OROZCO
Microstructural changes and phase transformations of eutectoid Zn-AI-based alloy ZnA120.2Cul.8 (wt pct) were studied under tensile stress by using X-ray diffraction and scanning electron microscopy (SEM) techniques. It was found that the lamellar microstructure of the heat-treated eutectoid Zn-AI based alloy changed partially into a spheroidized structure at the rupture part of the specimen after tensile testing, while the lamellar structure at the bulk part of the specimen remained stable in the original state. The X-ray diffraction identification results showed that two phase transformations, i.e., decomposition of metastable phase '_T and a four-phase transformation, + + , occurred during tensile testing. It was concluded that the tensile stress affected not only microstructural change but also phase transformation of the alloy. The SEM observation on the etched specimen showed clearly the morphology of the microstructural change.

Communication: Microstructural Stability of the (+) Solution-Treated And Quenched Near- Titanium Alloy Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.50Mo-0.35Si-0.06C
PARTHA GHOSAL, RAJESH PRASAD,and C. RAMACHANDRA

TRANSFORMATIONS

Interfacial Reactions in Al-Mg (5083)/Ai₂O₃P Composites During Fabrication and Remelting
W.M ZHONG, G L'ESPERANCE, and M SUERY
The interface reactions between -AI₂O₃ particles and 5083 Al-Mg alloys during fabrication by compocasting and subsequent remelting at 800°C for 30 minutes have been studied using analytical transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Experimental results show that MgO is the main reaction compound produced by the reaction 3Mg(l)+Al₂0₃ (s)=3MgO (s)+2AI(l). The MgO crystals formed by interfacial reaction are very small (about 5 to 20 nm in diameter), and the reaction zone is about 50 to 80-nm wide for as-cast materials and about 100 to 150 nm after remelting. The reaction kinetics is controlled by seepage of Mg and release of Al (l) along the newly formed MgAI₂O₄ or MgO grain boundaries. Because of the largevolume expansion during the formation of MgO from the reaction between Mg (l) and Al₂O₃ and because of the low diffusivity in MgO crystals, the reaction rate is very low.

Interfacial Reactions in Al-Mg (5083)/SiC Composites During Fabrication and Remelting
W.M. ZHONG, G. L'ESPERANCE, and M. SUERY
The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06 wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800°C for 30 minutes. The reaction mechanisms between SiC and liquid Al and between the SiO₂ Iayer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAI₂O₄) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO₂ layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing the SiC from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness of the initial SiO₂ layer.

Communication: Direct Evidence for Combustion Reaction on TiNi/TiC Composites Formation by Mechanical Alloying
J.Y. HUANG, L.L. YE, Y.K. WU, and H.Q. YE

MECHANICAL BEHAVIOR

Influence of Temperature and Strain Rate on Slip and Twinning Behavior of Zr
S.G. SONG and G.T. GRAY III
The stress-strain behavior of pure Zr was studied systematically at various temperatures and strain rates. At 76 K Zr deforms predominantly by twinning, whereas above room temperature (RT), slip is the controlling deformation mode. A transition in the rate-controlling deformation mode from slip to twinning has been observed to occur at intermediate temperatures during the course of plastic deformation. Above 373 K slip dominates the entire course of deformation. The transition from slip to twinning in the stress-strain behavior is linked to differing strain-hardening rates and temperature sensitivities of the two deformation modes.

ELECTRONIC, MAGNETIC & OPTICAL MATERIAL

MATERIALS PROCESSING

Constitutive Modeling for CdTe Single Crystals
J.C. MOOSBRUGGER and A. LEW
To understand the role of thermal stresses in the generation, multiplication, and propagation of dislocations in CdTe single crystals produced by directional solidification, constitutive models which accurately reflect the elastic-viscoplastic behavior of CdTe over a wide range of temperatures are needed. In this article, the relevant reported mechanical behavior of CdTe is reviewed and discussed. Constitutive equations developed for single slip, isothermal behavior of elemental semiconductor crystals by Haasen and co-workers, which include dislocation density as the important internal variable, are then extended to include an additional dislocation arrangement internal variable as well as a high-temperature, time-dependent recovery behavior. The constitutive framework is incorporated in a continuum slip framework to include the possibility of multiple slip and to relate slip system shear strain rates to the macroscopic plastic strain rate. Comparison of the model with available experimental data for the small strain case over a wide range of temperatures is presented. Slip system interaction is included. These constitutive equations can then be used in computational analyses of thermal stress generation for comparison with characterized crystals grown in microgravity and ground-based experiments.

Evolution of Microstructure During Fabrication of Zr-2.5 Wt Pct Nb Alloy Pressure Tubes
D. SRIVASTAVA, G.K. DEY, and S. BANERJEE
Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step. have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated + _l microstructure is required for obtaining the desired level of strength at 310°C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.

Communication: Intermetallic Sheets Synthesized From Elemental Ti, Al, And Nb Foils
D.E. ALMAN and C.P. DOGAN

COMPOSITE MATERIALS

The Transverse Creep Deformation and Failure Characteristics of SCS-6/Ti-6Al-4V Metal Matrix Composites at 482°C
M.R. EGGLESTON and A.M. RITTER
While continuous fiber unidirectional composites are primarily evaluated for their longitudinal properties the behavior transverse to the fibers often limits their application. In this study the tensile and creep behaviors of SCS-6/Ti-6A14V composites in the transverse direction at 482°C were evaluated. Creep tests were performed in air and argon environments over the stress range of 103 to 276 MPa. The composite was less creep resistant than the matrix when tested at stress values larger than 150 MPa. Below 150 MPa the composite was more creep resistant than the unreinforced matrix. Failure of the composite occurred by the ductile propagation of cracks emanating from separated fiber interfaces. The environment in which the test was performed affected the creep behavior. At 103 MPa the creep rate in argon was 4 times slower than the creep rate in air. The SCS-6 silicon-carbide fibers graphite coating oxidized in the air environment and encouraged the separation of the fiber-matrix interface. However at higher stress levels the difference in behavior between air- and argon-tested specimens was small. At these stresses separation of the interface occurred during the initial loading of the composite and the subsequent degradation of the interface did not affect the creep behavior. Finally the enrichment of the composites surface by molybdenum during fabrication resulted in an alloyed surface layer that failed in a brittle fashion during specimen elongation. Although this embrittled layer did not appear to degrade the properties of the composite the existence of a similar layer on a composite with a more brittle matrix might be very detrimental.

Anelastic Relaxation In Al-4 Wt Pct Cu-AI₂O₃ Fiber-Reinforced Composites
STEFANO SGOBBA, LORENZO PARRINI, HANS-ULRICH KUNZI, and BERNHARD ILSCHNER
In many industrial applications, Iike high precision weighing and positioning, the elastic and dimensional stability of materials is required at a nanometric scale. High-resolution laser interferometry and mechanical spectroscopy have been employed to measure low-temperature anelastic creep of the short-fiber-reinforced composite Al-4 wt pct Cu-AI₂O₃. The typical strain resolution of the laser interferometer is 10^-10. Fiber reinforcement has been found to increase the dislocation density in the metal matrix; in parallel, damping and anelastic creep are enhanced. This behavior has been explained on the basis of the structure of interparticle dislocations and ' relaxation.