Discussion of “the role of microstructure on strength and ductility of hot-extruded mechanically alloyed NiAl”

M. DOLLAR, S. DYMEK, SJ. HWANG, and P. NASH:Metall. Trans. A, 1993, vol. 24A, pp. 1993-2000.     

Structure of mechanically alloyed Ti-Al-Nb powders

Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5Al-12.5Nb, and 28.5Al-23.9Nb (at. pct) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffraction and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bec) and amorphous phases at longer milling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhombic (O-Ti₂AlNb) or a mixture of the orthorhombic (O) and hexagonal close-packed (α₂-Ti₃Al) phases, the proportion of phases being dependent on the powder composition. Milling beyond the amorphous phase formation resulted in the formation of an fee phase in all the powders, which appears to be TiN, formed as a result of contamination of the powder. Formerly Graduate Student, University of Idaho     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

Ductile fracture of mechanically alloyed iron-yttria alloys

The influence of Y₂0₃ particle content on the tensile fracture of mechanically alloyed iron has been studied for a series of dispersion-strengthened alloys containing up to 10 vol pct particles. When compared to the behavior of spheroidized steels, the present results indicate that at comparable volume fractions of particles, the Fe-Y₂0₃ alloys exhibit a much decreased tensile ductility. Observations of microscopic damage indicate that this is a consequence of rapid void nucleation at small strains, limited void growth, and enhanced void linking, especially at high particle contents. Analysis of these observations suggests (a) a surprisingly high oxide particle-matrix interfacial bond strength, (b) an influence of rapid strain hardening at small strains in creating high flow stresses, which assist void initiation, (c) enhanced void nucleation at high volume fractions of particles due to neighboring particles and voids, and (d) an accelerated void-linking process at high volume fractions of particles when interparticle spacing approaches particle/ void size. J.B. KOSCO, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Pennsylvania State University.     

Elevated-temperature stability of mechanically alloyed Cu-Nb powders

When two-phase mixtures of ductile metals are mechanically alloyed, they often assume a convoluted lamellar structure. Since these powders are consolidated at elevated temperatures, their structures (and, therefore, properties) are likely to be altered by consolidation processing. We have investigated microstructural changes that take place on heat-treating mechanically alloyed Cu −20 vol pct Nb alloys. The transition from a “platelike” to a spherical microstructure is described, and the kinetics of this process appear controlled by a type of boundary diffusion, even though the coarsening temperature was high in terms of the homologous temperature of Cu. Reasons for this behavior are suggested. Finally, during heat treatment (carried out in H), a Nb layer forms around the particles. The thickness of this layer (and the corresponding zone denuded of Nb within the particle) increases with continued elevated-temperature exposure, and at a rate consistent with the process being driven by curvature forces. Formerly Professor, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA     

Microstructural and fracture characterization of Nb-Cr-Ti mechanically alloyed materials

Three materials containing Nb, Cr, and Ti were fabricated by consolidating powders made by mechanical alloying. The Nb/Ti ratio was maintained at about 1.3 and Cr was increased to form the intermetallic Cr₂Nb. X-ray diffraction, metallography, and transmission electron microscopy were used to thoroughly characterize the microstructure and substructure of the materials. Fatigue and fracture toughness properties were also evaluated at ambient temperature. The alloyed powders contained only small amounts of intermetallic, but during the consolidation heat treatment, two of the materials precipitated large volume fractions of Cr₂Nb. In the third material, Cr₂Nb was precipitated by heat treatment, although this was not expected from the composition based on the Nb-Cr-Ti phase diagram. Maximum fracture toughness of the composutes was ≈ 11 MPa √m. The low fracture toughness was attributed to the high plastic constraint of matrix deformation by the Cr₂Nb and compositional change in the matrix.     

Compaction and characterization of mechanically alloyed nanocrystalline titanium aluminides

Blended elemental (BE) Ti-24 at. pct Al-11 at. pct Nb (Ti-24-11) and Ti-55 at. pct Al (Ti-55) powders and prealloyed (PA) Ti-24-11 powders were mechanically alloyed in a SPEX mill or an attritor. After SPEX milling for 10 hours, the BE Ti-24-11 powder contained the B2/bcc phase, while the BE Ti-55 powder showed the presence of an amorphous phase. The PA Ti-24-11 powder containing the B2 phase showed a decrease of crystal size on milling. These powders were consolidated by hot isostatic pressing (“hipping”), Ceracon process, and dynamic methods. On compaction, the B2/bcc phase in the Ti-24-11 sample transformed to a mixture of the B2 and orthorhombic (“O”) phases, while the amorphous phase in the Ti-55 powder crystallized to a mixture of the γ-TiAl and α ₂-Ti₃Al phases. The finest grain size in compacted material was obtained in the dynamically consolidated powder, and the grain size in the hot isostatic pressed (“hipped”) powder became larger with the increasing hipping temperature.     

Densification and structural change of mechanically alloyed W-Cu composites

Fine-grained, high-density (97+ pct of theoretical density (TD)), 80W-20Cu wt pct (58W-42Cu at. pct) composites have been prepared using nonconventional alloying techniques. The W and Cu precursor powders were combined by a high-energy ball-milling procedure in air or hexane. The mechanically alloyed W+Cu powder mixtures were then cold pressed into green compacts and sintered at 1523 K. The milling medium and milling time were varied to increase product densities with a concomitant order-of-magnitude decrease in grain size. For densification, air was found to be a more effective medium than hexane. From microhardness measurements, it was concluded that the W-Cu alloys were dispersion and solution hardened, but were sensitive to entrapped residual impurities. X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and scanning electron micros-copy (SEM) analyses were used to demonstrate that the as-milled and sintered W-Cu alloy structures were metastable, decomposing into the starting W and Cu components upon heating at or above 723 K.     

Mechanical behavior and properties of mechanically alloyed aluminum alloys

The fracture and deformation behaviors of several product forms produced from mechanically alloyed (MA) aluminum alloys 9052 and 905XL were studied. The main operative strengthening mechanism is strengthening due to the submicron grain size. Ductility and toughness were found to be controlled by the morphology of the prior particle boundaries. We propose that the work-hardening behavior of these MA alloys is similar to the behavior exhibited by a deformed fcc alloy that (a) contains rigid barriers to dislocation motion, (b) deforms by wavy slip, and (c) forms a cell substructure upon deformation.     

Shock consolidation of mechanically alloyed amorphous Ti-Si powders

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. Formerly Undergraduate Research Assistant, School of Materials Science and Engineering, Georgia Institute of Technology. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.