METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 28A, No. 6, June 1997 This Month Featuring: Alloy Phases, Transformations, Mechanical Behavior, Welding & Joining, Solidification, Composite Materials. View June 1997 Contents.

ALLOY PHASES

TRANSFORMATIONS

MECHANICAL BEHAVIOR

Solidification Structure and Abrasion Resistance of High Chromium White Irons
Ö.N. DOAN, J.A. HAWK, and G. LAIRD II
Superior abrasive wear resistance, combined with relatively low production costs, makes high Cr white cast irons (WCIs) particularly attractive for applications in the grinding, milling, and pumping apparatus used to process hard materials. Hypoeutectic, eutectic, and hypereutectic cast iron compositions, containing either 15 or 26 wt pct chromium, were studied with respect to the macrostructural transitions of the castings, solidification paths, and resulting microstructures when poured with varying superheats. Completely equiaxed macrostructures were produced in thick section castings with slightly hypereutectic compositions. High-stress abrasive wear tests were then performed on the various alloys to examine the influence of both macrostructure and microstructure on wear resistance. Results indicated that the alloys with a primarily austenitic matrix had a higher abrasion resistance than similar alloys with a pearlitic/bainitic matrix. Improvement in abrasion resistance was partially attributed to the ability of the austenite to transform to martensite at the wear surface during the abrasion process.

Modeling Solid Solution Strengthening in Nickel Alloys
H.A. ROTH, C.L. DAVIS, and R.C. THOMSON
The yield stress of multicomponent nickel solid solution alloys has not been modeled in the past with respect to the effects of composition and temperature. There have been investigations of the effect on the yield stress of solutes in binary systems at a fixed temperature, but the effects on the yield stress of multiple solute elements and temperature changes have not been investigated. In this article, two different forms of the trough model are considered for nickel-base alloys to determine the most applicable model for solid solution strengthening in the system. The yield stresses of three binary nickel-chromium and three ternary nickel alloys were determined at a range of temperatures. The yield stress of the alloys was then modeled using the Feltham equation. The constants determined in fitting the Feltham equation to the experimental data were then applied to other experimental solid solution alloys and also to published information on commercial solid solution nickel alloys. It was found that the yield stress of the nickel solid solution alloys could be modeled successfully using the Feltham equation.

Fractal Fracture and Transformation Toughening in CuNiAl Single Crystal
C.H. SHEK, G.M. LIN, J.K.L. LAI, and Z.F. TANG
The fracture toughness J_Ic() and fractal dimension D_f of fractured surfaces of CuNiAl single crystal have been measured at temperatures in the range 20°C to 120°C, in which thermoelastic martensitic transformation takes place. The parent phase has higher fracture toughness than the martensite phase due to stress-induced transformation. The relationship between J_Ic() and D_f has been studied. It was found that log J_Ic() is linearly related to D_f if the failure involves a single mechanism. The slope of the plot may either be positive or negative for brittle or ductile failure, respectively. The difference in the J_Ic()-D_f correlation can be understood in terms of the micromechanism of fracture.

The Entering Behavior of Environmental Gases into the Plastic Zone around Fatigue Crack Tips in Titanium
M. SHIMOJO, R. IGUCHI, T.H. MYEONG, and Y. HIGO
Fatigue crack growth tests were performed on titanium in vacuum, a nitrogen gas, inert gases (helium and argon), and air. Fracture surface morphologies were different from each other even if these environments were mild. Microcracks which were parallel to the striations were observed on the fracture surfaces, and the frequency of them increased with the environment becoming active (He < N₂ < Air). These results suggest that the environment has some effects on the deformation behavior in the plastic zone at the crack tip. The concentration of nitrogen in the plastic zone around a crack surface tested in nitrogen was analyzed using electron probe microanalysis (EPMA). The concentration of nitrogen in the plastic zone, especially in the cyclic plastic zone, increased significantly. The result indicates that nitrogen may be adsorbed on the fresh surfaces produced at the crack tip during loading and diffuse into the cyclic plastic zone with cyclic dislocation movement. Considering all the results, it can be thought that atoms of environmental gases including argon, as well as nitrogen, have some effects on the chemical composition of the cyclic plastic zone.

Creep and Low-Cycle Fatigue Behavior of Ferritic Fe-24Cr- 4Al Alloy in the Dynamic Strain Aging Regime: Effect of Aluminum Addition
S.C. TJONG and S.M. ZHU
Creep and low-cycle fatigue behavior of ferritic Fe-24Cr-4Al alloy was studied in the temperature range of 673 to 873 K, where dynamic strain aging (DSA) occurrence was found. The DSA of the alloy manifested in the form of serrated flow, negative strain rate sensitivity, and the peak or plateau in the variations of yield strength (YS) and ultimate tensile strength (UTS) with temperature. The characteristic creep behavior of the alloy was experimentally verified as that for a class I solid solution. However, this ferritic alloy showed an anomalous high stress exponent (n = 5.7) and high activation energy (Q_c = 285 kJ/mol) of the secondary creep, which were commonly exhibited by class II solid solutions. During cyclic deformation, the alloy displayed serration in the stress-strain hysteresis loops, increased cyclic hardening, and enhanced planarity of dislocations. On the basis of the observed experimental results and proper analysis, it was proposed that there was strong elastic interaction between solute aluminum atoms and dislocations in the DSA temperature domain. The anomalous creep and fatigue features were interpreted in terms of the interaction of aluminum with the dislocations.

Effects of Microstructure on the Fracture Toughness of Ti₃Al-Based Titanium Aluminides
X. WU and P. BOWEN
The influence of microstructure on the fracture toughness of Ti-23Al-9Nb-2Mo-1Zr-1.2Si (at. pct) and Ti-23Al-11Nb-0.9Si (at. pct) Ti₃Al-based alloys has been investigated. Basket-weave microstructures comprising different volume fractions of ₂ and retained phases were produced by systematic heat treatments. Besides the volume fraction of the retained phase, the average size of the laths has also been used to characterize these microstructures. The toughness of both alloys was examined at room temperature, and the brittle transgranular fracture modes were found to be controlled by microstructure. However, the toughness is not determined solely by the volume fraction of the retained phase, and a linear relationship has been obtained between the fracture toughness and the average size of the retained laths. It appears therefore that the toughness of Ti₃Al-based alloys at room temperature is controlled primarily by the width of retained laths rather than by the retained volume fraction.

WELDING & JOINING

SOLIDIFICATION

COMPOSITE MATERIALS