Ernest Levine

The aging behavior of orthorhombic marten site in Ti-6-2-4-6

In Ti-6-2-4-6 alloys, beta transforms to orthorhombic martensite when quenched from a temperature of 1188 or above, X-ray analysis showed that aging at 773 or 873 gradually reduces the degree of orthorhombicity until a hexagonal structure equivalent to alpha, but having the morphological characteristics of the prior martensite, is produced. The orthorhombicity is reduced by solute rejection to beta which forms as particles both homogeneously and heterogeneously within the martensitic structure. The structure at maximum hardness is a fine distribution of Burger’s oriented beta particles in a matrix of martensite of greatly reduced orthorhombicity. Overaging appears to occur as a result of coarsening of the homogeneous beta particles. It is shown that aging at temperatures from 873 to 1083 K results in growth of one particular varient of the beta which is located at the interface between twin related regions composing a martensite lath. This beta along with similarly oriented beta at lath interfaces forms a continuous beta matrix by a gradual growth process. It is shown that this matrix has the identical orientation and shape of the original beta grain prior to quenching. A mechanism is proposed to account for this “memory effect”.

Precipitation-hardening in Cu-3.6 wt PCT Ti

The room temperature tensile properties of age-hardened Cu-3.6 wt pct Ti were investigated. The resulting structures of both single- and double-aged samples were examined by transmission electron microscopy. Emphasis was placed on the correlation of structure with mechanical properties. Isothermal aging at 400°C resulted in an increase in yield strength from that of 40 ksi in the solution-treated alloy to a maximum of 98 ksi after aging for 10,000 min (167 hr). The structure associated with maximum strength was that of an aligned disordered cubic coherent precipitate. Examination of deformed specimens showed that slip was heterogeneous both within a grain and along a given shear band. Aging at 550°C resulted in the same aligned cubic coherent precipitate as at 10 min, but continued-isothermal holding caused the growth of a weaker recrystallized structure and thus a decrease in strength. It was concluded that overaging in this system was due entirely to the consumption of the continuous precipitate by the discontinuous. This recrystallized constituent was found to be composed of a copper-rich solid solution and Cu3Ti arranged in alternate parallel lamellae. The hcp Cu3Ti precipitate had its close-packed plane parallel to the {Ill} plane of the matrix, and edge dislocations were found to take up the misfit between these two structures. A series of single- and multiple-aging treatments gave three completely recrystallized structures with approximately the same yield strength. However, the total elongation of these structures decreased as the final aging temperature was reduced. This was attributed to an ordering reaction in the Cu3Ti plates at lower temperatures. Direct evidence of a change in deformation characteristics was obtained by examining specimens deformed to the same extent, but having different degrees of order induced by varying the final aging temperature. The disordered plates were shown to plastically deform along with the matrix, but the ordered plates fractured.

Mercury embrittlement of age-hardened cu-1.9wt Pct cobalt and Cu-3.6 wt Pct Titanium

The embrittlement of age-hardened Cu-1.9 wt pct Co and Cu-3.6 wt pct Ti by mercury was investigated at room temperature. The embrittlement effect on alloys subjected to different aging conditions is correlated with slip mode and precipitate morphology. Copper-cobalt failures in mercury occurred at a stress above conventional yield strength, but below the ultimate tensile strength. The solution-treated condition exhibited the greatest loss in fracture strength. Here, the slip lines were distinct and heterogeneous, while in aged samples, they were diffuse and homogeneous. Heterogeneous slip results in large stress concentrations which can account for the greater embrittlement in the solution-treated structure. In copper-titanium, all failures in mercury occurred in a relatively narrow stress band of 52 to 66 ksi, independent of the state of aging. The yield strength attainable in air ranged from 40 to 98 ksi. In those samples which were aged to form coherent precipitates, failure in mercury occurred below the conventional yield stress. It is thus concluded that the mode of slip and yield strength do not appear to influence significantly the fracture characteristics of copper-titanium in a liquid mercury environment.

Grain size and grain growth in an equiaxed alpha-beta titanium alloy

Methods of revealing grain size in a two-phase α-β titanium alloy have been examined and observations on beta grain growth in the presence of alpha have been carried out. The technique proposed by Greenfield and Margolin1 for revealing β matrix grain sizes has been shown not to produce grain growth. However, for grain sizes of about 10 µm the G.M. technique does not reveal all the grains because of the similarity in orientation in neighboring grains. These clusters of similarly oriented grains are shown to persist as grain growth takes place but the misorientation between grains within a cluster decreases. Both the beta grain growth and alpha particle coarsening follow the same time dependency from which it is shown that a linear relationship exists between α particle size and β grain size. It is proposed that α particles must dissolve from theβ grain edges for β grain growth to occur. The linear dependency between beta grain size,Dβ, and alpha particle size,dα, can be rationalized either on the basis of geometrical or surface tension considerations.

Fracture characteristics of polycrystalline AgCl wet with aqueous solutions

Polycrystalline AgCl, normally ductile in air when deformed in tension, is observed to suffer as much as an 80% loss of ductility with a corresponding decrease in fracture stress when deformed in specific aqueous environments. The fracture path is observed to be intergranular as opposed to a transgranular fracture which is observed when fracture occurs in air or non-embrittling solutions. The selectivity of the embrittling solutions is related to the adsorption of ions which are compatible with Ag+ or Cl− in the host lattice. A mechanism for the embrittlement based on the lowering of the strain energy by replacement of strained bonds at a crack nucleus by strain-free adsorbed-ion bonds is proposed. The adsorbed-ion double layer also serves to stabilize the crack faces.

Deformation behavior of an omega hardened alpha-beta titanium alloy

Abstract Using Ti-Mn alloys, an investigation was made into the characteristics of slip and cracking in a structure containing various amounts of alpha in equilibrium with beta phase which was hardened by the precipitation of omega. Precipitation of omega in the beta resulted in total embrittlement of alloys containing 70 per cent beta or more but increasing the amount of alpha to 55 per cent resulted in an elongation of 5 per cent even though beta was still the matrix. Surface observations showed intense heterogeneity of slip within the omega hardened beta which appeared to initiate failure parallel to the slip band. Crack growth is arrested by the softer alpha phase accounting for the increase in ductility with increasing amount of alpha. Thin film transmission electron microscopy revealed a decreased density of omega particles within a slip band suggesting that slip causes a localized dissolution of omega. This would lead to a preferred path for further slip and account for the large heterogeneous offsets observed. It is concluded that the sequence of slip avalanche leading to localized shear failure or cracking is present for all volume per cents of hardened beta, but the observation of this phenomenon becomes more difficult with increased amount of beta due to the increased localization of slip.

The embrittlement of single crystals and bicrystals of agcl in aqueous salt solutions

Abstract The degree of embrittlement of bicrystal AgCl deformed in aqueous NaBr was studied as a function of the orientation of adjacent grains that make up a boundary and the angle that the grain boundary makes with the tensile axis. Embrittlement was observed when the angle between slip directions in adjacent grains exceeded about 20°. The degree of embrittlement was insensitive to variations in grain boundary angle to the tensile axis except for a boundary completely parallel to the tensile axis. It was concluded that the stress concentration at the head of a slip band with the aid of the environment causes initiation of cracks and that the resolved normal stress is active in propagating these cracks to cause intergranular failure. The embrittlement of notched single crystals of AgCl is attributed to the combined effects of stress concentration at the notch root, environment and the applied normal stress since crack propagation always occurred perpendicular to the applied tensile stress.