Texture Strengthening and Fracture Toughness of Titanium Alloy Sheet at Room and Cryogenic Temperatures

Abstract

SUMMARYPreferred crystallographic orientation or texturing in metallic sheet can, under conditions of biaxial stress, result in strengthening considerably greater than that predicted f o r a n isotropic material. Certain titanium alloys exhibit texturing. After it is rolled into sheet form, titanium alloyed with about 4 percent aluminum gives close to the ideal texture f o r maximum biaxial strengthening. The commercially available titanium alloy, Ti-5A1-2. 5Sn, also exhibits texture strengthening. The effect of texturing on the yield, ultimate, notch, and weld strengths of both Ti-4A1-0.20 and Ti-5A1-2.5Sn extra-lowinterstitial (ELI) grade was determined by testing 0.020-inch-(0.051-cm-) thick sheet in uniaxial and 1 to 2 biaxial stress fields at 20°, 77' , and 294' K. The biaxial test specimens were cylinders 6 inches (15.2 cm) in diameter and 18 inches (45.7 cm) long.In addition, plastic Poisson's ratios were determined at the three test temperatures.The ratio of 1 to 2 biaxial to uniaxial yield strength ranged from 1. 36 at 20' K to 1.53 at 294' K for the Ti-4A1-0.20 alloy (room-temperature plastic Poisson's ratio 0.845) and f r o m 1.24 at 20' K to 1. 37 at 294' K for the Ti-5A1-2.5Sn ELI alloy (room temperature p 0.732). Substantial increases in ultimate strength were also obtained. Although the Ti-4A1-0.20 alloy had less uniaxial strength than did the Ti-5A1-2.5Sn ELI alloy, its more heavily textured condition generally resulted in greater 1 to 2 biaxial strength.strength. Tests of through-cracked cylinders at 20' and 77' K showed the Ti-4A1-0.20 alloy to have greater notch sensitivity. However, f o r through cracks 0.10 inch (0.25 cm) o r less, the 1 to 2 biaxial notch strengths of the two alloys are about equal. The use of plastic Poisson's ratio to predict the increase in biaxial yield strength resulted in good agreement with experimental values. pP7 P'The Ti-4A1-0.20 alloy a l s o exhibited superior biaxial weld INTRO D UCT IONCertain anisotropy in sheet materials can cause appreciable strengthening under conditions of biaxial stress. The form of anisotropy r e f e r r e d to is called texturing and is characterized by isotropy in the plane of the sheet with considerable anisotropy normal to this plane. For the purely isotropic case, yielding in a 1 to 2 stress field is predicted to occur at 1.15 times the uniaxial yield strength. For textured materials, however, yield strengths 1 . 5 times uniaxial yield strength or greater have been predicted for the same s t r e s s field (ref. 1). Uniaxial studies of texturing in titanium alloys have been made by several investigators (refs. 2 to 4). Sliney, e t al. (ref. 5) and Babel, et al. (ref. 6) have experimentally investigated the biaxial strengthening of Ti-5A1-2.5Sn alloy cylindrical p r e s s u r e vessels at room temperature. This work r e p o r t s the r e s u l t s of a n investigation of biaxial strengthening in a n experimental textured titanium alloy, Ti-4A1-0.20, at temperatures as low as 20' K.of the hexagonal close-picked crystals are parall...

Properties of Textured Titanium Alloys

“…Their work on Ti-5AI-2.5Sn alloy vessels has also shown large increases in both biaxial yield strength and biaxial burst strength. One of their programs [94] resulted in the curves shown in Figures 38 and 39. These curves clearly demonstrate the vastly improved biaxial properties achievable through texture strengthening.…”

“…Very little information has been published on the influence of texture upon fracture toughness, K1c or Kc• Although there has been some discussion [95,124,125], most of the reports have not had pole figures and fracturetoughness values for the same material. Probably the most complete investigation was carried out on the SST program at the Boeing Co. [ 1•8] This program has shown that a large anisotropic variation in fracture toughness is present in Ti•6AI-4V extrusions, plate and sheet.…”

Moiré interferometry applied to fracture in titanium tubes

“…The material here had certainly been annealed but although samples of its microstructure were observed to be fine-grained and equiaxed, these facts alone are not sufficient to ensure three-dimensional isotropic texture; unfortunately, a detailed metallurgical study of the threedimensional structure was beyond the scope of this investigation. However, the effects on texture and subgrain-scale features arising from processes involving cold-working and annealing of α-titanium have long been regarded as complex [2,16], and this is an area which continues to be the subject of research by materials scientists, as exemplified by references [17] and [18]. Indeed, the anisotropy induced by such processes is now being tailored to suit particular component applications in order to exploit the advantageous fatigue resistance of particular orientations [19].…”