Yoshihisa Shirai

A Study of High Temperature Viscoplastic Deformation of Beta Titanium Alloy Considering Yield-Point Phenomena

In this paper, the high temperature, deformation behaviour of beta titanium alloy Ti-20V-4Al-1Sn sheet is studied by performing uniaxial tension experiments at three different strain rates at high temperatures of 700°C, 750°C and 800°C. The stress-strain curves for these temperatures show strain rate sensitivity, yield point phenomena and continuous flow, softening patterns. Microstructures of deformed specimens at several representative deformation stages and different strain rates are studied using an optical microscope. Dynamic recovery does not occur at the early stage of deformation including the yield-point and the subsequent yield drop regime, but it is activated at a large deformation stage, where it is affected by both strain rate and strain. A viscoplastic, constitutive model, based on the assumption of rapid dislocation multiplication, is proposed to describe such high temperature, yield-point phenomena. In this modelling, the softening effect due to dynamic recovery is also considered. The stress-strain responses, predicted by the constitutive model, well capture the yield-point phenomena, strain rate sensitivity and subsequent continuous flow, softening behaviour of the beta titanium alloy.

Difference of Microstructure and Fatigue Properties between Forged and Rolled Ti-6Al-4V

In the Present Study, the Effects of the Microstructural Morphologies of a Ti-6Al-4V (Ti-64) Alloy on its Fatigue Behavior Were Investigated. Ti-64 Bars Were Subjected to Two Different Thermo-Mechanical Processing Methods. The First Sample, Referred to as Material-A, Had a Forged Microstructure with the Average Primary α Volume Fraction of 44%. The Second One, Referred to as Material-B, Had a Hot-Rolled Microstructure with the Average Primary α Volume Fraction of 43%. Fatigue Tests Were Performed on each Sample to Obtain S-N Curves. The Microstructure of each Sample Was Observed Using an Optical Microscopy in Order to Measure the Grain Sizes of the Primary α and Secondary α Phases. The Results of the Fatigue Tests Indicated that Material-B Demonstrates Better Fatigue Strength than Material-A. The Microstructure of the Longitudinal Section of each Material Was Also Observed to Analyze the Results of the Fatigue Tests. The Measured Diameters and Volume Fractions of the Primary α Phases of the Two Types of Materials Are Similar. On the other Hand, the Secondary α Width of each Material Is Different. It Is Found that Fatigue Strength Is Related to the Width of the Secondary α Phase.

Superplasticity in the Aerospace Titanium Alloy Ti-5553

Titanium alloys are preferentially used in aerospace industry mainly because of the highest strength to density in metals and alloys. A variety of titanium alloys such as Ti-6Al-4V, Ti-10V2Fe-3Al and some heat-resistant titanium alloys have widely been used for airframe structures and jet engines [1]. Ti-6Al-4V designated as Ti-64 is an α+β type alloy which has enormously been used as the most conventional titanium alloy even in aerospace industry and it is well known that Ti-64 can exhibit good superplasticity with the dual phase microstructure consisting of fine α grains in β matrix. In addition, one of the present author reported that Ti-3Al-5V alloy exhibiting microstructure consisting of fine α grains small as 1μm in β matrix, which was obtained by conventional cold rolling with subsequent low temperature annealing, also exhibits excellent superplasticity with tensile test conditions of much lower temperature and much higher strain rates than those for Ti-64. On the other hand, Ti-10V-2Fe-3Al designated as Ti-1023 is classified into near β type alloy and was developed in the late 1970s especially for structural airframes and landing gears produced by forging and subsequent heat treatments and exhibits good combination of high strength and high fracture toughness by designing microstructure[2,3].