Method of liquid phase sintering a two-phase alloy

Abstract

Liquid phase sintering method for a two-phase alloy includes forming a green body billet of a two-phase alloy, solid state sintering the green body billet, surrounding the solid state sintered billet with a refractory barrier medium within a refractory container to form a charge, optionally flowing wet hydrogen through at least a portion of the charge, equilibrating a charge temperature below a solidus temperature of the two-phase alloy, changing the charge temperature to a liquid phase sintering temperature of the two-phase alloy, maintaining the liquid phase sintering temperature for a period of time of ≦four hours, reducing the charge temperature to less than the solidus temperature of the two-phase alloy, and optionally holding the charge stationary as the charge temperature passes through the solidus temperature. Optionally, the charge can be rotated about an axis of symmetry during liquid phase sintering and a portion of the charge can be zone heated.

What is claimed is:  
     
         1 . A method to liquid phase sinter a two-phase alloy, the method comprising: 
 forming a green body billet of a two-phase alloy;    solid state sintering the green body billet;    forming a charge by surrounding the solid state sintered billet by a refractory barrier medium within a refractory container, wherein the refractory barrier medium prevents contact between the solid state sintered billet and the refractory container;    equilibrating a temperature of the charge below a solidus temperature of the two-phase alloy;    changing the temperature of the charge to a liquid phase sintering temperature of the two-phase alloy;    maintaining the liquid phase sintering temperature for a period of time of less than or equal to four hours; and    reducing the temperature of the charge to less than the solidus temperature of the two-phase alloy.    
     
     
         2 . The method of  claim 1 , wherein solid state sintering the green body billet results in at least 80% theoretical density.  
     
     
         3 . The method of  claim 1 , wherein the refractory barrier medium is a ceramic liner, a ceramic sand, or an open cell ceramic foam.  
     
     
         4 . The method of  claim 3 , wherein the ceramic sand is Al 2 O 3 , ZrO 2 , or MgO.  
     
     
         5 . The method of  claim 3 , wherein the ceramic sand has a grain size of −325 to 80 mesh  
     
     
         6 . The method of  claim 1 , wherein the refractory container is formed of a metallic material.  
     
     
         7 . The method of  claim 6 , wherein the metallic material is a Mo-based alloy or a W-based alloy.  
     
     
         8 . The method of  claim 1 , wherein the refractory barrier medium is permeable to a wet hydrogen atmosphere and the method comprises flowing wet hydrogen through at least a portion of the charge.  
     
     
         9 . The method of  claim 8 , wherein the wet hydrogen atmosphere contacts at least a portion of the two-phase alloy.  
     
     
         10 . The method of  claim 8 , wherein the wet hydrogen has a pressure of 3 to 4 psi.  
     
     
         11 . The method of  claim 1 , wherein equilibrating a temperature of the charge below a solidus temperature of the two-phase alloy is equilibrating at less than 20° C. below the solidus temperature.  
     
     
         12 . The method of  claim 1 , wherein changing the temperature of the charge to a liquid phase sintering temperature of the two-phase alloy is changing the temperature at a rate of from 40° C./hr to 400° C./hr.  
     
     
         13 . The method of  claim 1 , wherein the period of time for maintaining the liquid phase sintering temperature is from 0.3 hours to 1.5 hours.  
     
     
         14 . The method of  claim 1 , wherein reducing the temperature of the charge to less than the solidus temperature of the two-phase alloy is reducing the temperature at a rate of from 20° C./hr to 100° C./hr.  
     
     
         15 . The method of  claim 1 , wherein the two-phase alloy is a tungsten heavy alloy.  
     
     
         16 . The method of  claim 15 , wherein the tungsten heavy alloy includes less than or equal to 93 wt. % tungsten.  
     
     
         17 . The method of  claim 15 , wherein the tungsten heavy alloy includes less than or equal to 93 wt. % tungsten and the balance at least one secondary element selected from the group consisting of Ni, Fe and Co.  
     
     
         18 . The method of  claim 15 , wherein the solidus temperature is 1475±20° C.  
     
     
         19 . The method of  claim 15 , wherein the liquid phase sintering temperature is 1535±20° C.  
     
     
         20 . The method of  claim 1 , wherein the charge has an axis of symmetry and the method comprises rotating the charge about the axis of symmetry during at least a portion of the method during which the temperature of the charge is above the solidus temperature.  
     
     
         21 . The method of  claim 20 , wherein a rotation rate of the rotating charge is from 1 to several cycles per minute.  
     
     
         22 . The method of  claim 1 , wherein the charge has a cylindrical shape with an axis in a height dimension and the method comprises rotating the charge about the axis of symmetry during at least a portion of the method during which the temperature of the charge is above the solidus temperature.  
     
     
         23 . The method of  claim 22 , wherein a rotation rate of the rotating charge is from 1 to several cycles per minute.  
     
     
         24 . The method of  claim 1 , comprising holding the charge stationary as the temperature passes through the solidus temperature during the step of reducing the temperature of the charge to less than the solidus temperature.  
     
     
         25 . The method of  claim 1 , wherein the charge is placed in a partial vacuum or an atmospheric furnace.  
     
     
         26 . The method of  claim 1 , wherein the temperature of the charge is equilibrated, changed, or maintained by radiative heating, resistive heating, or electromagnetic heating.  
     
     
         27 . The method of  claim 26 , wherein electromagnetic heating includes RF heating or MW heating.  
     
     
         28 . The method of  claim 1 , comprising zone heating a portion of the charge to liquid phase sinter the two-phase alloy.  
     
     
         29 . The method of  claim 28 , wherein zone heating comprises heating the portion of the charge to the liquid phase sintering temperature to form a heating zone and traversing the heating zone from a first end of the charge to a second end of the charge by relative motion between the charge and a heating element.  
     
     
         30 . The method of  claim 29 , wherein the relative motion is step-wise or continuous.  
     
     
         31 . The method of  claim 29 , wherein the relative motion is at a rate of 1 to 5 cm per hour.  
     
     
         32 . The method of  claim 28 , wherein zone heating occurs during the step of changing the temperature of the charge to a liquid phase sintering temperature of the two-phase alloy.  
     
     
         33 . The method of  claim 28 , wherein the charge has an axis of symmetry and the method comprises rotating the charge about the axis of symmetry during at least a portion of the method during which the temperature of the charge is above the solidus temperature.  
     
     
         34 . The method of  claim 33 , wherein a rotation rate of the rotating charge is from 1 to several cycles per minute.  
     
     
         35 . The method of  claim 28 , wherein the charge has a cylindrical shape with an axis in a height dimension and the method comprises rotating the charge about the axis of symmetry during at least a portion of the method during which the temperature of the charge is above the solidus temperature.  
     
     
         36 . The method of  claim 35 , wherein a rotation rate of the rotating charge is from 1 to several cycles per minute.