Abstract:
A method and apparatus are provided which enable the nondestructive testing of strength of a heat treated alloy. An alloy is insonified with an ultrasonic signal. The resulting convoluted signal is detected and the acoustic nonlinearity parameter is determined. The acoustic nonlinearity parameter shows a peak corresponding to a peak in material strength.

Description:
CROSS-REFERENCE 
     Pursuant to 35 U.S.C. §119, the benefit of priority from provisional application No. 60/050,915, with a filing date of Apr. 24, 1997, is claimed for this non-provisional application. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to nondestructive measurement of strength of heat-treated precipitation-strengthened alloys and specifically to the monitoring of the harmonic content of an acoustic signal passed through the specimen during the heat treatment process. 
     2. Discussion of the Related Art 
     Generally, heat-treatment is performed according to compiled data. A recipe is followed, according to previous experience, to arrive at a heat treatment time which will produce a maximum strength for a given alloy. The compiled data method assumes, however, that the material being treated is identical to those used to compile the data. This is not usually the case as the material is generally not homogeneous in constituent composition and the composition from batch to batch is generally different. Thus such methods can only provide an estimate as to appropriate heat treatment parameters. 
     Methods of determining strength are known which are destructive, such as tensile or torsional strength tests. Other methods are not as destructive, but only assess surface strength of the material such as surface hardness tests. These methods are static and generally require that the material be removed from the heat treatment process. 
     A large class of alloys are strengthened by precipitates which contain a different local chemical composition from that of the bulk metal matrix. It has been well established that optimal strength levels may be achieved in certain alloy systems when relatively large strains become set up at or near the interfaces between the precipitates and the surrounding matrix. The strain fields are very effective in blocking the motion of point and line defects through the metal when a load is applied. From a processing standpoint, the maximum strength is achieved by heat treating alloys at the proper temperature for an optimal length of time. During initial hardening, precipitates begin to cluster together in very small groups known as zones. With increased time, the zones grow larger and are known as particles or precipitates and the strain fields become stronger up to a maximum. Aging for too long leads to a decrease in material strength as the strain fields diminish in strength due to continuing growth of the precipitates. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for monitoring a heat treatment process in a nondestructive manner. 
     It is a further object of the present invention to provide a method which allows for monitoring of the heat treatment process without first stopping the process. 
     To achieve the forgoing objects a heat treatable alloy is provided. While undergoing heat treatment, also known as artificial aging or precipitation hardening, the material is insonified with ultrasonic waves. The resulting signal is monitored and the acoustic nonlinearity parameter is calculated. The acoustic nonlinearity parameter is then used to predict the strength of the material being interrogated. 
     The method may of course be used on a specimen that has already undergone heat treatment with similar results. The greatest advantage, however, is realized in real time monitoring of the heat treatment process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a graph of a series of measurements of average Vickers hardness against precipitation heat treatment time for 2024 Aluminum alloy. It also shows a series of calculated values for the normalized acoustic nonlinearity parameter against precipitation heat treatment time; 
     FIG. 2 is a flow chart illustrating steps in one example of the application of the present invention; and 
     FIG. 3 schematically illustrates one possible embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A workpiece, made from a heat treatable metallic alloy, is prepared for heat treatment. A transducer is acoustically coupled to the workpiece. The transducer is preferably capable of producing an acoustic signal having a wavelength much larger than the grain and precipitate size of the material to be monitored. The transducer also preferably is capable of producing a signal which is substantially sinusoidal. The transducer is also preferably selected to be able to withstand high temperatures such as those used in heat treatment processes. 
     As the workpiece is heat treated, it is insonified by the transducer. The resulting signal is monitored. The monitored signal may be treated in a variety of ways. The amplitude of the fundamental signal may be monitored and via a feedback system kept at a constant amplitude. Then, the second harmonic of the signal may be monitored and used to indicate the changes in material strength. 
     Another method makes use of the same two measurements to calculate a value for the acoustic nonlinearity parameter (see Eq. 1, below for the calculation of the acoustic nonlinearity parameter). Since the acoustic nonlinearity parameter is proportional to the amplitude of the second harmonic signal when the fundamental signal is constant, either value may be conveniently used to monitor changes in material strength. It is important to note, however that if the second harmonic signal is used alone that it must be normalized as described above, by keeping constant the fundamental signal. 
     For the sake of clarity, only the acoustic nonlinearity parameter will be discussed in the following explanation, however it is evident that the second harmonic signal amplitude could be used in its place. As the heat treatment progresses, the acoustic nonlinearity parameter will display a series of peaks. Each peak corresponds to the dominance of a particular precipitate in its contribution to material strength. In some cases more than one precipitate will form at about the same time so a single peak could correspond to more than one precipitate. In a given alloy, there are a known number of precipitates which contribute to the material&#39;s heat treated strength. Thus, for a given alloy there are a given number of peaks expected. Once the peak which corresponds to maximum material strength is determined, the heat treatment process can be controlled through a feedback system, the heat treatment ending when the appropriate peak in acoustic nonlinearity parameter is reached. 
     EXAMPLE 1 
     In one example of the application of the present invention, the artificial aging of aluminum alloy 2024 from the T4 to the T6 temper was monitored (see FIG.  2 ). Samples of stock aluminum alloy were heat treated in 72 minute increments at a temperature of 190° C. (step  1 ) for 12 hours according to ASM standards to obtain the transformation from T4 to the T6 temper (step  7 ). In order to monitor the changes in the nonlinearity parameter and the hardness during the transformation eleven sets of samples were sectioned in sequence from bar stock, each set consisting of a pair of disks. One set was removed from heat treatment every 72 minutes for the 12 hour duration of the heating and quenched in cold running tap water (steps  2  and  3 ). 
     A Vickers hardness test was performed on one of each pair (step  4 ), the other was insonified by a transducer axially aligned with the sample and producing a 5 MHz ultrasonic signal (step  5 ). Acoustic harmonic generation measurements were made on each sample and the acoustic nonlinearity parameters were calculated from these measurements in accordance with Eq. 1. 
      β=8 /k   2   d B   2   /A   1   2   Eq. 1 
     Where β is the acoustic nonlinearity parameter, A 1  is the amplitude of the acoustic wave fundamental signal, B 2  is the amplitude of the second harmonic signal, d is the wave propagation distance, and k is the wave number. 
     The results of the measurements are given in FIG. 1 which show graphs of the acoustic nonlinearity parameter and the measured Vickers hardness, both plotted as a function of heat treatment time (step  6 ). The nonlinearity parameter is normalized with respect to the value for the T4 temper. 
     It can be seen that two distinct peaks appear on the graph of nonlinearity parameter. The first peak can be explained in terms of the precipitation and reversion of GP zones. When heat treatment begins, the samples are warmed from approx. 25° C. to a temperature of 190° C. As the temperature rises in the samples, GP zones begin to precipitate more rapidly than had been occurring in the samples while stored at room temperature due to natural aging. The more rapid precipitation of GP zones produces an increase in the strength and in the value of the nonlinearity parameter as the result of the coherency strains generated. This process continues until the GP zone solvus temperature of approximately 180° C. is reached. At this point a dissolving of the GP zones back into the matrix occurs, resulting in decreases in the coherency strains and thus in the material strength and in the value of the nonlinearity parameter. 
     At the dissolution of the GP zones, the growth of S′ precipitates begins. The S′ precipitates are the primary strengthening precipitates of the material. A second peak in the nonlinearity parameter curve appears, corresponding to the growth of these precipitates. The second peak corresponds to a maximum Vickers hardness measured for the material. Beyond this time, over aging begins to occur and the nonlinearity parameter drops as does the hardness of the material. 
     FIG. 3 is a schematic representation of one possible embodiment of the present invention. In this embodiment, a workpiece  13 , made from a heat treatable metallic alloy is prepared for heat treatment by heat treatment apparatus  11 . An acoustic source and transducer  12  is acoustically coupled to the workpiece. As the workpiece  13  is heat treated, it is simultaneously insonified by the acoustic source and transducer  12 . The resulting signal is monitored by the receiver/monitor  14 . For example, in at least one embodiment, in a known manner, the receiver/monitor  14  can include a transducer, a filter for isolating and measuring the fundamental signal, and a filter for isolating and measuring the second harmonic signal of the resultant signal. A controller and recorder device  15  (which in at least one embodiment can be in the form of a computer) can be operatively connected to the heat treatment apparatus  11 , the acoustic source and transducer  12  and the receiver/monitor  14 . In at least one embodiment, the controller and recorder device  15  can be supplied predetermined data  16 , which predetermined data  16  can include, for example, one or more expected measurements of at least a portion of the monitored resulting signal, such as the second harmonic. 
     In one possible embodiment, the controller and recorder device  15  can utilize the signal measurements provided by the receiver/monitor  14  to calculate the acoustic nonlinearity parameter, which nonlinearity parameter, as described hereinabove, can be used to indicate the changes in material strength or hardness (see FIG.  1 ). In another possible embodiment of the present invention, the heat treatment apparatus  11  can heat treat the metallic alloy  13  prior to the insonification by the acoustic source and transducer  12 . 
     Other variations will be readily apparent to those of skill in the art. The forgoing is not intended to be an exhaustive list of modifications but rather is given by way of example. It is understood that it is in no way limited to the above embodiments, but is capable of numerous modifications within the scope of the following claims.